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The carmine spider mite, Tetranychus cinnabarinus (Boisduval) and the twospotted spider mite, Tetranychus urticae Koch, are serious pests of strawberries and many other horticultural crops. Control of these pests has been heavily dependent upon chemical acaricides. Objectives of this study were to determine the resistance status of these two pest species to commonly used acaricides on strawberries in a year-round intensive horticultural production region. LC90 of abamectin for adult carmine spider mites was 4% whereas that for adult twospotted spider mites was 24% of the top label rate. LC90s of spiromesifen, etoxazole, hexythiazox and bifenazate were 0.5%, 0.5%, 1.4% and 83% of their respective highest label rates for carmine spider mite eggs, 0.7%, 2.7%, 12.1% and 347% of their respective highest label rates for the nymphs. LC90s of spiromesifen, etoxazole, hexythiazox and bifenazate were 4.6%, 11.1%, 310% and 62% of their respective highest label rates for twospotted spider mite eggs, 3%, 13%, 432,214% and 15% of their respective highest label rates for the nymphs. Our results suggest that T. cinnabarinus have developed resistance to bifenazate and that the T. urticae have developed resistance to hexythiazox. These results strongly emphasize the need to develop resistance management strategies in the region.

Susceptibility to spiromesifen, a tetronic acid derivative, was determined for three imidacloprid-resistant strains and 12 geographically discrete natural populations of Bemisia tabaci (Gennadius) (=Bemisia argentifolii Bellows & Perring) (Hemiptera: Aleyrodidae) from California and Arizona by laboratory bioassays. Newly emerged first instars were sprayed with aqueous serial dilutions of spiromesifen and evaluated for toxicity to establish baseline susceptibility data. Interpopulation variability in susceptibility to spiromesifen was observed among the natural populations of whiteflies up to 29-fold; however, there was only 30-fold difference in susceptibility among natural and resistant populations tested. In general, spiromesifen was quite toxic to first instars across most of their geographic range, with LC50 values ranging from 0.210 to 6.08 μg (AI)/ml. The magnitude of variation was smaller among the three-resistant strains. These results suggest that the observed variability reflect natural variation in spiromesifen susceptibility among all the test populations, possibly due to previous exposure to insecticides at each location. The effectiveness of spiromesifen also was evaluated against all immature stages of whiteflies from three field and two resistant strains. Spiromesifen was significantly more active against early instars of whiteflies based on lower LC50 values recorded compared with the fourth instars. Spiromesifen was effective against the resistant strains including a Q-biotype of B. tabaci from Spain, which is highly resistant to neonicotinoids. Results of this study indicate absence of cross-resistance between spiromesifen and more commonly used neonicotinoids. Our findings suggest that spiromesifen should be considered an ideal candidate for whitefly resistance management programs in rotation with other effective chemistries.

Xylella fastidiosa causes Pierce’s disease (PD), a serious disease in grapevines, and grapevine cultivars vary in susceptibility to X. fastidiosa in the field. The mechanism(s) by which this occurs has not been clearly elucidated. To explore possible mechanisms, X. fastidiosa cells from a PD strain were grown in pure xylem fluid of PD-susceptible grapevines, Vitis vinifera and V. labrusca, versus PD-resistant grapevines, V. champinii and V. smalliana. When grown in xylem fluid from the susceptible species, X. fastidiosa cells formed a heavier biofilm compared to those in xylem fluid from the resistant species. Differential expression of selected genes of X. fastidiosa cultured in the xylem fluids of V. vinifera and V. smalliana was analyzed using a DNA macroarray. Compared with xylem fluid of V. smalliana, xylem fluid of V. vinifera stimulated the expression of X. fastidiosa genes involved in virulence regulation, such as rpfC, gacA, xrvA, gcvR, and cysB, and genes involved in biogenesis of pili and twitching motility, such as pilI, pilU, pilE and pilG. Increased expression of virulence genes likely contributes to the expression of PD symptom in the susceptible grapevines, whereas reduced expression of these genes may lead to limitation of symptoms in resistant grapevines.

Two systemic neonicotinoids, imidacloprid and thiamethoxam, are widely used for residual control of several insect pests in cotton (Gossypium spp.), vegetables, and citrus (Citrus spp.). We evaluated their impact on six species of beneficial arthropods, including four parasitoid species--Aphytis melinus Debach, Gonatocerus ashmeadi Girault, Eretmocerus eremicus Rose & Zolnerowich, and Encarsia formosa Gahan--and two generalist predators--Geocoris punctipes (Say) and Orius insidiosus (Say)--in the laboratory by using a systemic uptake bioassay. Exposure to systemically treated leaves of both neonicotinoids had negative effects on adult survival in all four parasitoids, with higher potency against A. melinus as indicated by a low LC50. Mortality was also high for G. ashmeadi, E. eremicus, and E. formosa after exposure to both compounds but only after 48 h posttreatment. The two predators G. punctipes and O. insidiosus were variably susceptible to imidacloprid and thiamethoxam after 96-h exposure. However, toxicity to these predators may be related to their feeding on foliage and not just contact with surface residues. Our laboratory results contradict suggestions of little impact of these systemic neonicotinoids on parasitoids or predators but field studies will be needed to better quantify the levels of such impacts under natural conditions.

Xylella fastidiosa causes a serious Pierce's disease (PD) in grapevine. Xylella fastidiosa cells from a PD strain were grown in a pure xylem fluid of a susceptible grapevine cultivar vs. xylem fluid from citrus, which is not a host for this strain of X. fastidiosa. When grown in grapevine xylem fluid, cells of the PD strain formed clumps and biofilm formed to a greater extent than in citrus xylem fluid, although the PD strain did grow in xylem fluid of three citrus varieties. The differential expression of selected genes of a PD X. fastidiosa strain cultured in the two xylem fluids was analyzed using a DNA macroarray. Compared with citrus xylem fluid, grapevine xylem fluid stimulated the expression of X. fastidiosa genes involved in virulence regulation, such as gacA, algU, xrvA, and hsq, and also genes involved in the biogenesis of pili and twitching motility, such as fimT, pilI, pilU, and pilY1. Increased gene expression likely contributes to PD expression in grapevine, whereas citrus xylem fluid did not support or possibly suppressed the expression of these virulence genes.

A study was conducted to determine the lethal effects of systemic imidacloprid against the egg stage of the glassy-winged sharpshooter Homalodisca vitripennis , and its mymarid egg parasitoid Gonatocerus ashmeadi . During the development of the sharpshooter embryo within the egg, imidacloprid did not have a lethal effect, even at very high concentrations. However, upon emergence from the egg mass, the 1st instar nymph encountered residues of imidacloprid in the surrounding leaf tissues. When mortality was assessed within 48 h of emergence, there was an excellent dose–response between the imidacloprid concentration and nymph mortality, giving an LC 50 of 39 ng imidacloprid per cm 2 leaf. In the same way, G. ashmeadi did not succumb to lethal levels of imidacloprid during its development within the sharpshooter egg. However, parasitoid adults were sensitive to imidacloprid residues during emergence from the sharpshooter egg. When mortality was assessed within 48 h of emergence, the LC 50 for the parasitoid was 66 ng imidacloprid per cm 2 leaf. In a survey of commercial citrus trees that were treated with imidacloprid, the mean residues of the insecticide within the leaves did not exceed the LC 50 concentrations for either insect. The implications of these results for the integration of biological and chemical controls are discussed.

In laboratory studies we evaluated lethal effects of two insect growth regulators (IGRs) on the egg stage and five larval instars of the glassy-winged sharpshooter (GWSS). The effect of both IGRs was dose-dependent and varied according to the age of the life stage. The activity of buprofezin, a chitin synthesis inhibitor, was significantly greater when topical, residual treatment was applied to the 1st (LC 50 =30.3 μg/ml) or 2nd (LC 50 =39 μg/ml) instars compared to later instars. No significant differences in LC 50 's were observed between older instars and adults. Toxicity responses of GWSS immature stages to buprofezin were compared over 2 years with higher susceptibility observed in the second year. Buprofezin did not delay developmental times from the 1st to the next two instars upon treatment. Pyriproxyfen, a juvenile hormone mimic, had a lethal effect on 1–2 d old GWSS eggs but did not deter late-aged eggs from hatching to 1st instars indicating that it's action was age and stage specific. Tests conducted in the second year showed GWSS eggs to be more sensitive to pyriproxyfen compared to the previous year. No toxicity was observed when older immatures or adults were treated with pyriproxyfen. Results from this study cannot only serve as baselines for buprofezin and pyriproxyfen in resistance-monitoring programmes for GWSS, but can also have significant implications for development of management programs, including accurate timing of the applications of the two IGRs that target the most susceptible stages, eggs and early instars.

The greenhouse whitefly, Trialeurodes vaporariorum (Westwood), is an important pest of strawberries in California, USA. The adults and nymphs feed on phloem sap of leaves to remove the photo-assimilates. The objective of this study is to test the impact of whitefly management with insecticides on strawberry fruit quality. Applications of imidacloprid, thiamethoxam, buprofezin and pyriproxyfen decreased the mean adult white-fly numbers by 2.80-, 2.17-, 1.69- and 1.39-fold, respectively, compared to the untreated control. Similarly, the mean numbers of first and second instar whiteflies were reduced 4.36-, 2.20-, 1.90- and 2.02-fold, respectively, while the mean numbers of third and fourth instars were reduced 5.48-, 2.28-, 2.71- and 1.43-fold, respectively, in plants treated with imidacloprid, thiamethoxam, buprofezin and pyriproxyfen. The mean soluble solids content in imidacloprid, thiamethoxam, buprofezin and pyriproxyfen treatments was 1.04-, 1.06-, 1.03- and 1.04-fold greater, respectively, than that in the control. The whitefly reduction enhanced the mean fruit titratable acidity by 4%-6%. Mean glucose levels in imidacloprid and thiamethoxam treatments were significantly higher than in other treatments. However, the whitefly management did not affect the mean fructose levels. Imidacloprid, thiamethoxam and pyriproxyfen treatments boosted the ascorbic acid levels by up to 4%. The impact of whitefly management on strawberry fruit nutrition and antioxidant capacity is discussed.

Since 1998, the greenhouse whitefly, Trialeurodes vaporariorum Westwood (Homoptera: Aleyrodidae), has emerged as a major insect pest of many horticultural crops in coastal California. Control of this pest has been heavily dependent upon chemical insecticides. Objectives of this study were to determine the status of the greenhouse whitefly susceptibility to neonicotinoid and conventional insecticides on strawberries in Oxnard/Ventura, a year-round intensive horticultural production area of southern California. For bioassay tests, adult whiteflies were collected from commercial strawberry crops, and immatures were directly developed from eggs laid by these adults. LD(50) values of soil-applied imidacloprid, thiamethoxam and dinotefuran were respectively 8.7, 3.2 and 4.9 times higher for the adults, 1.8, 1.2 and 1.5 times higher for the first-instar nymphs and 89.4, 390 and 10.4 times higher for the third-instar nymphs than their top label rates. LC(50) values of foliar-applied imidacloprid, thiamethoxam and acetamiprid were respectively 6.1, 6.0 and 1.7 times higher for the adults and 3.8, 8.7 and 4.4 times higher for the second-instar nymphs than their top label rates. For the adults, LC(90) values of endosulfan, malathion, methomyl, bifenthrin and fenpropathrin were 2.2, 1.2, 1.9, 2.3 and 4.9 times lower than their respective top label rates. Chlorpyrifos was not very effective against the adults, as indicated by its LC(90) being 120% higher than its top label rate. The present results strongly emphasize the need to develop resistance management strategies in the region.

Susceptibility of immatures of the glassy-winged sharpshooter, Homalodisca coagulata (Say) (Hemiptera: Cicadellidae), to 10 insecticides that included chlorpyrifos, dimethoate, endosulfan, bifenthrin, cyfluthrin, esfenvalerate, fenpropathrin, acetamiprid, imidacloprid, and thiamethoxam was evaluated in the laboratory. All five instars were exposed to different doses of each foliar insecticide by the petri dish technique, whereas a systemic uptake method was used to assess the toxicity to imidacloprid and thiamethoxam. All test insecticides exhibited high toxicity to all immature stages of H. coagulata at concentrations below the field recommended rates of each insecticide. Although all five instars were susceptible to test insecticides, mortality was significantly higher in first instars than in the older immatures based on low LC50 values (ranging from 0.017 to 5.75 ng(AI)/ml) with susceptibility decreasing with each successive stage. Fifth instars were generally the least sensitive (LC50 values ranging from 0.325 to 216.63 ng (AI)/ml). These results show that mortality was directly related to age of the insect and suggest that chemical treatment at early stages is more effective than at late stages. Acetamiprid (neonicotinoid) and bifenthrin (pyrethroid) were the most toxic to all five instars, inducing most mortality within 24 h and showing lower LC50 values ranging from 0.017 to 0.686 ng/ml compared with other insecticides (LC50 values ranging from 0.191 to 216.63 ng(AI)/ml). Our data suggest that a diverse group of very effective insecticides are available to growers for controlling all stages of H. coagulata. Knowledge on toxicity of select insecticides to H. coagulata immatures may contribute to our understanding of resistance management in future for this pest by targeting specific life stages instead of the adult stage alone.

Female Homalodisca coagulata (Say) were collected from October 2001 to February 2005 from citrus at the University of California, Riverside. Between 5 and 20 females per sampling date were dissected, and each was assigned an ovarian rank: previtellogenic, vitellogenic, or postvitellogenic. Ovarian ranking was used to characterize H. coagulata reproductive activity. Results of these dissections revealed consistent annual patterns in the proportion of previtellogenic females present in this field population. These patterns indicate that there are two distinct generations annually, with an occasional third generation. A time-dependent model of H. coagulata vitellogenesis cycles in Riverside, CA, was developed, which makes it possible to predict the appearance of the subsequent generation based on previous observed peaks in the proportion of vitellogenic females.

The uptake and persistence of a systemic formulation (240 g l −1 SC) of imidacloprid was studied in grapevines treated by chemigation, with the objective of defining suitable application rates for control of the glassy-winged sharpshooter Homalodisca coagulata Say. The sharpshooter is an important vector of Pierce's Disease in southern California, and insecticide treatments are necessary for effective management of insect populations and disease transmission. Uptake of imidacloprid was most rapid at the highest rates of application (281 and 562 g ha −1 ), reaching target threshold levels within the xylem fluid of 10 μg l −1 within 2 days in younger vines (4 years old). At 141 g ha −1 , however, uptake was slow and threshold levels were not achieved in every vine. In older vines (20 years old), 6–8 days elapsed before threshold levels were detected in vines treated with 281 and 562 g ha −1 . Despite the initial delay in uptake, once the target threshold was reached, it was maintained throughout the season. It is clear from available information on the population dynamics of the glassy-winged sharpshooter, particularly relating to its seasonal movement from citrus orchards to neighboring vineyards, that appropriate timing of insecticide treatments can play a crucial role in the management of this pest.

Homalodisca coagulata Say, adults from three locations in California were subjected to insecticide bioassays to establish baseline toxicity. Initially, two bioassay techniques, petri dish and leaf dip, were compared to determine the most useful method to establish baseline susceptibility data under laboratory and greenhouse conditions. Comparative dose-response data were determined by both techniques to endosulfan, dimethoate, cyfluthrin, and acetamiprid. Toxic values were similar to some insecticides with both techniques but not for all insecticides, revealing susceptibility differences among the three populations of H. coagulata. In subsequent tests, the petri dish technique was selected to establish baseline susceptibility data to various contact insecticides. A systemic uptake bioassay was adapted to estimate dose-mortality responses to a systemic insecticide, imidacloprid. A 2-yr comparison of toxicological responses showed all three populations of H. coagulata to be highly susceptible to 10 insecticides, including chlorpyrifos, dimethoate, endosulfan, bifenthrin, cyfluthrin, esfenvalerate, fenpropathrin, acetamiprid, imidacloprid, and thiamethoxam. In general, two pyrethroids, bifenthrin and esfenvalerate, were the most toxic compounds, followed by two neonicotinoids, acetamiprid and imidacloprid. The LC50 values for all insecticides tested were lower than concentrations used as recommended field rates. Baseline data varied for the three geographically distinct H. coagulata populations with the petri dish technique. Adult H. coagulata collected from San Bernardino County were significantly more susceptible to select pyrethroids compared with adults from Riverside or Kern counties. Adults from San Bernardino County also were more sensitive to two neonicotinoids, acetamiprid and imidacloprid. The highest LC50 values were to endosulfan, which nonetheless proved highly toxic to H. coagulata from all three regions. In the majority of the tests, mortality increased over time resulting in increased susceptibility at 48 h compared with 24 h. These results indicate a wide selection of highly effective insecticides that could aid in managing H. coagulata populations in California.

Gonatocerus ashmeadi is an important egg parasitoid of the glassy-winged sharpshooter, Homalodisca coagulata , in California. Although adults of this parasitoid are easily identified from morphological characteristics, their identification as immatures within eggs is more difficult, particularly during the early stages of development. In this study, we show that esterase profiles determined by polyacrylamide gel electrophoresis (PAGE) can be used to detect this species in parasitized eggs within two days of parasitization. The expression of esterase E 0.17 occurred at a time when mymarid parasitoids would be expected to hatch within the host egg. The intensity of this band increased as the parasitoid developed within the sharpshooter egg. After five days, a second parasitoid esterase, E 0.13 , was detected. These two esterases were clearly distinct from all sharpshooter esterases. The electrophoretic technique was also sensitive enough to detect a polymorphic esterase locus in adult insects originating from San Juan Capistrano in California.

The glassy-winged sharpshooter (GWSS) is a xylem feeder that develops conspicuous, year-round populations in many citrus-producing regions of California. Field studies were conducted in a combined lemon and orange orchard to determine the influence of changing amino acid concentrations on relative densities of GWSS. Nineteen protein amino acids were detected in xylem fluid of both lemon and orange trees. Although annual profiles of total and essential amino acids were similar for each citrus species, mean concentrations were consistently higher in lemons than in oranges for most of the year, except for one critical period in late winter-early spring when concentrations were higher in oranges. Principal component analysis followed by factor analysis was performed individually on lemon and orange data sets to identify a reduced number of orthogonal factors composed of amino acids having similar seasonal profiles. Four factors were identified for each citrus species that accounted for 85% and 79% of the total variation in the orange and lemon analyses, respectively. These were then examined with respect to shifts in GWSS numbers that occurred asynchronously in lemons and oranges over the annual population cycle. Three distinctive number shifts were identified that included a peak in adult numbers in lemons during August 2001, significantly higher numbers in lemons relative to oranges during midwinter, and finally an increase in oranges of both adults and nymphs during spring 2002. Various groups of amino acids, i.e., factors, displayed peak annual or elevated levels during the intervals when shifts in GWSS numbers were occurring. Soluble protein levels in oranges and lemons did not correspond to shifting GWSS numbers as certain amino acids. However, soluble protein levels were higher in oranges during late winter/early spring when GWSS adults were sexually active. Potential roles of these amino acids and proteins in GWSS host selection are discussed.

The impacts of planting date and nitrogen fertilization on cotton (Gossypium hirsutum L.) photosynthesis and soluble carbohydrate contents in relation to silverleaf whitefly, Bemisia tabaci (Gennadius) biotype “B”, populations were examined in field experiments. Cotton planted in late April and early June was treated with 0,112, 168 and 224 kg/N hectare in soil using urea fertilizer. The mean photosynthetic rate of April-planted cotton was 4%J20% higher than that of June-planted cotton early in the season, but 10%J 18% lower than that of June-planted cotton late in the season. The photosynthetic rates for both planting dates were positively correlated with levels of added nitrogen. While levels of glucose for both planting dates were positively correlated with nitrogen levels, fructose and sucrose levels were not. The mean levels of fructose were up to 40% lower, while that of sucrose were up to 59% higher, in April-planted cotton than in June-planted cotton. Levels of photosynthetic rate or stomatal conductance were not correlated with adult whitefly densities for either planting date. Levels of glucose and fructose were positively correlated with whitefly densities only for June-planted cotton late in the season.

The silverleaf whitefly (Bemisia argentifolii Bellows and Perring) is a widely distributed pest of cotton (Gossypium hirsutum L.) and the population levels may be affected by rates of nitrogen fertilization and planting date. Field experiments were conducted to investigate the impact of cotton planting date and nitrogen fertilization on silverleaf whitefly population dynamics. Cotton was planted on 26 April and 8 June, for the early and late plantings, respectively. Nitrogen treatments consisted of soil applications of 0, 112, 168 and 224 kg of nitrogen per hectare. The population levels of adult whiteflies were much higher on early-planted cotton than on late planting. Also, increased numbers of adult whiteflies on both early and late plantings occurred with increasing amounts of applied nitrogen. Applied nitrogen increased seed cotton yields of early plantings but had no effect on the yields of late plantings.

Titers of two systemic neonicotinoid insecticides in citrus trees were measured in conjunction with conventional evaluations of their impact on glassy-winged sharpshooter (Homalodisca coagulata (Say); GWSS) populations. Xylem fluid samples were collected at regular intervals and from multiple locations within field-grown citrus trees to determine imidacloprid and thiamethoxam concentrations using commercial ELISA kits. Uptake profiles varied considerably with peak mean titers of imidacloprid occurring 6-8 weeks after application compared with 2 weeks for thiamethoxam. The persistence of each compound also varied as near-peak levels of imidacloprid were sustained for another 6-10 weeks before gradually declining. In contrast, thiamethoxam titers declined more rapidly after the initial peak, possibly reflecting an application rate only one-quarter of that used for imidacloprid. Within-tree distributions were more similar for the two compounds, with no significant effect due to height of the sample (upper or lower half) or to the quadrant location within the tree, with the exception of one quadrant in the thiamethoxam-treated trees. Substantial reductions in GWSS nymphs and adults were observed in imidacloprid-treated trees during the 2001 trial and were sustained for 4-5 months after treatment. Treatment effects on nymphs were not as well pronounced in the 2002 trial, when overall GWSS infestations were much reduced from the previous year. However, consistently lower adult infestations were still observed in 2002 for both treatments compared with untreated trees. Information on the spatial and temporal profiles in citrus trees was obtained for both compounds to complement field impact data and improve understanding of their pest management potential.

The glassy-winged sharpshooter Homalodisca coagulata Say (Homoptera: Cicadellidae) is a new pest to California agriculture. It is the principal vector of several plant pathogenic diseases, particularly Pierce's Disease in grapevines, and oleander leaf scorch. A microplate-based assay is described that measures the sensitivity of acetylcholinesterase (AChE) activity to inhibition by organophosphorus (OP) insecticides in this important pest. The technique provides users with an accurate measure of the efficacy of OP binding to this target site, and is a valuable tool for monitoring field populations of the insect to determine whether the use of OP insecticides has selected for resistant individuals. The technique will also measure AChE sensitivity in the smoke-tree sharpshooter, H. lacerta Fowler. This species is native to California and is regarded only as a minor pest. Both inhibition and kinetic measurements for the AChE enzymes in these sharpshooters demonstrate the close phylogenetic relationships between the two species.

The impact of urea nitrogen fertilization on silverleaf whitefly, Bemisia argentifolii Bellows & Perring, population dynamics was examined in field-grown cotton (Gossypium hirsutum L.). Five urea nitrogen treatments were tested, consisting of soil applications of 0, 112, 168, and 224 kg nitrogen per hectare, and a combined soil–foliar application of 112:17 kg nitrogen per hectare. A positive response was observed between N application rates and the measured levels of nitrate N in petioles from mature cotton leaves. Similarly, a positive response was observed between N application rates and the numbers of adult and immature whiteflies appearing during population peaks. To determine whether these positive responses were related, we measured the levels of dietary N compounds (proteins and free amino acids) that would be available for insect nutrition in cotton petioles at the different N application rates. Sampling dates and N application treatments affected levels of soluble proteins in cotton petioles, and interactions between sampling dates and treatments were significant. Across all sampling dates, the relationship between N application rates and levels of soluble proteins was linear. Sampling dates also affected levels of total and individual free amino acids. Fertilizer treatments only affected levels of total amino acids, aspartate, asparagine, and arginine plus threonine. Levels of aspartate or asparagine and the N application rates were linearly correlated. No significant correlations were observed between levels of dietary N compounds in cotton petioles and numbers of whiteflies, either adults or immatures, on the cotton plants.

Systemic uptake bioassays using excised cotton leaves confirmed resistance to imidacloprid in a Guatemalan population of the tobacco whitefly Bemisia tabaci Gennadius. Polyacrylamide gel electrophoresis of naphthyl esterases identified the insects as B-types. Upon collection from the field, resistance was determined to be 58-fold relative to a susceptible strain originating in the Imperial Valley of California. Resistance levels increased to 126-fold in this population during its continuous exposure to systemically treated cotton. In biochemical investigations, there was no detectable NADPH-dependent mixed function oxidase metabolism of 14C-imidacloprid at any time during the selection process. In contrast, microsomal preparations from housefly abdomens readily produced significant amounts of the mono-hydroxy and olefin derivatives of the parent compound. Detoxification of imidacloprid by housefly MFOs may account for reports of lower toxicity of the insecticide towards this insect compared with whiteflies, despite similar binding properties between imidacloprid and the nicotinic acetylcholine receptors in both species. © 2003 Society of Chemical Industry

The greenhouse whitefly, Trialeurodes vaporariorum (Westwood), has recently become a major insect pest of strawberries in Southern California. The population dynamics of this pest were monitored over 2 yr in six commercial strawberry fields near the coastal communities of Oxnard and Ventura under two crop-production regimes, summer- and fall-planted strawberries. Adult whitefly numbers generally peaked during the February through May period for fall-planted strawberries and during the October through November period for summer-planted strawberries. Population densities varied greatly among fields within each regime and the differences were likely caused by surrounding alternate host crops.

The efficacy of six novel insecticides against the greenhouse whitefly, Trialeurodes vaporariorum (Westwood), on strawberries was evaluated in greenhouse and field experiments. Soil applied imidacloprid at 10, 20 and 40 mg per plant caused adult mortalities of 82%, 93% and 96%, respectively, at 48 h post-treatment in the greenhouse. A significant residual activity resulted in 84–87% mortality after 40 days and 58–78% mortality after 60 days. Topical applications of pyriproxyfen at rates of 16–65 μg AI/ml completely inhibited egg hatch, whereas lower rates of 0.05–0.20 μg AI/ml resulted in 38–70% egg hatch. However, most nymphs that hatched failed to develop into adults. Using 0.5, 1.0 and 2× their greatest label concentration applications of buprofezin and pyridaben caused 100% mortality of nymphs whereas pymetrozine caused only 70%, 74% and 78%, mortality respectively. Thiamethoxam at 0.5 and 1.0× its greatest label concentration killed 67% and 90% of nymphs, respectively. In field experiments, imidacloprid applications decreased adult densities by 31–61% from 21 to 42 days post treatment compared with controls, and suppressed densities of third and fourth instars by 34–68% after 42 days of treatment. Pyriproxyfen and buprofezin did not significantly affect adult densities on most sampling dates. An application of buprofezin followed by an application of pyriproxifen decreased nymphal densities by 98% on the sampling dates of 4 and 11 May and maintained 61–85% control after the sampling on 1 June. Applications of pyriproxyfen followed by buprofezin had no effect on the nymphal densities on 4 and 11 May but densities had fallen by 78–90% from early to late June. Applications of pyridaben and pymetrozine had no effect on adult densities on any sampling dates. However, pyridaben suppressed densities of third and fourth instars by 39–44% on 4 and 11 May sampling dates, and by 47–73% in late June. Pymetrozine had no effect on the densities of nymphs. Applications of thiamethoxam had no effect on adult densities on most sampling dates and did not significantly change the immature densities compared to the control. The potential roles of these insecticides in integrated greenhouse whitefly management programs are discussed.

The carmine spider mite, Tetranychus cinnabarinus (Boisduval) and the twospotted spider mite, Tetranychus urticae Koch, are serious pests of strawberries and many other horticultural crops. Control of these pests has been heavily dependent upon chemical acaricides. Objectives of this study were to determine the resistance status of these two pest species to commonly used acaricides on strawberries in a year-round intensive horticultural production region. LC90 of abamectin for adult carmine spider mites was 4% whereas that for adult twospotted spider mites was 24% of the top label rate. LC90s of spiromesifen, etoxazole, hexythiazox and bifenazate were 0.5%, 0.5%, 1.4% and 83% of their respective highest label rates for carmine spider mite eggs, 0.7%, 2.7%, 12.1% and 347% of their respective highest label rates for the nymphs. LC90s of spiromesifen, etoxazole, hexythiazox and bifenazate were 4.6%, 11.1%, 310% and 62% of their respective highest label rates for twospotted spider mite eggs, 3%, 13%, 432,214% and 15% of their respective highest label rates for the nymphs. Our results suggest that T. cinnabarinus have developed resistance to bifenazate and that the T. urticae have developed resistance to hexythiazox. These results strongly emphasize the need to develop resistance management strategies in the region.

A survey of 53 Bemisia argentifolii BellowsPerring populations from different agricultural regions in California and Arizona was conducted from 1997 to 1999 to establish baseline toxicological responses to buprofezin and pyriproxyfen. Although both compounds proved to be highly toxic even in minute quantities to specific stages, geographical and temporal differences in responses were detected using a leaf spray bioassay technique. Monitoring for three years revealed that six to seven populations had higher LC50 values but not greater survival when exposed to these two insecticides. A significant difference in relative susceptibility to buprofezin was first observed in late season 1997 in San Joaquin Valley populations with LC50s ranging from 16 to 22 microg (AI)/liter(-1) compared with IC50s of 1 to 3 mg (AI)/liter(-1) in Imperial, Palo Verde Valley and Yuma populations. Whiteflies collected in subsequent years from these and other locations showed an increase in susceptibility to buprofezin. Regional differences in susceptibilities to pyriproxyfen were minimal within the same years. Three years of sampling revealed consistently higher LC50s to pyriproxyfen in populations from Palo Verde Valley, CA, compared with whiteflies from Imperial, San Joaquin Valley or Yuma. As was the case with buprofezin, a decline in LC50s to pyriproxyfen was observed in whiteflies from all locations sampled in 1999. However, no correlation was observed between buprofezin and pyriproxyfen toxicity in any of the strains. The variable toxicities observed to both compounds over a period of 3 yr may be due principally to inherent differences among geographical populations or due to past chemical use which may confer positive or negative cross-resistance to buprofezin or pyriproxyfen.

Two forms of acetylcholinesterase were identified in field populations of the beet armyworm, Spodoptera exigua (Hubner), collected from cotton in San Joaquin Valley, CA. Strains (BESS and BKRR) homogeneous for each variant were isolated and their relative susceptibilities to methomyl, chlorpyrifos, and chlorpyrifos-oxon assessed by topical application bioassay. In comparisons with a laboratory susceptible strain (DOW), BKRR and BESS expressed 68-fold and sevenfold resistance, respectively, to the carbamate methomyl. Neither strain was cross-resistant to chlorpyrifos or its oxygen analog (chlorpyrifos-oxon). In biochemical studies, the BKRR AChE enzyme was ≈30-fold and sixfold more insensitive to methomyl and chlorpyrifos-oxon, respectively, compared with the DOW enzyme. The correlation between the toxicological and biochemical studies provides strong evidence that target-site insensitivity is an important mechanism of resistance to methomyl. The lack of significant cross-resistance to chlorpyrifos suggests that the insensitive AChE in these field populations was selected by methomyl alone and not by the organophosphate.

The impact of nitrogen fertilization on cotton plants, Gossypium hirsutum L., silverleaf whitefly, Bemisia argentifolii Bellows & Perring, population dynamics and honeydew production were investigated in the field at Riverside, California, USA. Treatments were soil applications of 0, 112, 168 and 224 kg nitrogen per hectare, and a soil application of 112 kg of nitrogen plus a foliar application of 17 kg nitrogen per hectare. Increased numbers of both adult and immature whiteflies occurred during population peaks with increasing amounts of applied nitrogen. Higher numbers of whiteflies resulted in increased levels of honeydew. Increasing plant nitrogen also enhanced cotton foliar photosynthetic rates and stomatal conductance, and altered concentrations of glucose, fructose and sucrose in cotton petioles. However, at our treatment levels nitrogen had no effect on seedcotton yield. Petiole glucose levels were significantly correlated with numbers of whitefly adults on leaves during their peak populations. Significant correlations between whitefly numbers and other cotton physiological parameters occurred on only a few sampling dates.

A 3-yr project was initiated in 1993 to examine the effects of insecticides and sustained whitefly, Bemisia argentifolii Bellows & Perring (aka. B. tabaci Gennadius (Strain B)), feeding on alfalfa plant growth and vigor in greenhouse cage studies, and to determine the impact of natural Bemisia whitefly populations on alfalfa forage yields and quality in a large-plot field experiment. Alfalfa plant growth and vigor after exposure to imidacloprid and a mixture of fenpropathrin and acephate insecticides did not differ from untreated plants in the greenhouse. Consequently, foliar and soil applied insecticides were used to manipulate whitefly densities on alfalfa plants to measure whitefly feeding effects on plant growth and forage yield. Heavy whitefly densities on untreated alfalfa plants in the greenhouse resulted in significant reductions in relative growth rates and net assimilation rates as compared with imidacloprid-treated plants that were maintained relatively whitefly-free. Reductions in alfalfa plant growth measured between infested and treated plants were proportional to whitefly densities. Field plot results derived from three crop seasons were relatively consistent with our greenhouse trials. Both experimental approaches clearly showed that alfalfa plants exposed to high densities of whitefly immatures and adults grew at a significantly slower rate andproducedlessfoliage.Asaresultofreducedgrowthrates,alfalfamaturityinthenaturallyinfested plots was estimated to be '7—10 d behind managed plots. Delays in maturity resulted in significant reductions in forage yields of 13—18% during August—September harvests when whitefly populations reached peak abundance. Whitefly feeding stresses also effected hay quality through the reduction of crude protein content and contamination of foliage with honeydew and sooty mold. The status of the Bemisia whiteflies as an economic pest to alfalfa is clearly evident from these studies, but the damage potential of whiteflies in the southwestern United States appears to be restricted to one or two harvest periods during the summer coinciding with peak adult populations and their dispersal from alternate host crops.

Honeydew droplets were collected from nymphal and adult Bemisia argentifolii Bellows and Perring. Nymphs produced a regular pattern of honeydew droplets consisting of periods of production followed by periods where no droplets were produced. The mean length of individual production periods increased from the second (40 ± 1 min) to the fourth (162 ±11 min) stadium, until the late fourth stadium when honeydew production by most nymphs became continuous. In contrast, the periods of no production decreased in length as nymphs developed from the second (35 ± 1 min) to the fourth (23 ± 1 min) stadium. The frequency of droplets during the production periods decreased from the first (80 ± 1 drops per h) to the fourth (22 ± 1 drops per h) stadium. This periodic pattern is consistent with previous reports for Trialeurodes vaporariorum (Westwood), however, the functional significance of this pattern remains unknown. If this cyclic pattern should prove critical to the successful feeding and development of whiteflies, its disruption may decrease survival and provide a novel target for pest control strategies. Second and third instars had higher percentages of di- and trisaccharides (sucrose, trehalulose, and melezitose) and lower percentages of monosaccharides (fructose and glucose) compared to first and fourth instars. This could indicate a change in metabolic needs of nymphs during periods of rapid growth, or indicate that trehalulose synthase producing endosymbiotic bacteria are more abundant, or more actively producing enzyme during the second and third stadia.

A field population of Bemisia argentifolii Bellows & Perring was subjected to selective pressure with 5 insecticide regimens: continuous applications of bifenthrin, endosulfan, chlorpyrifos; bifenthrin- endosulfan- chlorpyrifos rotation and a 1:2bifenthrin + endosulfan mixture. Insecticide resistance developed at variable rates in all whitefly populations under selection during this study. Selection with bifenthrin resulted in early resistance development that steadily increased under continuous selection to 752-fold by generation F27. With chlorpyrifos and endosulfan selection treatments, low levels of resistance developed in generations F7 and F5.respectively (resistance ratios of 8-fold). Resistance development in the rotation and mixture regimens was delayed for an additional 10 generations. The magnihlde of resistance ranged from a high of 752-fold for the single continuous use of bifenthrin to a low of 16-fold resistance for the bifenthrin-endosulfan-chlorpyrifos rotation. The level of resistance was also low under selection by the bifenthrin + endosulfan mixture after 23 generations (resistance ratio = 17-fold). Compared with single, continuous insecticide selections in the greenhouse, the rate of resistance development was significantly reduced by rotations and mixtures of insecticides. For all insecticide-selection regimens, slopes of regression lines increased over many generations. The whitefly population treated with the bifenthrin + endosulfan mixture generated the steepest lines.

Results of earlier studies designed to determine adult whitefly, Bemisia argentifolii Bellows & Perring, densities at which cotton, Gossypium hirsutum L., should be treated to reduce honeydew contamination have been variable. In this study, we determined the relationship between frequency of insecticide applications triggered by thresholds of 5, 10, 15, and 25 adult whiteflies per leaf and honeydew production by B. argentifolii feeding on cotton. Honeydew production was monitored on 15 dates from 21 May to 27 August 1996 in Brawley, CA, using water-sensitive papers placed on upper and lower leaves and bolls. On 3 dates, lint was collected and analyzed for sugars using high-performance liquid chromatography. Densities of adults per 5th mainstem leaf, adults per 10-s vacuum sample, and whitefly immature instars per square centimeter of leaf were generally higher in control plots (1 insecticide application), plots treated at 25 adults (4 applications), and untreated plots than in plots treated at 5, 10, and 15 adults (11, 6, and 5 applications, respectively). In general, numbers of honeydew drops per square centimeter were higher in control, 25-adult, and untreated plots than in 5-, 10-, and 15-adult plots. Papers placed on uppermost bolls received more drops later in the season than did papers placed on the lowest bolls. Honeydew drop production was generally lowest throughout the season in the 5-adult treatment. However, it was not significantly different than the 10- or 15-adult treatments on 13, 20, and 27 August. Amounts (micrograms) of trehalulose on top and bottom bolls were generally highest in the control treatment and lowest in the 5-adult treatment, but there were generally no significant differences among amounts from 5-, 10-, and 15-adult treatments. Honeydew drop densities were unexpectedly low in control and untreated plots on some later dates. Control and untreated plants also had fewer leaves than other plants during these times, suggesting leaf loss may account for some variability in honeydew production during late season. Seedcotton yields differed only between 10- and 25-adult threshold plots. At Brawley, CA, insecticide treatments at 5 and 10 adults per leaf seemed equally sufficient for reducing whitefly honeydew production. However, by treating at the 10 threshold, fewer applications were needed. This would reduce the immediate costs to growers, and also would reduce long term problems associated with insecticide resistance.

Laboratory studies were conducted to test the effects of synthetic and natural insecticides on lettuce leaf consumption by beet armyworm, Spondoptera exigua (Hi.ibner). Two synthetic and 4 natural insecticides we compared: Capture (bifenthrin), Lannate (methomyl), Success (spinosad), Xentari (Bacillus thuringiensis suhsp. aizawai) , Kryocide (cryolite), and Trilogy (90% neem oil extract). Capture, Lannate, Success, and Xentari at 50 ppm (AI) caused 100% mortality within 1 d after exposure and prevented any detectable feeding on leaf disks by neonate S. exigua larvae. Capture, Lannate, Success, and Xentari at 50 ppm allowed only minor feeding by 3rd and 5th instar S. exigua before causing nearly 100% mortality. Xentari at 50 ppm appeared less effective than the synthetic insecticides and Success, but statistical differences among them were not significant. Neither Kryocide nor Trilogy at 50 ppm reduced leaf consumption by neonate, 3rd instar, and 5th instar S. exigua compared with controls. Higher concentrations of Kryocide and Trilogy (1,000 and 10,000 ppm) did significantly reduce leaf consumption by 3rd instar S. exigua . The most effective natural insecticides we tested for reducing lettuce consumption by S. exigua were Success and Xentari.

A field-collected population of the silverleaf whitefly, Bemisia argentifolii, was selected with the nicotinyl compound, imidacloprid, over 32 generations to determine if resistance would develop when maintained under continuous selection pressure in a greenhouse. Resistance was slow to increase at first with low to moderate levels of resistance (RR from 6- to 17-fold) in the first 15 generations of selection. Further selection steadily led to higher levels of resistance, with the greatest resistance ratio at 82-fold, the gradual rise suggesting the involvement of a polygenic system. At the end of the selection, slopes of probit regressions were substantially steeper than earlier, indicating increased homogeneity of imidacloprid resistance in this strain. A hydroponic bioassay featuring systemic uptake of imidacloprid through roots was developed to monitor the changes in resistance to imidacloprid in the selected whitefly strain and in seven field-collected strains from Imperial Valley, California. Six out of seven field-collected strains exhibited low LC50 values (0·002 to 0·512 mg ml-1) compared to the selected resistant strain, with one exception where the LC50 was 0·926 mg ml-1 (RR=15·0). Variation in responses to imidacloprid in the field strains suggest that this technique is sufficiently sensitive to detect differences in susceptibilities of whitefly populations. The imidacloprid-resistant strain showed no cross-resistance to endosulfan, chlorpyrifos or methomyl (RR ranging from 0·4- to 1·5-fold). A low level of cross-resistance was observed to bifenthrin in the IM-R strain at 7-fold. The success of selection for resistance to imidacloprid has serious implications for whitefly control programs that rely heavily on imidacloprid. ©1997 SCI

The effects of pyrethrum and azadirachtin on the ovipositional responses of Bemisia argentifolii Bellows & Perring and Bemisia tabaci (Gennadius) were determined in the laboratory. Choice and no-choice tests revealed that neither B. argentifolii nor B. tabaci females preferred to alight and oviposit on bean plants treated with pyrethrum. In contrast, the number of B. argentifolii females that alighted on control versus azadirachtin-treated plants was not significantly different in all but 1 instance. When given a choice, significantly fewer B. tabaci females landed on azadirachtin-treated plants at observation hours 6, 8, and 24. In no-choice trials, numbers of B. tabaci females alighting on control versus azadirachtin-treated plants were equivalent throughout the entire test period. Moreover, although B. argentifolii oviposited equal numbers of eggs on control and azadirachtin-treated plants in choice and no-choice trials, B. tabaci females laid significantly fewer eggs on plants treated with azadirachtin. Percentage nymphal establishment of B. argentifolii on control versus azadirachtin-treated plants was not significantly different in choice and no-choice trials. Although the percentages of established B. tabaci nymphs were the same on control and azadirachtin-treated plants in the no-choice test, a significantly lower percentage of nymphs were found on treated plants in the choice trials.

Whiteflies and other homopterous insects contain endosymbiotic organisms that are essential for their development and reproduction. Reduction or elimination of the endosymbiotic organisms in insects by antibiotic therapy results in reduced insect growth, death, or lack of reproduction. Thus, endosymbionts provide a potential target for insect control with systemic antibacterial materials, or transgenic plants that produce antibacterial proteins. We investigated the effects of various antibiotics, with different ranges of activity and modes of action, on the whitefly Bemisia argentifolii Bellows & Perring (also known as Bemisia tabaci B biotype) oviposition rate, growth, survival, emergence as adults, and sex ratio. The materials tested were oxytetracycline hydrochloride, rifampicin, penicillin, ampicillin, lysozyme, and chloramphenicol. Method of application (feeding on diet or excised leaves) showed similar effects on adults or nymphs. Of the materials tested, those that had significant negative effects on growth and development of whiteflies (tetracycline and rifampicin) interfere with bacterial protein synthesis. Those materials that primarily attack the bacterial cell walls cell membranes (penicillin, ampicillin, and lysozyme) did not have any significant effect on growth and development. Treatments with antibacterial materials had no significant effect on oviposition rates of adults or the sex ratio of their offspring. Delays in development, and reduction in the percentage of offspring emerging as adults provide ample evidence that antibacterial materials have potential role in management strategies against both adult and immature stages of whiteflies.

The whitefly Bemisia argentifolii Bellows & Perring has been a major pest of crops in the southwestern United States since 1991. However, there has been no published information on whitefly populations on alfalfa, Medicago sativa L., a major crop found throughout this region. In this study, adult and immature Bemisia argentifolii were collected every 2 or 4 wk in 22 commercial fields of alfalfa within localized sites in the Imperial Valley, CA, and in Yuma, AZ, from June 1993 to January 1996. Populations of all stages of whiteflies on alfalfa were low or nonexistent from December to May at both sites. In the Imperial Valley, adult populations increased in June and peaked during September, and decreased from October to November. In Yuma, adult populations peaked during August, and decreased late October to November. Populations of eggs, 1st to 4th instars, late 4th instars, and exuviae at both sites increased in June, July, and August. Compared with densities of eggs and nymphs, densities of exuviae were low on alfalfa. In the Imperial Valley, adult populations were significantly higher in 1993 than in 1995, and in Yuma, adult populations were higher in 1993 than 1994. Greater populations of whiteflies were found in Imperial Valley than in Yuma in 1993 and 1994. Densities of whiteflies of all stages were similar among fields within sites during the season. Eggs were distributed evenly among leaves from top, middle, and bottom strata of plants, whereas nymphs were found mostly on those from the middle and bottom strata. Low percentages of parasitized whitefly nymphs (

Action thresholds for whiteflies, Bemisia argentifolii Bellows & Perring on cotton, Gossypium hirsutum L., are being developed to determine the whitefly densities at which cotton should be treated with insecticides to prevent lint yield losses. However, the effects action threshold treatments have on cotton photosynthesis have not been determined. Changes in the photosynthetic rate of cotton can be used to assess damage to cotton early in the season before lint is harvested. In this study photosynthetic rates and stomatal conductance of cotton treated at 4 different action thresholds for whiteflies were measured 12 times during July and August 1995 in the Imperial Valley of California using a portable photosynthesis system. Action thresholds were 2.5, 5, 10, and 20 adults per leaf. Untreated control plots were also used for comparison in a 5 × 5 Latin square design. Adult, egg, and nymphal whitefly densities on untreated cotton were significantly higher than those on all other treatments. In general, no differences in densities of any stage were seen among the 2.5, 5, and 10 treatments. Densities from the 20 treatment were variable. Photosynthetic rates of untreated cotton leaves were usually significantly lower compared with those of 2.5, 5, and 10 treatments. In general, no significant differences in photosynthetic rates were seen among the 2.5, 5, and 10 treatments. Photosynthetic rates of leaves from the 20 treatment were significantly lower and higher than leaves from the other treatments and the untreated control, respectively, on some dates. In general, no significant differences were detected in stomatal conductance among the 2.5, 5, and 10 treatments, and results were similar to those seen with photosynthesis. Based on photosynthetic rates and stomatal conductance, measurable physiological damage to cotton leaves occurs between action thresholds of 10–20 adult whiteflies per leaf.

A series of field experiments were conducted to evaluate Bemisia tabaci (Genn.) infestations in both sprinkler and furrow irrigated cantaloupe and cotton plots under conditions of intense whitefly pressure in Imperial Valley, CA. Various fungal pathogen and insecticide treatments also were compared within each irrigation regime. A consistent finding throughout all experiments was that densities of immature whiteflies were significantly reduced in sprinkler irrigated plots. This was most evident in sprinkler irrigated cantaloupe plots treated with the insecticide imidacloprid, as plants survived whitefly infestations to produce marketable fruit in these plots only. Similarly, whitefly densities in imidacloprid-treated/sprinkler-irrigated cotton were significantly lower than all other treatments. Spray applications of insect fungal pathogens were for the most part indistinguishable from the untreated control in terms of densities of whiteflies. Results from the first cantaloupe trial indicate that sprinkler irrigation on a daily schedule resulted in consistently lower whitefly infestations compared to a biweekly schedule.

This study investigates the process of transfer of endosymbiotic organisms from adult silverleaf whitefly, Bemisia argentifolii Bellows & Perring (Bellows et al. 1994) (also known as Bemisia tabaci B biotype), into the developing ova. In dissected females, individual mycetocyte cells containing microorganisms were scattered singly among developing oocytes. Oocytes with mycetocytes included were first observed in females 16 h after emergence. The mean number of oocytes >0.1 mm in length per female increased through the 4th d after adult eclosion then leveled off. The number of oocytes containing mycetocytes followed a similar pattern. Stages of mycetocyte inclusion followed a pattern based on the size of the oocyte. Oocytes became associated with a single mycetocyte cell when they were a mean of length of 0.135 ± 0.003 mm (minimum of 0.11 mm). All mature oocytes examined contained a mycetocyte. Mycetocytes were observed inside a common membrane with oocytes, at what becomes the pedicel end of the ova, when oocytes were an average of 0.147 ± 0.004 mm long. In the final stages of ovum development, the plasma of the oocyte completely surrounded the mycetocyte and the chorion was thickened. Mycetocytes included in oocytes had a mean length of 33 ± 0.4 µm and width of 27 ± 0.5 µm. Although the females that were dissected had the opportunity to oviposit, some retained several fully developed ova. Because inclusion of mycetocytes into oocytes is a continuous process, manipulations of endosymbionts before inclusion into the oocytes should be possible at any time during adult life; however, earlier treatment of an individual would probably affect a greater proportion of their offspring.

A field survey from February to June 1995 in the Imperial Valley, California, revealed lower densities of silverleaf whitefly, Bemisia argentifolii Bellows & Perring, eggs and nymphs on alfalfa than on broccoli, zucchini, cantaloupe, and cotton. To determine if this pattern was caused by ovipositional preference, studies relating preference to alfalfa and the other crops were conducted under greenhouse conditions. Developmental time, survival, and fecundity were also determined. Mean densities of adult females per square centimeter of foliage were significantly lower on alfalfa than on zucchini and cantaloupe, and mean densities of eggs and nymphs were significantly lower on alfalfa than on zucchini. Egg hatching rates on different plants did not differ significantly. In a greenhouse maintained at 27 +/- 2 degrees C, mean developmental time from egg to adult was not significantly different on any host: 225 d on alfalfa, 22.2 d on broccoli, 21.7 d on cantaloupe, 21.3 d on cotton, and 23.0 d on zucchini. Under cooler outdoor temperatures (11-35 degrees C) and conditions, whiteflies developed significantly slower from egg to adult on alfalfa (39.4 d) than on cotton (37.4 d). Percentage of whitefly survival from egg to adult on alfalfa, broccoli, cotton, and zucchini did not differ significantly in the greenhouse study. Under outdoor conditions, the percentage of adults eclosing on alfalfa (56.3) was significantly less than on cotton (84.3).

Honeydew secretion by nymphal field populations of silverleaf whitefly, Bemisia argentifolii Bellows & Perring, on cotton was documented during 10-, 14-, or 24-h periods in the Imperial Valley, California. Honeydew drops were collected on water-sensitive paper by placing 5th main stem node leaves inside open-ended plastic bags with strips of paper. Honeydew drops were also collected directly on open-ended plastic bags placed on leaves, and honeydew sugars were analyzed using gradient anion-exchange high performance liquid chromatography. No consistent diurnal patterns of honeydew drop secretion emerged, but on 1 of the 4 d drop numbers were higher in the morning (0500-1100 hours) than in the afternoon and evening (1500-1900 hours), and drops were collected throughout one 24-h period. On 3 of the 4 d, higher quantities of honeydew sugars were collected in the morning (0500 hours) than in the evening (1500-1900 hours). Quantities and percentages of honeydew sugars ranked as follows: trehalulose > melezitose > sucrose > fructose > glucose. This ranking, with minor exceptions, did not change during the day (0500-1900 hours). Trehalulose accounted for approximately 30% of the total sugar mass in the honeydew. Honeydew sugar secretion and composition did not show a clear or consistent relationship with either leaf water potentials or temperatures.

The silverleaf whitefly (Bemisia argentifolii Bellows & Perring) continues to be a difficult pest to control in California's desert valleys. To gain a better understanding of the possible role that insecticide resistance plays in its annual outbreaks, a resistance monitoring program was established to document susceptibilities of whiteflies to various insecticides through time. Continuous monitoring during 1993 and 1994 detected no trend toward higher resistance levels. Higher toxicities of insecticide mixtures compared to single insecticides were regularly observed in bioassay results. Various factors including diverse insecticide use and altered cropping patterns may have helped to avoid serious insecticide resistance problems in the Imperial Valley so far.

Adult females of the silverleaf whitefly, Bemisia argentifolii Bellows & Perring, and the sweetpotato whitefly, B. tabaci (Gennadius), were given access to three different host plants in choice and no-choice experiments to determine if oviposition preference could account for field observations of host use. The experimental host plants were zucchini, which is a preferred host for both species; sugar beet, which is not favored by either species; and cabbage, which is preferred by B. argentifolii and rarely used by B. tabaci . In choice trials, both whitefly species laid the greatest proportion of eggs on zucchini. The lowest proportion of eggs laid by B. argentifolii was on sugar beet, whereas B. tabaci laid its lowest proportion on cabbage. The proportion of eggs laid on cabbage by B. argentifolii was significantly greater than th.at laid by B. tabaci . In bihourly counts, females of both whitefly species were found on each host in approximate proportion to their oviposition choice. In no-choice trials both whitefly species laid approximately the same numbers of eggs on zucchini; yet on cabbage and sugar beet, B. argentifolii laid significantly more eggs. An additional experiment revealed that B. tabaci would oviposit on cabbage to the same degree as B. argentifolii if the leaf surface was rubbed smooth.

The sweetpotato whitefly, Bemisia tabaci (Gennadius), was trapped throughout the southern desert agricultural region of California during two consecutive growing seasons. Trap data revealed changes in whitefly population densities that provide insight into the epidemiology of lettuce infectious yellows virus (LIYV) in fall melon and lettuce. Whitefly abundance increased rapidly from July to September in cotton. During that period, there were significant correlations between number of cotton fields in a region and number of whiteflies trapped in that region. Beginning in August and September, whitefly densities increased in melon, and the proportion of viruliferous whiteflies increased in cotton and melon. After the defoliation of cotton was initiated in September, whiteflies migrated to melons, which not only served as their host but also as a reservoir for LIYV. In October and November high numbers of viruliferous whiteflies were found in melon and lettuce. As melons were harvested and the fields dried, viruliferous whiteflies migrated to newly emerged lettuce.

In a greenhouse experiment we documented the effects of low, medium, and high nitrogen in cotton on development and honeydew production by the whitefly, Bemisia argentifolii Bellows and Perring. Nitrogen status of cotton plants was manipulated by irrigation with 0.5, 2.5, or 5.0 mmol/liter nitrogen fertilizers. Although the fertilization treatments produced large differences in plant growth, subtle differences were found in whitefly development. No differences among treatments were found in stage-specific survival or the time whiteflies spent in each stadium. Time to adult emergence by whiteflies increased with decreasing nitrogen fertilization. Although early-instar whiteflies on high nitrogen plants initiated the production of honeydew droplets before whiteflies on medium and low nitrogen treatments, they subsequently generated fewer droplets through 24 h periods.

Diet-incorporation bioassays were conducted to evaluate effects of selected natural insecticides on the tobacco budworm, Heliothis virescens (F.). Compounds evaluated were azadirachtin, kryocide, pyrethrum, rotenone, ryania, and sabadilla. Survivorship within the rotenone (87.3%) treatment was not significantly different from that in the control (83.3%); larvae exposed to ryania and azadirachtin rarely survived beyond the first stadium. Survivorship among the kryocide, pyrethrum, and sabadilla treatments was 56.7, 40.0, and 35.0%, respectively. With the exception of rotenone, developmental time was Significantly longer for all treatments compared with the control. Larvae maintained on the pyrethrum diet required ≈20 additional days to reach the pupal stage. Similarly, development among larvae reared on kryocide and sabadilla was lengthened ≍9 and 13 d, respectively. Ensuing pupal weight within the pyrethrum, kryocide, and sabadilla treatments were also significantly lower. Significant differences in the consumption and utilization of food by fourth instars also were found among the treatments. In general, relative consumption and growth rates, as well as utilization efficiencies, were significantly lower among larvae confined to the sabadilla, azadirachtin, kryocide, and ryania treatments.

Multiple treatments of mevinphos, methomyl, and permethrin were applied to head lettuce ( Latuca sativa L.) under commercial field conditions, and harvest residues compared with those of single treatments made at identical periods before harvest. Single treatments resulted in harvest residue levels below current legal tolerances for these materials on head lettuce. Two consecutive treatments of all insecticides 8 d apart resulted in similarly low harvest residues. However, at shorter treatment intervals of 4 and 2 d, the same materials showed extensive residue accumulation. This trend was greatest at successive treatment intervals of 2 d, where mevinphos increased by 79%, methomyl by 161%, and permethrin by 242%. The importance of this residue loading phenomenon is discussed in light of management of insecticide residues on lettuce in commercial production.