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A resistance monitoring program conducted for the polyphagous whitefly, Bemisia tabaci (Gennadius), in Imperial Valley, CA, USA generated a large set of LC50s for adults collected from broccoli, cantaloupe and cotton crops over a four-year period. A vial bioassay and, subsequently, a yellow-sticky card bioassay produced similar temporal profiles of relative susceptibilities to the pyrethroid insecticide bifenthrin. Both bioassays revealed that whiteflies collected from broccoli were significantly less susceptible to bifenthrin compared to the other two crops. A similar finding was observed for endosulfan and the mixture of bifenthrin+endosulfan in the yellow-sticky card bioassay. The possibility that seasonal differences contributed to the observed differences in susceptibility provided the impetus to conduct a reciprocal transfer experiment using broccoli (or kale) and cantaloupe grown simultaneously in the field and greenhouse. Whitefly adults collected from an organic farm over three consecutive weeks had significantly higher LC50s on kale than those collected the same day on cantaloupe. After culturing in the greenhouse on broccoli or cantaloupe and testing again, LC50s remained significantly higher on broccoli after one week and again at the F1 generation. In contrast, whiteflies originating on kale in the field and transferred to cantaloupes in the greenhouse had significantly reduced LC50s at the F1 generation. When tested against the bifenthrin+endosulfan mixture, significantly higher LC50s were generated for whiteflies reared on broccoli in the greenhouse at one week and the F1 compared to the field source from cantaloupes. The consistently higher LC50s for whiteflies on broccoli and other Brassica spp. crops, compared to cantaloupes or cotton, point to statistically significant host-plant influences that are expressed in both field-collected and greenhouse-reared populations of whiteflies.

Bt cotton plants are genetically engineered to produce insecticidal toxins from the Bacillus thuringiensis (Bt) Berliner (Bacillales: Bacillaceae) bacterium and target key lepidopteran pests. In all previous strains of pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae) selected in the laboratory for resistance to insecticidal Cry1Ac toxin using an artificial diet containing the toxin, resistance to Cry1Ac and to Bt cotton is linked to three cadherin alleles (r1, r2, and r3). In contrast, the BG(4) pink bollworm strain was selected for resistance to Bt cotton by feeding larvae for four days in each of 42 generations on bolls of 'NuCOTN33B(R)' that expressed Cry1Ac toxin. After additional selection for eleven generations on Cry1Ac-incorporated diet, the susceptibility to Cry1Ac, fecundity, egg viability, and mating of this strain (Bt4R) was compared with the unselected Cry1Ac-susceptible parent strain. Some larvae of the Bt4R strain survived on diet containing >or= 10 microg Cry1Ac per milliliter artificial diet, but none survived on transgenic cotton bolls. In contrast to strains selected exclusively on Cry1Ac diet, some survival of progeny of reciprocal moth crosses of Bt4R resistant and Bt-susceptible strains occurred on Cry1Ac-treated diet, suggesting differences in levels of dominance. The Bt4R resistant strain does not have the r1, r2, or r3 mutant cadherin genes as do all previous strains of pink bollworm selected on Cry1Ac-treated artificial diet. The combined results suggest a mechanism of resistance to Cry1Ac that is different from previously described cadherin mutations.

Laboratory studies were carried out to compare the toxicity of seven foliar insecticides to four species of adult beneficial insects representing two families of Hymenoptera: Aphelinidae (Aphytis melinus Debach, Eretmocerus eremicus Rose & Zolnerowich, and Encarsiaformosa Gahan) and Mymaridae (Gonatocerus ashmeadi Girault) that attack California red scale, Aonidiella aurantii (Maskell); sweetpotato whitefly, Bemisia tabaci (Gennadius) (both E. eremicus and E. formosa); and glassy-winged sharpshooter, Homalodisca vitripennis (Germar), respectively. Insecticides from four pesticide classes were evaluated using a petri dish bioassay technique across a range of concentrations to develop dosage-mortality regressions. Insecticides tested included acetamiprid (neonicotinoid); chlorpyrifos (organophosphate); bifenthrin, cyfluthrin, and fenpropathrin (pyrethroids); and buprofezin and pyriproxyfen (insect growth regulators [IGRs]). Chlorpyrifos was consistently the most toxic pesticide to all four species of beneficial insects tested based on LC50 values recorded 24 h posttreatment compared with 48-h LC50 values with the neonicotinoid and pyrethroids or 96 h with the IGRs. Among the three pyrethroids, fenpropathrin was usually less toxic (except similar toxicity to A. melinus) than was cyfluthrin, and it was normally less toxic (except similar toxicity with E. formosa) than was bifenthrin. Acetamiprid was generally less toxic than bifenthrin (except similar toxicity with G. ashmeadi). The IGRs buprofezin and pyriproxyfen were usually less toxic than the contact pesticides, but we did not test for possible impacts on female fecundity. For all seven pesticides tested, A. melinus was the most susceptible parasitoid of the four test species. The data presented here will provide pest managers with specific information on the compatibility of select insecticides with natural enemies attacking citrus and cotton, Gossypium hirsutum L., pests.

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.

Laboratory bioassays were carried out with four neonicotinoid insecticides on multiple strains of Bemisia tabaci (Gennadius) to evaluate resistance and cross-resistance patterns. Three imidacloprid-resistant strains and field populations from three different locations in the southwestern USA were compared in systemic uptake bioassays with acetamiprid, dinotefuran, imidacloprid and thiamethoxam. An imidacloprid-resistant strain (IM-R) with 120-fold resistance originally collected from Imperial Valley, California, did not show cross-resistance to acetamiprid, dinotefuran or thiamethoxam. The Guatemala-resistant strain (GU-R) that was also highly resistant to imidacloprid (RR=109-fold) showed low levels of cross-resistance when bioassayed with acetamiprid and thiamethoxam. However, dinotefuran was more toxic than either imidacloprid or thiamethoxam to both IM-R and GU-R strains as indicated by low LC50s. By contrast, a Q-biotype Spanish-resistant strain (SQ-R) of B. tabaci highly resistant to imidacloprid demonstrated high cross-resistance to the two related neonicotinoids. Field populations from Imperial Valley (California), Maricopa and Yuma (Arizona), showed variable susceptibility to imidacloprid (LC50s ranging from 3.39 to 115 microg ml(-1)) but did not exhibit cross-resistance to the three neonicotinoids suggesting that all three compounds would be effective in managing whiteflies. Yuma populations were the most susceptible to imidacloprid. Dinotefuran was the most toxic of the four neonicotinoids against field populations. Although differences in binding at the target site and metabolic pathways may influence the variability in cross-resistance patterns among whitefly populations, comparison of whitefly responses from various geographic regions to the four neonicotinoids indicates the importance of ecological and operational factors on development of cross-resistance to the neonicotinoids.

Equipping the standard plastic cup trap, also known as the CC trap, with lime-green light-emitting diodes (LED-plastic cup trap) increased its efficacy for catching Bemisia tabaci by 100%. Few Eretmocerus eremicus Rose and Zolnerowich and Encarsia formosa Gahan were caught in LED-plastic cup traps. The LED-plastic cup traps are less expensive than yellow sticky card traps for monitoring adult whiteflies in greenhouse crop production systems and are more compatible with whitefly parasitoids releases for Bemisia nymph control.

Host plant performance, esterase, and virus transmission tests revealed cassava-strain and sweetpotato-strain populations of whitefly Bemisia tabaci (Gennadius) biotypes in India. Individuals from the sweetpotato-reared population did not breed on cassava, Manihot esculenta Crantz, and the cassava-strain-reared individuals failed to develop on sweetpotato, Ipomoea batatus (L.) Lam. Eggplant, Solanum melongena L., and tobacco, Nicotiana tabacum L., were common hosts for both biotypes. The cassava-strain whiteflies but not the sweetpotato-reared whiteflies successfully transmitted cassava mosaic virus from disease-infected cassava seedlings to healthy cassava seedlings. Presence of biotypes in B. tabaci is reported for the first time from India.

Various insecticide use strategies including rotations, sequential use, and mixtures were evaluated experimentally on Bemisia tabaci (Gennadius) in California and Arizona (U.S.A.) cotton fields. Toxicological responses of adult B. tabaci were measured along with preimaginal densities and cotton yields from plots subjected to different insecticide regimens. Weekly monitoring for susceptibility changes over ten consecutive weeks in four different trials failed to detect significant differences between sequential use and rotation regimens, nor in comparison to the control plots. There were, however, significant differences among study-site locations and between study years as well as significant within-season time effects. Relative infestations in insecticide-treated plots expressed as a percentage of preimaginal densities in control plots indicated that better control was obtained by all insecticide treatments in conjunction with higher susceptibility levels observed in the second year. Lower preimaginal densities of B. tabaci were measured in the rotation treatment in comparison to sequential treatments of endosulfan, chlorpyrifos, or amitraz, but all were less effective than sequential treatments of bifenthrin or the mixture of bifenthrin + endosulfan. Cotton lint yields were inversely related to B. tabaci densities, with highest yields in the bifenthrin and mixture plots and lowest yields in the control plots. Suppression of B. tabaci infestations in insecticide-treated plots relative to untreated control plots also improved under conditions of lower B. tabaci pressure. The increases in cotton yield and susceptibility to insecticides seen in the current study support the trend observed in the southwestern USA of improved management of B. tabaci despite continuing intensive use of insecticides.

Studies were conducted in 1994 and 1995 to examine the effects of a range of action thresholds for managing Bemisia tabaci (Gennadius) Biotype B (= B. argentifolii Bellows & Perring) with insecticides in cotton on populations of arthropod predators in Imperial Valley, CA, and Maricopa, AZ. Application of insecticides significantly reduced population densities of spiders, Geocoris punctipes (Say), G. pallens (Stål), Orius tristicolor (White), Nabis alternatus Parshley, Zelus renardii Kolenati, Hippodamia convergens Guerin-Méneville, Spanogonicus albofasciatus (Reuter), Drapetis sp., and Chrysoperla carnea Stephens in one or both years and sites compared with untreated controls. Use of higher B. tabaci thresholds conserved some species and groups relative to lower thresholds. Stepwise regression analyses indicated that reductions in predator populations were generally influenced more strongly by the timing of the first insecticide application than by the total number of sprays necessary to maintain suppression of the pest below any given action threshold. A predation index, which weights the importance of each predator species based on their known frequency of predation on B. tabaci and another key pest, Pectinophora gossypiella (Saunders), was developed and analyzed. Patterns were similar to results based on changes in abundance alone, but the index generally revealed less severe effects of insecticides on overall predator function. The current action threshold for conventional insecticidal control of B. tabaci in Arizona and southern California is five adults per leaf. Results here suggest that predator conservation may be enhanced by raising the initial threshold to delay the first application or initially using more selective materials such as insect growth regulators.

We compared smooth-leaf okra- and normal-leaf upland cotton (Gossypium hirsutum L.) strains and cultivars for susceptibility to colonization by Bemisia tabaci (Gennadius) biotype B. Experiments were conducted at seven field sites, five at Holtville, CA, and two at Maricopa, AZ, during 1996-2000. Okra-leaf strains and cultivars, as a group, had lower numbers of adults, eggs, and nymphs compared with normal-leaf strains and cultivars indicating the potential of okra-leaf genetic traits for reducing colonization by B. tabaci. Results also suggest that okra-leaf shape may provide less favorable micro-environmental conditions for the habitat of B. tabaci because of more open canopy as evidenced by higher leaf perimeter to leaf area ratio. The okra-leaf cultivar 'Siokra L-23' appears to have genetic traits that should be examined further as a source of B. tabaci resistance.

The study was conducted in the northern Texas Rolling Plains in 1999 to define the relationship between number of cotton aphids, Aphis gossypii Glover, and resulting contamination of cotton lint by honeydew. Whole-plot treatments were three furrow irrigation management treatments: cotton grown without supplemental irrigation (dryland), irrigated cotton with last irrigation in mid August, and irrigated cotton with last irrigation in late August. Subplots within each irrigation treatment included an untreated check, a plot treated with lambda-cyhalothrin to stimulate aphid population increase, a plot treated with lambda-cyhalothrin followed by pymetrozine after aphids began to increase, and a plot treated with lambda-cyhalothrin followed by thiamethoxam after aphids began to increase. Cotton aphids were counted on leaves picked from the top and bottom half of the plant. Cotton lint was analyzed for contamination by glucose, fructose, sucrose, and melezitose secreted by cotton aphids, and percentage leaf moisture and nitrogen and leaf sucrose concentrations were determined. The manual sticky cotton thermodetector was used to determine degree of lint stickinesss. There was a significant relationship between thermodetector counts and melezitose contamination on lint, and a melezitose concentration of 90.9 microg/g of lint was associated with a thermodetector count of 10, the threshold for sticky lint problems in textile mills. An equation was developed to estimate melezitose concentration on lint as a function of average numbers of aphids per leaf and the interaction between percentage leaf moisture and nitrogen. The number of aphids per leaf associated with a melezitose concentration of 90.9 microg/g of lint ranged from 11.1 to 50.1, depending on percentage leaf moisture and nitrogen. The threshold for sticky lint problems occurred when aphid numbers ranged between 11.1 and 50.1 per leaf after bolls open.

For the silverleaf whitefly, Bemisia argentifolii (Bellows and Perring) (Homoptera: Aleyrodidae), scanning and transmission electron microscopic techniques were used to observe the characteristics of egg oviposition into both plant cells/tissues and artificial membranes, and to document the morphology of mature egg pedicles removed from the ovaries of females. The exterior of the distal portion of the pedicel consisted of a tangled array of fibrous structures (0.2-0.3 microm in diameter) that constituted about 20-25% of the outer diameter of the pedicel. The attachments of the fibers to the core of the pedicle suggested that the pedicel functions as the collector and conduit for water (vapor), and perhaps solute movement into the egg. Silverleaf whitefly eggs on membranes were incubated at various levels of relative humidity and the eggs were scored for egg hatch. At 98-100% rh, the percentage egg hatch was 86-98%. At lower humidity ranges of 0-20, 55-65, and 75-85% rh, none of the eggs hatched. Media (solute) uptake by silverleaf whitefly egg pedicels was determined by exposing the pedicel side of eggs oviposited on membranes to media solutions containing the high molecular weight polysaccharide, [(14)C]-inulin. Solute uptake by the pedicel and movement into developing silverleaf whitefly eggs were demonstrated using [2-(14)C]-acetate, and assaying for radioactivity in hatched nymphs. These studies, using exposure of pedicels to relative humidity and radiolabeled materials, demonstrate that whitefly egg hatch is dependent upon water uptake by the pedicel, and that the pedicel has the ability to transport solutes into the developing egg.

A survey of 53 Bemisia argentifolii Bellows & Perring 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.

We investigated the effect of exogenous methyl jasmonate (MeJA) on the emission of herbivore-induced volatiles; these volatile chemicals can signal natural enemies of the herbivore to the damaged plant. Exogenous treatment of cotton cv. Deltapine 5415 plants with MeJA induced the emission of the same volatile compounds as observed for herbivore-damaged plants. Cotton plants treated with MeJA emitted elevated levels of the terpenes (E)-beta-ocimene, linalool, (3E)-4,8-dimethyl-1,3,7-nonatriene, (E,E)-alpha-farnesene, (E)-beta-farnesene, and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene compared to untreated controls. Other induced components included (Z)-3-hexenyl acetate, methyl salicylate, and indole. Methyl jasmonate treatment did not cause the release of any of the stored terpenes such as alpha-pinene, beta-pinene, alpha-humulene, and (E)-beta-caryophyllene. In contrast, these compounds were emitted in relatively large amounts from cotton due to physical disruption of glands by the herbivores. The timing of volatile release from plants treated with MeJA or herbivores followed a diurnal pattern, with maximal volatile release during the middle of the photoperiod. Similar to herbivore-treated plants, MeJA treatment led to the systemic induction of (Z)-3-hexenyl acetate, (E)-beta-ocimene, linalool, (3E)-4,8-dimethyl-1,3,7-nonatriene, (E,E)-alpha-farnesene, (E)-beta-farnesene, and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene. Our results indicate that treatment of cotton with MeJA can directly and systemically induce the emission of volatiles that may serve as odor cues in the host-search behavior of natural enemies.

Adult whitefly Bemisia argentifolii Bellows & Perring trap (CC trap) catches were compared with suction type trap catches. CC trap catches were significantly correlated to the suction trap catches. Higher numbers of B. argentifolii adults were caught in CC traps oriented toward an untreated, B. argentifolii-infested, cotton field as compared with traps oriented toward Bermuda grass fields, farm roads, or fallow areas. CC trap catches at five heights above ground (from 0 to 120 cm) were significantly related to each other in choice and no-choice studies. CC trap catches were low in the Imperial and Palo Verde Valleys from late October to early June each of 1996, 1997, and 1998. Trap catches increased with increasing seasonal air temperatures and host availability. Trap catches were adversely affected by wind and rain. Abrupt trap catch increases of 40- to 50-fold for 1-2 d in late June to early July followed by abrupt decreases in adult catches suggest migrating activity of adults from other nearby crop sources.

During 1996, 1997, and 1999, studies were conducted in cotton, sugar beets, alfalfa, yardlong bean, and peanut fields to compare insect catches in CC traps equipped with different trap base colors. The studies were conducted in southwestern United States, China, and India. The nine colors, white, rum, red, yellow, lime green, spring green, woodland green (dark green), true blue, and black, varied in spectral reflectance in the visible (400-700 nm) and near-infrared (700-1050 nm) portions of spectrum. Lime green, yellow, and spring green were the three most attractive trap base colors for silverleaf whitefly, Bemisia argentifolii Bellows & Perring, and leafhopper, Empoasca spp. adults. The three trap base colors were moderately high in the green, yellow, and orange spectral regions (490-600 nm), resembling the spectral reflectance curve of the abaxial (underleaf) surfaces of green cotton leaves. True blue and white were the most attractive trap base colors for western flower thrips, Frankliniella occidentalis (Pergande), adults. The true blue and white trap bases were moderately high in the blue spectral region (400-480 nm).

The effect of soil moisture on the distribution of Steinernema riobrave in a sand column was determined. Larvae of Pectinophora gossypiella were used to detect S. riobrave infective juveniles (IJ) in each 2.5-cm section of 30-cm-long soil columns. Soil moisture was determined for each section and related to the numbers of nematodes recovered from infected insect baits. Infective juveniles of S. riobrave applied on the sand column surface showed some degree of positive geotaxis. IJ in soil columns with a consistent moisture gradient grouped in the upper 12.7 cm within a water potential range of 40 to 0.0055 MPa (2% to 14% moisture). Nematodes in sand columns that were gradually dehydrating moved down the soil column, aggregating on the 28th day between 15-23 cm in depth. Nematode redistribution over time allowed IJ to remain within a water potential range of 0.1 to 0.012 MPa (5.2% to 9.5% moisture).