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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.

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.

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.

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.

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.

An electronic feeding monitor was used to examine the feeding behavior of the green peach aphid, Myzus persicae (Sulzer), on lettuce, Lactuca sativa L. ‘Empire’ sprayed with a number of behavior-modifying compounds. The number of probing events recorded per hour was significantly higher on plants treated with an insecticidal soap(M-Pede), azadirachtin (Margosan-O), and an acrylic copolymer (Forevergreen) than on controls. Ingestion events occurred significantly less often on plants treated with nicotine sulfate (Black Leaf 40), the acrylic copolymer, and the insecticidal soap than on control plants. Aphids spent significantly more time in preprobing activities on plants treated with the acrylic copolymer compared with the untreated control. Aphid feeding behavior on plants treated with sabadilla was not significantly different from that on plants treated with distilled water. The use of these results in an IPM context is discussed.

Studies were conducted in 1980 to evaluate the effects of low and high population densities of Tetranychus urticae on photosynthesis, transpiration, productivity, and vegetative growth of strawberry plants under field conditions. All factors were significantly reduced by high mite densities. A sequenced mite-day model is presented and includes pertinent points on the population curve where the plant processes were reduced by 10%. The significance of the sequence in which the plant processes were reduced is discussed.

During the summer of 1974 in the Imperial and Palo Verde Valleys of southern California, population densities of arthropod predators and Hetiothis spp. were studied after insecticide treatments for the pink bollworm. Geocoris spp., Orius tristicolor (White) and spiders were the most numerous arthropod predators. Chrysopa carnea Stephens and Collops marginellus LeConte were least affected by increased insecticide usage. The mean density of arthropod predators was significantly reduced while the mean density of Heliothis spp. was significantly increased by increased insecticide usage.

Effects of various levels of feeding injury caused by the avocado brown mite, Oligonychus punicae (Hirst) (Acari: Tetranychidae), on avocado leaves were examined. Mite feeding from the upper leaf surface reduced chlorophyll content of the upper mesophyll layer of avocado leaves, whereas chlorophyll content remained unchanged in the lower spongy parenchyma layer. Stomatal and mesophyll conductance values, photosynthesis, and transpiration were found to be negatively correlated with increasing levels of spider mite injury. Avocado photosynthesis rates were reduced 56% when 100% feeding injury was sustained on the upper leaf surface.

Overwintering emergence of pink bollworm moths was monitored for 4 years in the Imperial Valley, Calif. Simultaneous temperatures were recorded under emergence cages and field conditions. All temperatures were converted to a cumulative degree-day summation. Accumulating degree-days was seen to be an accurate means of forecasting cumulative moth emergence under both cage and field conditions. These findings also were verified with data from Yuma, Ariz.