Section A: Biology, Ecology and Population Dynamics (Part One) - 2000

Section A: Part Two (2000)

Section A (1999)

Plenary Session Summary:

Investigator's Name(s): Linda L. Walling

Affiliation & Location: Department of Botany and Plant Sciences, University of California, Riverside, CA 92521

Research & Implementation Area: Section A: Biology, Ecology and Population Dynamics

Dates Covered by the Report: 1996-1999.

New Genes and New Signals: The Bemisia argentifolii -Squash Interaction

Many previous studies in plant defense have concentrated on either responses to wounding, pathogen infections or responses to chewing insects. To determine the nature of plant responses to phloem-feeding insects, we have studied whitefly interactions with tomato and squash plants. Tomato-whitefly interactions were attractive to study given the availability of genes that are activated by pathogens or mechanical stress. We compared changes in tomato gene expression following feeding by two species of whiteflies: Bemisia argentifolii Bellows and Perring (the silverleaf whitefly) and Trialeurodes vaporariorum Westwood (the greenhouse whitefly). In response to pathogens, defense-response genes are activated by salicylic acid (SA), reactive oxygen species (ROS) or a combination of jasmonic acid (JA) and ethylene. RNA blot analyses showed that pathogenesis-related (PR) protein genes regulated by both JA/ethylene and SA are activated after feeding by both species of whiteflies. Both local and systemic accumulation of PR RNAs were observed. RNAs for PR genes regulated by JA/ethylene (basic chitinase, basic b-1,3-glucanase, PAL) accumulated to the highest levels. Increases in the RNAs for PR genes regulated by SA (acidic b-1,3-glucanase, acidic chitinase) were also detected but these RNAs were not as abundant. Wound-response gene RNAs (leucine aminopeptidase and proteinase inhibitor) were not detectable. In addition, transgenic tomato plants expressing a LapA promoter:GUS fusion gene were used to further elucidate the wound response following whitefly feeding. Whitefly feeding did not induce a wounding response.

Novel genes induced by whitefly feeding were identified by differential RNA display. After feeding by the silverleaf and greenhouse whiteflies, transcripts for a JA/ethylene regulated Wfi1 gene was identified. Wfi1 encodes a subunit of the NADPH oxidase that is involved in generation of superoxide anion in the plant oxidative burst. The studies on this gene are more completely described in Puthoff et al (this volume). Interesting, Wfi1 is induced by whitefly but not aphid feeding. These data suggest that the signals generated by these different phloem-feeding insects may be different.

Interactions between squash plants and the silverleaf and sweetpotato (B. tabaci Type A) whiteflies were also examined in order to understand the similarities and differences in plant responses to feeding by two closely related species of whiteflies. Silverleaf whitefly nymphs induce the squash developmental disorder called leaf silvering, while silverleaf whitefly adults and sweetpotato whitefly nymphs and adults do not. For this reason, differential RNA display was used to identify squash genes preferentially induced by the silverleaf whitefly and not the closely related sweetpotato whitefly. Genes induced in apical, non-infested silvered leaves after silverleaf whitefly (Bemisia argentifolii) feeding were isolated. The identification of genes expressed in apical, silvered leaves from silverleaf whitefly-infested plants and not expressed in leaves of similar age from sweetpotato whitefly-infested plants may allow the identification of defense-response genes that are systemically expressed. Alternatively, the genes may be involved in the establishment, maintenance or as a response to leaf silvering. Two genes (SLW1 and SLW3) preferentially induced by the silverleaf whitefly have been identified. The expression and characterization of SLW1 was detailed in van de Ven et al. (this volume).

SLW3 was regulated in a different manner than the JA/ethylene regulated SLW1. Temporal and spatial expression studies showed that SLW3 RNAs accumulated only after nymph feeding. SLW3 RNAs accumulated in more distal leaves only in response to silverleaf whiteflies, not in response to sweetpotato whitefly feeding. SLW3 RNAs also accumulated in infested leaves from silverleaf and sweetpotato whitefly infested plants and in the leaves most proximal to the infested leaves. These data suggest that SLW3 may be regulated by two sets of signals. One set of signals may induce local expression by both whitefly species. A second set of signals in silverleaf whitefly-infested plants may induce expression in the most distal leaves. Alternatively, the same signal may induce local and systemic expression but this factor must be less potent, produced in lower quantities or transported inefficiently in sweetpotato whitefly infested plants. Unlike SLW1, which is expressed in both flowers and fruit, SLW3 RNAs were not detected in any organ examined. Silverleaf and sweetpotato whitefly infestations did not alter this developmental programming. SLW3 encodes a b-glucosidase-like protein and is regulated by a novel defense-signaling pathway. SLW3 RNA levels were not influenced by Pseudomonas syringae pv syringae infection, wounding, MeJA, ethylene, SA, ABA, hydrogen peroxide or nitrous oxide treatments. These data indicate that SLW3 is not activated by the known defense- or wound-signaling pathways, which are characterized in tomato and Arabidopsis. The source of the signal that activates SLW3 expression is not known. It may be of insect, endosymbiont or plant origin. Alternatively, it may be synthesized by the coordinate activities of the insect and plant, as has been seen for the volatile-inducing elicitor volicitin. Interestingly, SLW3 RNAs accumulated to high levels in response to water-deficit stress suggesting that SLW3 is regulated by two different signaling pathways or there is overlap in the signals generated by whitefly feeding and water-deficit stress.

Acknowledgements: I wish to acknowledge the important contributions of D. P. Puthoff, W.T.G. van de Ven, C.S. LeVesque, and T. M. Perring to this research initiative. This research was partially supported by USDA Grant 95-37301-2081 and a University of California Biotechnology Grant to T.M. Perring and L.L. Walling and by an USDA grant 99-35301-8077 to L.L. Walling.

Investigator’s Name(s): Jacquelyn L. Blackmer¹, Elizabeth W. Davidson², & Linda L. Lee¹.

Affiliation & Location: ¹USDA--ARS, Western Cotton Research Lab, Phoenix, AZ; ²Department of Zoology, Arizona State University, Tempe, AZ.

Research & Implementation Area: Section A: Biology, Ecology and Population Dynamics.

Dates Covered by the Report: December 1998 – December 1999

Evaluation of Artificial Diets for Rearing Bemisia tabaci (Biotype B)

(Homoptera: Aleyrodidae)

Bemisia tabaci (Biotype B), also known as B. argentifolii Bellows & Perring, is one of the principal pests of food and fiber crops. In 1990, a change in feeding behavior, an expanded host range, and rapidly developing resistance to insecticides made this species the primary autumn pest in the southwestern USA. Despite the important pest status of B. tabaci, very little is known about the nutritional ecology of this insect. Understanding the role that nutrients, phagostimulatory cues and/or feeding deterrents have in whitefly host selection and acceptance, growth and reproduction could lead to the development of plant genotypes resistant to whiteflies, as well as provide information that might be useful in explaining whitefly distributions and outbreaks. The objectives of our studies were to optimize a recently developed meridic diet, which could be used as a standard for comparison, while various holidic diets were examined for their suitability in sustaining whiteflies through to the adult stage. A polycarbonate feeding chamber, equipped with a Teflon membrane, was used to test a series of diets that evaluated the effect of sucrose concentration (10-30%), diet pH (6.5-8.0), the ideal number of eggs/chamber(<50->600) and the optimal egg age (3-9 d) on hatch rate, development and emergence rates of B. tabaci. Hatch rates and survivorship were significantly higher for diets with sucrose concentrations of 15-20%, when compared to diets containing 10 and 30% sucrose. Response to diet pH was variable. Survivorship was initially higher on diets with a pH of 6.5-7.0 than on diets with a pH of 7.5-8.0, but by Day 10 this difference was negligible. Five- to six-day-old eggs had a significantly higher hatch rate, and nymphs survived better than in all other age groups. There was a strong negative association between the number of eggs placed on the membranes and both hatch rate and survivorship. The ideal number of eggs was less than 200. Preliminary trials with various holidic diets found that one developed for rearing Myzus persicae and Aphis fabae was comparable, in terms of whitefly nymphal development, to the meridic yeast-extract diet.

Investigator’s Name(s): ¹C. C. Chu, ²T. P. Freeman, ³J. S. Buckner, ¹T. J. Henneberry, ³D. R. Nelson, ⁴G. P. Walker, & ⁵E. T. Natwick.

Affiliation & Location: ¹USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ; ²Electronic Microscopy Center, North Dakota State University, Fargo, ND; ³USDA--ARS, Biosciences Research Laboratory, Fargo, ND; ⁴Department of Entomology, University of California, Riverside, CA; ⁵University of California, Imperial County Cooperative Research and Extension Center, Holtville, CA.

Research & Implementation Area: Section A: Biology, Ecology, and Population Dynamics.

Dates Covered by the Report: 1998

Silverleaf Whitefly Colonization on Upland Cottons and Relationships to Leaf Morphology and Leaf Age

Silverleaf whitefly colonization on Stoneville (ST) 474 and Deltapine (DPL) 5415 cottons in the field was examined in relation to leaf trichome density, leaf age and leaf morphological characteristics as possible factors influencing cultivar host selection. The increased numbers of all silverleaf whitefly life stages on ST 474 in the field appeared to be related to the higher trichome density on abaxial leaf surfaces compared with DPL 5415. In both cultivars, leaves from node #1 below the terminals were smaller and had higher vascular bundle densities and numbers of lysigenous glands than older, larger leaves. Younger leaves also had smaller leaf areole areas, more terminal vein endings per unit leaf area, and shorter distances from abaxial leaf surfaces to minor vein phloem tissues compared with older leaves. These younger leaf morphological characteristics may contribute to the higher silverleaf whitefly densities on younger leaves compared with older leaves. In the laboratory, electronically monitored adult females and visually monitored settled 1st and 4th instar nymphs preferred to probe into secondary and tertiary leaf veins as compared with main and primary leaf veins.

Investigator’s Name(s): ¹C. C. Chu, ¹T. J. Henneberry, ²E. T. Natwick, ³D. Ritter, & ³S. L. Birdsall.

Affiliation & Location: ¹USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ; ²University of California, Imperial County Cooperative Research and Extension Center, Holtville, CA; ³Imperial County Agricultural Commissioners’ Office, El Centro, CA.

Research & Implementation Area: Section A: Biology, Ecology, and Population Dynamics.

Dates Covered by the Report: 1996 - 1998

Seasonal Activity of Adult Silverleaf Whiteflies in the Imperial and Palo Verde Valleys, California

Trap studies were conducted in Imperial and Palo Verde Valleys with the objective of determining the seasonal activity pattern and dispersal activity of adult silverleaf whiteflies. Year round trapping in the Imperial and Palo Verde Valleys, California in 1996, 1997 and 1998 showed low adult trap catches from late October to early June and increasing trap catches with increasing seasonal air temperatures and host availability. Trap catches were adversely affected by wind and rain. CC trap catches were significantly correlated to yellow sticky card and suction trap catches. Higher numbers of silverleaf whitefly adults were caught in CC traps directionally oriented to a silverleaf whitefly infested, untreated cotton field as compared with traps oriented toward Bermuda grass fields, farm roads or fallow areas. Abrupt increases in trap catches of 40 to 50 fold more adults for one to two days followed by abrupt decreases in adult catches suggested dispersal activity of adults from other nearby crop sources.

Investigator’s Name(s): ¹C. C. Chu, ²E. H. Erickson, ¹S. J. Crafts-Brandner, & ¹T. J. Henneberry.

Affiliation & Location: ¹USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ; ² USDA--ARS, Honeybee Research Laboratory, Tucson, AZ.

Research & Implementation Area: Section A: Biology, Ecology, and Population Dynamics.

Dates Covered by the Report: 1999

Evaluation of Leaf-clip Cage and Development of A Single Leaf Plant Technique to Study Photosynthetic Traits of Cotton Under Silverleaf Whitefly Infestation-stress Conditions

Cotton leaf temperatures having leaf clip-cages had leaf temperatures under leaf cage rings 12% higher compared with leaf temperatures outside the leaf clip-cages. This probably occurred because of the pressure of the leaf clip cage rings on leaf tissues and the prevention of transpiration on the parts of leaf blades under the cage rings. Leaf blade temperatures adjacent to the edges of clip-cage rings and inside the cages were 9% and 6% higher, respectively, compared with temperatures outside leaf clip cage rings. A single leaf plant technique, with the stem terminal and all other leaves removed, was developed to evaluate the impact of silverleaf whitefly infestations on cotton leaf physiology. Silverleaf whitefly infestation-stressed plants had reduced photosynthesis and transpiration, and increased leaf temperatures under greenhouse conditions compared with uninfested plants.

Investigator’s Name(s): ¹C. C. Chu, ²T. P. Freeman, ³J. S. Buckner, ¹T. J. Henneberry, ³D. R. Nelson, & ⁴E. T. Natwick.

Affiliation & Location: ¹USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ; ²Electronic Microscopy Center, North Dakota State University, Fargo, ND; ³USDA--ARS, Biosciences Research Laboratory, Fargo, ND; ⁴University of California, Imperial County Cooperative Research and Extension Center, Holtville, CA.

Research & Implementation Area: Section A: Biology, Ecology, and Population Dynamics.

Dates Covered by the Report: 1999

Cotton Leaf Trichomes and Silverleaf Whitefly Density Relationship

Higher trichome density on underleaf surfaces has been attributed to higher silverleaf whitefly density on different cotton cultivars. Studies were conducted to determine whether the positive trichome-insect density relationship holds true among the leaves on main stem leaf nodes. The hairy Stonville (ST) 474 leaves had higher branched trichome densities as well as silverleaf whitefly densities compared with those on smooth NuCOTN (Nu) 33B leaves in a field study at Maricopa, AZ. Mean numbers for trichome and silverleaf whitefly nymphs were 6.9 and 6.0 per 37 mm² leaf area for ST 474 compared with 1.7 and 2.9 for Nu 33B, respectively. However, this trichome-insect density relationship did not hold true when leaves from main stem leaf nodes #1, 2, 3, 4, 5, 7, 10 and 15 were compared. The branched trichome densities were 12.3, 8.4, 7.8, 4.9, 7.4, 6.0, 4.6 and 3.5 per 37 mm² leaf area on leaves of those leaf nodes for ST 474, but nymph densities were 0.1, 4.1, 13.1, 13.4, 7.1, 7.5, 1.8 and 0.9 per 37 mm² leaf area. For Nu 33B the branched trichome densities were 3.5, 2.9, 1.8, 1.9, 1.4, 0.9, 0.9 and 0.2 per 37 mm² leaf area and 0.1, 1.8, 4.6, 4.5, 5.6, 3.0, 1.8 and 2.0 per 37 mm² leaf area. It appears that factors other than trichome density also influence silverleaf whiteflies colonization. Studies are in progress to examine factors influencing leaf age-silverleaf whitefly density relationship.

Investigator’s Name(s): ¹C. C. Chu, ²T. P. Freeman, ³J. S. Buckner, ¹T. J. Henneberry, & ³D. R. Nelson.

Affiliation & Location: ¹USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ; ²Electronic Microscopy Center, North Dakota State University, Fargo, ND; ³USDA--ARS, Biosciences Research Laboratory, Fargo, ND.

Research & Implementation Area: Section A: Biology, Ecology, and Population Dynamics.

Dates Covered by the Report: 1999

Do Silverleaf Whiteflies Use Leaf Surface Cues for Feeding Sites Selection?

Crawlers have limited mobility and limited time to find feeding sites that assure access to vascular leaf tissues and their survival. We reported earlier that crawlers spend about 80% of their time in contact with vein-associated, elongated epidermal cells and non-glandular branched trichomes located on cotton underleaf surfaces. We also reported that leaf trichomes located in proximity to vascular bundles may provide directional orientation for whiteflies to acceptable feeding sites. Using EPG technique we found lately that adult females probe into secondary and tertiary leaf veins as well as between veins of cotton leaves. It appears that whiteflies may use leaf surface cues for feeding site selection. However, recent descriptions of cotton leaf morphology show that whiteflies can reach phloem tissues from almost any position on the underleaf surface of young leaves and thus may not require the presence of surface cues. Thus, whether whiteflies do use leaf surface cues for feeding site selection remains a question warrant more studies.

Investigator’s Name(s): Steven J. Crafts-Brandner.

Affiliation & Location: USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ.

Research & Implementation Area: Section A: Biology, Ecology, and Population Dynamics.

Dates Covered by the Report: 1999

Effect of N Nutrition of Free Amino Acid Levels in Silverleaf Whiteflies

Our objective was to determine how short-term alterations in N nutrition influenced the free amino acid composition of adult silverleaf whiteflies. Insect N nutrition was altered by feeding whiteflies for 2-to-4 days on N-stressed muskmelon plants or on artificial feeders. Free amino acid levels and composition were similar in whiteflies fed on N-sufficient and N-stressed plants. Glutamine was the predominant amino acid in the whitefly bodies, but in all cases we detected a very high level of an amino compound that eluted with the same retention time as proline. In contrast to amino acid levels in the whitefly bodies, the honeydew amino N was markedly reduced for insects feeding on N-stressed plants. This effect was even more pronounced for insects feeding on an artificial diet of 15% sucrose, where amino N was essentially non-detectable after 2 days. As for the insect bodies, glutamine was the predominant amino acid in the honeydew. Proline, however, was detected in very low levels in the honeydew. These results indicate that honeydew composition is quite sensitive to insect N nutrition, thus providing a good indication of N supply to the insect. Although insect amino-N level was not altered by short-term alterations in N nutrition, the rapid effect of N supply on honeydew amino N levels indicates that longer-term exposure to a low-N diet may influence insect N composition and, perhaps, performance.

Investigator’s Name(s): Thomas P. Freeman ¹, Dennis R. Nelson ², James S. Buckner ², C. C. Chu ³, & Thomas J. Henneberry³.

Affiliation & Location: ¹Electron Microscopy Center, Plant Pathology Department, North Dakota State University, Fargo, ND; ²USDA--ARS, Bioscience Research Laboratory, Fargo, ND; ³USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ.

Research & Implementation Area: Section A: Biology, Ecology and Population Dynamics.

Dates Covered by the Report: 1999

Determination of Stylet Length and the Extent of Stylet Penetration for Silverleaf Whiteflies

Using light and electron microscopy we have determined that the adult Bemisia argentifolii Bellows & Perring (Homoptera: Aleyrodidae) stylet lengths are considerably longer than have been previously reported. Adult whitefly stylets range from 100 µm to over 300 µm in length. Stylet penetration was determined by rapidly killing and fixing feeding silverleaf whiteflies in acidified DMP (2,2-dimethoxypropane) and then removing them from the leaves and measuring the stylet extended beyond the distal tip of the labium. The portion of the stylet extended into the leaf ranged from 43 µm to over 150 µm with a mean penetration of 90 µm. Using the same technique to kill and fix nymphs feeding on leaves we found their stylets to also be considerably longer than previously reported. Stylet lengths were found to be shorter in crawlers than in 4^th instar nymphs. Crawler stylets measured as long as 113 µm whereas some 4^th instar nymphs had stylets over 200 µm long. With stylets of these lengths and the arrangement of minor veins in cotton leaves, both adults and nymphs may be able to reach phloem tissue from almost any point on the abaxial epidermis of the youngest expanding leaves or even leaves located at nodes 7-15 below the apex. Thus, stylet length and phloem depth may not be a determining factor in successful whitefly feeding.

Investigator’s Name(s): Thomas P. Freeman ¹, Dennis R. Nelson ², James S. Buckner ², C. C. Chu ³, & Thomas J. Henneberry³.

Affiliation & Location: ¹Electron Microscopy Center, Plant Pathology Department, North Dakota State University, Fargo, ND; ²USDA--ARS, Bioscience Research Laboratory, Fargo, ND; ³USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ.

Research & Implementation Area: Section A: Biology, Ecology and Population Dynamics.

Dates Covered by the Report: 1999

Mechanism and Site of Silverleaf Whitefly Stylet Penetration

The mechanism involved in adult silverleaf whitefly stylet penetration into cotton and hibiscus leaves was determined using light and electron microscopic techniques. The adult silverleaf whitefly can extend, essentially the entire length of its stylet into a leaf. The depth of penetration can range from 100 µm to over 300 µm. The portion of the stylet within the leaf can be determined by examining the position of the head in relationship to the labium.

Utilizing the tilt and rotation capabilities of the scanning electron microscope stage, and by examination of sectioned leaves at both the light and transmission electron microscope level we have determined that the greatest number of penetration sites are directly through epidermal cells rather than through the common wall between epidermal cells or through stomatal pores. Most stylet penetrations that appear to be through the common wall between cells at the light microscope level actually enter directly into the epidermal cell and not through the common anticlinal wall.

Investigator’s Name(s): C. Joel Funk & Michael E. Salvucci.

Affiliation & Location: USDA--ARS, Western Cotton Research Lab., Phoenix, AZ.

Research & Implementation Area: Section A: Biology, Ecology, and Population Dynamics.

Dates Covered by the Report: January - December 1999

Localization of Whitefly Enzymes

Specific enzymes mediate many of the interactions between the silverleaf whitefly and its host plant. Two whitefly enzymes, alkaline phosphatase and sucrase, have been localized and their potential roles will be discussed. Alkaline phosphatase (ALP) was initially detected in fixed and paraffin-embedded whitefly sections. The most prominent activity is associated with primary salivary glands and could be detected with both colorimetric and fluorescent ALP substrates. Additional activity was detected in accessory salivary glands, salivary ducts, the cibarium and mouth, regions of the oviduct surrounding the terminal oocyte, the colleterial gland, and occasionally within the midgut. Salivary ALP was obtained by collecting and concentrating sucrose diet from whitefly feeder units. Optimal ALP activity was found between pH 10-10.5. Possible functions of whitefly ALP may include a role in nutrient uptake or sclerotization of whitefly structures such as the salivary sheath.

Sucrase plays a key role in the flow of energy from the plant to the insect. Sucrose, the major constituent of cotton sap, is converted to glucose and fructose which are utilized by the whitefly or excreted in honeydew. The enzyme sucrase has been localized in the esophagus and midgut using a precipitating colorimetric substrate. However, the greatest concentration of activity was found in the filter chamber region along the coiled section of the midgut.

Investigator’s Name(s): Dale B. Gelman, Michael B. Blackburn, & Jing S. Hu.

Affiliation & Location: USDA--ARS, Insect Biocontrol Laboratory, Beltsville, MD.

Research & Implementation Area: Section A: Biology, Ecology, and Population Dynamics.

Dates Covered by the Report: 1999

Ecdysteroid Regulation of Molting in 4th Instars of the Greenhouse (Trialeurodes vaporariorum) and Silverleaf (Bemisia argentifolii) Whiteflies

A system of markers designed to track the development of 4^th instar greenhouse whiteflies has been used to track the development of 4th instar silverleaf whiteflies. The system is based on the measurement of body depth and observed changes in the size and color of the developing adult whitefly eye. Stages 0, 1, 2, 3, 4, and 5 have body depths of less than 0.1, 0.1, 0.15, 0.2, 0.25 and 0.27-0.30 mm, respectively. Stages 6, 7, 8, and 9 are characterized by slightly diffuse eye pigmentation, a light red, bright red bipartite and red-black bipartite eye, respectively. Since adult eye development is first observed in stage-6 4^th instars, it was hypothesized that molting (i.e., apolysis, the separation of the larval cuticle from the epidermis) occurred shortly before this stage. Histological studies revealed that for the greenhouse whitefly, adult wing and eye development do begin in stage-5 4th instars. (Histological preparations of silverleaf whiteflies are currently being processed.) Metamorphosis is rapid during stage 6. The wing buds become highly convoluted and by stage 7, cuticular spines have been formed. Adult eye formation was also observed to begin in stage-5 4th instars. In stage-6 preparations, the larval cuticle is either missing or is separated from the epidermis.

Since in other insect orders an increase in ecdysteroid levels has been found to be associated with the molt, an Enzyme Immunoassay was used to determine whole body ecdysteroid (molting hormone) titers in staged greenhouse and silverleaf whiteflies. For the greenhouse whitefly, mean whole-body ecdysteroid titers fluctuated between 0.068 and 0.34 pg/whitefly. For the silverleaf whitefly, titers fluctuated between 0.079 and 1.35 pgs/whitefly. Thus, at peak periods (occurred in stage-5 greenhouse and stage 4/5 silverleaf whiteflies), the ecdysteroid titer in 4^th instar silverleaf whiteflies is approximately four times greater than the titers in greenhouse whiteflies. Experiments are in progress to identify the ecdysteroids present at the time of the larval-adult molt.

Together, the results described above indicate that pharate adult formation occurs in stage-5 greenhouse whiteflies. It is noteworthy that whole-body ecdysteroid levels of last instar whiteflies are considerably lower than those reported for last instars of other insect orders.