Section A: Biology, Ecology, and Population Dynamics (Part One) - 1999

Section A: Part Two (1999)
Section A (2000)

Plenary Session Summary:

Authors: Jacquelyn L. Blackmer1 & David N. Byrne2

Affiliation & Location: 1USDA-ARS, Western Cotton Research Lab, Phoenix, AZ; 2University of Arizona, Tucson, AZ

Developmental and Behavioral Effects of Dietary Constituents on Bemisia tabaci

It is generally believed that nitrogen is the limiting factor for phloem-feeding insects. Not only is it present in low concentrations, but the ratio of essential: nonessential amino acids is unbalanced. Despite these apparent obstacles, phloem-feeding insects have adapted to this niche and are in fact quite successful. Whiteflies, aphids and scale insects are considered to be some of our most important agricultural and horticultural pests. Various morphological or physiological modifications (i.e., gut modifications and bacterial symbionts), as well as considerable behavioral flexibility (i.e., compensatory feeding, creation of nutrient sinks, ability to migrate) are in part responsible for their success. For Bemisia tabaci reared on Euphorbia pulcherrima, host quality significantly affected survivorship, adult weight, and flight behavior. For senescent compared to vegetative hosts, survivorship and adult weight were significantly reduced and long-duration flights were restricted to the first few days after emergence. These types of responses were examined in much greater detail in Cucumis melo with respect to fluctuations in amino acids. During a 12-wk study, 23 amino acids were detected, but these varied over time. For most essential amino acids ("rat 10"), there were two peaks observed, an initial large peak from wk 1-4, and a smaller peak associated with senescence during wk 10 and 11. For histidine, ornithine and citrulline, one large peak was observed from wk 5-8. Arginine peaked during the first few weeks and was no longer detectable after wk 7. Serine and glutamine/ glutamic acid were the only amino acids that peaked during visible senescence. We hypothesized that these different trends in amino acids were, at least in part, responsible for observed differences in life-history traits and flight behavior.

Multiple regressions were used to test the influence of amino acids on life-history traits of B. tabaci. However, to eliminate difficulties due to high intercorrelations among certain amino acids, a factor analysis using the principal extraction technique and varimax rotation was first performed. Factor analysis created a reduced number of orthogonal factors that, for the most part, corresponded to the trends that were observed for the various groups of amino acids. Factor 1 was comprised of the essential amino acids, with the exception of histidine and methionine. Factor 2 was comprised of glutamine/glutamic acid and serine. Factor 3 was predominantly histidine and ornithine; citrulline separated out as factor 5. Factor 4 was comprised of methionine and alanine, and factor 6 was comprised principally of aspartic acid. These 6 factors explained 86% of the variance among the amino acids. No single or combination of factors explained a significant amount of the variability in oviposition. For both male and female whiteflies, factor 1 was the single most important factor for explaining the variability in weights (R2=0.25, P < 0.005; R2=0.57, P < 0.005, respectively). As concentrations of essential amino acids decreased, so did weights. Factors 1 and 3 were the most important factors influencing development time (R2=0.54, P < 0.005). As these amino acids increased in relative concentration, developmental time decreased. Percent emergence was positively associated with factor 1 and negatively associated with factor 6 (R2=0.28, P < 0.005). These analyses reveal relationships among variables, but do not imply causality. However, a recently developed feeding chamber and artificial diet for whiteflies, will allow us to begin to test hypotheses originating from this exploratory approach.

In the context of these studies, flight behavior also was examine, and although long-distance fliers were present throughout the 12 week study, the frequency of long-distance flights significantly increased during the last three weeks (X2=30.2, P < 0.001). Additionally, through paired comparisons of leaves with and without developing whiteflies, we were able to demonstrate that whiteflies create a nutrient sink through aggregative feeding. Total amino acids as well as several individual amino acids (Cys, Ile, Leu, Tyr, Phe, Gaba, Orn, Lys, His, Arg) were significantly elevated in leaves that contained an average of 40.4 + 4.2 developing nymphs (P < 0.05). Once flowering and fruit initiation began, however, no significant differences were detectable between leaves with or without whiteflies.

Investigator's Name(s): 1C. C. Chu, 1C. G. Jackson, 2E. T. Natwick, 1T. J. Henneberry, & 3G. S. Simmons.

Affiliation & Location: 1USDA-ARS, Western Cotton Research Laboratory, Phoenix, AZ, 2University of California Imperial County Cooperative Research and Extension Center, Holtville, CA, and 3USDA-APHIS PPQ WR, Brawley, CA.

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

Dates Covered by the Report: 1997 - 1998

Selectivity of CC Trap Catches of Whitefly Adults and Whitefly Parasites Eretmocerus eremicus and Eretmocerus emiratus

Studies were conducted at Holtville, CA and Phoenix, AZ to compare silverleaf whitefly, Bemisia argentifolii Bellows and Perring, and whitefly parasite catches with CC traps and yellow sticky card traps in greenhouses. In the 1997 study at Holtville, CA, cantaloupe and watermelon plants were raised in three greenhouses (12.5 x 9 ft) (= three replicates) in soil-mix plastic container. The study was conducted from February to April. Five 3 x 5 inch yellow sticky card traps with both side exposed and five yellow trap base CC traps were installed in each greenhouse. Pairs of each type were placed within 50 cm apart. Whitefly adults and Eretmocerus eremicus were released in the greenhouses periodically. Adult whitefly trap catches and parasitized nymphs on leaves were counted. Few adult whiteflies and E. eremicus adults were caught in CC traps compared with yellow sticky card traps. This probably occurred because of the close proximity and competition for catches between the two trap types. In 1998, eight cloth-covered cages (8 x 8 x 8 ft) were placed in a large greenhouse. The 4 cage treatments were: no traps, 2 yellow trap base CC traps, 2 yellow sticky card traps (3 x 5 in.) with one surface exposed, and 2 yellow base CC traps plus 2 yellow sticky card traps. CC traps and yellow sticky card traps were placed 6 or more ft apart. Whitefly adults and E. emiratus adults were released periodically. Parasite to whitefly adult catch ratios were 6.9 and 0.5 for yellow sticky card traps and CC traps in individual cages. With the two trap types in the same cages, the parasite to whitefly adult catches were 3.5 and 0.3, respectively. Results indicate the potential of using CC traps for adult greenhouse whitefly control in combination with parasites releases for whitefly nymph control.

Investigator's Name(s): 1C. C. Chu, 1T. J. Henneberry, & 2E. T. Natwick.

Affiliation & Location: 1USDA-ARS, Western Cotton Research Laboratory, Phoenix, AZ, and 2University 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: 1996 - 1997

Silverleaf Whitefly Adults Caught in CC Traps at Different Trap Heights and Trap Catch Relationships to Leaf-turn Counts on Cotton

The effects of trap placement on silverleaf whitefly, Bemisia argentifolii Bellows and Perring, adult catches in CC traps in cotton fields were studied in California and Arizona in 1996 and 1997. In a no-choice study in 1996, more adults were caught in traps placed 15 cm below the top of the cotton canopy compared with traps placed at canopy top or 15 cm above the plant canopy. Traps caught more whitefly adults in the Stoneville 474 and Louisiana 887 plots compared with traps placed in the Deltapine 5415 and 5461 plots, reflecting the same differences in adult populations on the leaves, as determined using the leaf-turn method. In a no-choice trap study in 1997, trap catches were significantly correlated with leaf-turn counts with traps placed at the top of cotton beds, 30, 60, 90 and 120 cm above plant beds. Significant correlations occurred from 21 August to 18 September when the leaf-turn adult counts were 54 or more adults per ten leaves. In a choice study in 1997, adult whitefly trap catches from 14 August to 18 September in traps placed from 30 to 120 cm above cotton beds were significantly correlated with adult leaf-turn counts.

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

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

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

Dates Covered by the Report: 1996 - 1997

Silverleaf Whitefly Adult Catches in CC and Suction Traps Above Bare Ground and Implementation of CC Traps for Monitoring Whitefly Adult Populations in Imperial and Palo Verde Valleys in California

Fewer (20%) adult silverleaf whiteflies, Bemisia argentifolii Bellows and Perring, were caught in CC traps compared with suction traps. However, trap catches in the two type traps were significantly correlated. In choice and no-choice trap studies on bare ground, more adult whiteflies were caught in CC traps placed at the ground level compared to traps placed from 30 to 120 cm above ground. Year round CC trap catches averaged on a weekly basis for Imperial Valley, California in 1996 and 1997 and Palo Verde Valley in 1997, showed patterns of population fluctuation that are typical in the southwestern United States. Short term whitefly adult population fluctuations in most cases appeared due to occasional wind and rains, whereas overall fluctuations reflected seasonal temperature and host density effects. Results indicate that the CC trap may be a useful tool for monitoring seasonal whitefly adult populations in any specific period of time.

Investigator's Name(s): C. C. Chu, T. J. Henneberry, & M. A. Boykin.

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: 1996 - 1997

Attraction of Silverleaf Whiteflies to White Fluorescent and Incandescent Light Under Laboratory Conditions

Studies were conducted at the USDA-ARS Irrigated Dessert Research Station laboratory at Brawley, CA, to determine the attractiveness of fluorescent and incandescent light sources to adults of silverleaf whitefly, Bemisia argentifolii Bellows and Perring. Individuals moved from a release chamber through plastic tubes to white fluorescent and incandescent light sources 1.5 ft distant from the release point. Silverleaf whitefly adults response to light under laboratory conditions was minimal at light intensities of 2 lux or less as measured by traps catches at the fluorescent light source. More adults were attracted to higher intensity compared with low intensity fluorescent light. Fewer adults were attracted to low (5 lux) intensity incandescent light compared with higher (137 lux) intensity fluorescent light when both were at the energy level of 0.3-0.4 W/m2 . Minor movement of B. argentifolii adults occurred under dark or very low light intensity (< 2 lux) conditions. More adults were attracted to cotton and cantaloupe leaves and yellow sticky card traps adjacent to light sources (highest reflected light intensity) than to leaves and yellow sticky card traps distant from light sources (lower reflected light intensities).

Investigator's Name(s): 1C. C. Chu, 2P. J. Pinter, Jr., 1T. J. Henneberry, 3K. Umeda, 4E. T. Natwick, 5Y. Wei, 6V. R. Reddy, & 6M. Shrepatis.

Affiliation & Location: 1USDA-ARS, Western Cotton Research Laboratory and 2Water Conservation Laboratory, Phoenix, AZ, 3University of Arizona Maricopa County Cooperative Extension Service, Phoenix, AZ, 4University of California Imperial County Cooperative Research and Extension Center, Holtville, CA, 5Guangxi University, Guangxi, China, and 6Interational Crops Research Institute of the Semi-Arid Tropics, Andhra Radesh, INDIA.

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

Dates Covered by the Report: 1996 - 1997

Catches of Silverleaf Whiteflies, Thrips and Leafhoppers with Different Trap Base Colored CC Traps

Seven field studies in 1996 and 1997 were conducted in cotton, sugar beets, alfalfa, yardlong bean and peanut to compare insect catches in CC traps equipped with different trap base colors. 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 to 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 and Perring, and leafhopper, Empoasca spp. adults. The three trap base colors were moderately high in the green, yellow and orange spectral regions (490 to 600 nm), resembling the spectral reflectance curves of the underleaf of a green cotton leaf surface. True blue and white were the most attractive trap base colors for western flower thrips, Frankliniella occidentalis Pergande adults. Both of these trap base colors were moderate to high in the blue spectral region (400 to 480 nm).

Investigator's Name(s): 1A. C. Cohen, 1C. C. Chu, 1T. J. Henneberry, 2T. P. Freeman, 3D. R. Nelson,3J. Buckner, 4D. Margosan, 4P. Vail, & 4L. H. Aung.

Affiliation & Location: 1USDA-ARS, Western Cotton Research Laboratory, Phoenix, AZ, 2Electronic Microscopy Center, North Dakota State University, Fargo, ND, 3USDA-ARS Bioscience Research Laboratory, State University Station, Fargo, ND, & 4USDA-ARS Post Harvest Quality & Genetic Research Laboratory, Fresno, CA.

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

Dates Covered by the Report: 1993 and 1997

Studies of Silverleaf Whitefly Nymphs Feeding Behavior

Whitefly feeding is complex and includes the location of appropriate sites to probe leaves so that minor veins can be located. Nymphal stage survival of the silverleaf whitefly Bemisia argentifolii Bellows and Perring requires stylet penetration of the smallest veins in host plant leaves. Light and electron microscopy as well as confocal imaging have revealed that successful feeding always involves probing of no more than three xylem elements minor veins. Surface structures such as lamina trichomes and elongated epidermal cells provide cues for a first instar nymphs to initiate probing at appropriate places. A specialized saliva produces a sleeve-like salivary sheath that forms around the stylets to the minor veins. The sheaths are often sinuous and branched. Branching takes place both in the mesophyll and veins. Most of the sheath material inside the leaf is extra-cellular and in the extensive air space between spongy parenchyma cells. Only a small portion of the sheath is found inside cells, that portion being in epidermal cells. We found almost no evidence of stylet or sheath penetration into parenchyma or palisade cells. Leaf sectioning technique results suggested some feeding sheaths from the plant surface to sites other than vascular tissues. Stained and cleared non-sectional leaves provide a view of entire intact sheaths which showed that nymphs that developed beyond the first instar always made contact with veins as evidenced by the presence of the salivary sheaths. Fused bead appearing sheaths were up to 140 mm long and about 2 mm in diameter at their widest dimension. Sheaths were occasionally glued to cell walls and made contiguous contact between the plant leaf surface and veins. Some sheaths terminated blindly without reaching a vascular bundle. These were invariably sealed at the end. It appeared that successful feeding always involved intact sheaths. The relative success of silverleaf whitefly on different hosts is, in part, attributable to the geometry of the feeding arrangement in relationship to the availability of minor veins in the host plants. For example, a preferred host cantaloupe has 2X the amount of vascular bundle tissue compared with a poor host lettuce.

Investigator's Name(s): Elizabeth W. Davidson, Mark D. Lavine, Marc Mathews1, & Donald L. Hendrix2.

Affiliation & Location: 1Department of Biology, Arizona State University, Tempe, AZ 85287-1501; 2USDA-ARS, Western Cotton Research Laboratory, Phoenix, AZ 85040.

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

Dates Covered by the Report: January 1, 1997 - December 31, 1998

Improvements to the Artificial Feeding System for Bemisia argentifolii

Our goals in dietary improvement are not to bring B. argentifolii to adults on the artificial system, but rather to bring the greatest possible proportion to third instar within the shortest time, in order to use these larvae as hosts for parasitic wasps. However, within the last 6 months we have successfully produced at least 50 adult whiteflies on artificial feeders, having been reared through their entire development on artificial diet. To this end, we have added many different agents to the standard larval diet (5% yeast extract, 30% sucrose; Jancovich et al., 1997, Feeding chamber and diet for culture of nymphal Bemisia argentifolii. J. Econ. Entomol. 90: 628-633) and using standard diet as the control, have assessed changes in percentage survival and development to third or fourth instar within 13-14 days.

Slight improvement in development was observed with added alanine and with reduction of sucrose to 15%; these items will be assessed further. Marked differences were noted among batches and between manufacturers of yeast extract, Difco and BBL being the most useful. Development appeared to be similar at 27o or 31 oC in the light or in the dark, but temperatures above 31oC inhibited growth. Although development was much slower, third instar larvae were obtained on the standard Akey and Beck diet for aphids; this is the only diet other than yeast extract and sucrose which has ever permitted development of whitefly larvae beyond the first instar. We have analyzed amino acid composition of larvae reared on plants and on feeders, and have compared this analysis to yeast extract. We are basing additions of amino acids to the diet on these analyses, which technique has proven useful in diet improvements for aphids.

Major improvements have been achieved in two areas, egg sterilization including reduction in fungal contamination, and new membrane. Ten% chlorox has routinely been used to surface-sterilize eggs washed from leaves, but we have learned that substitution of the antibacterial-antifungal agent roccal appears to lead to much better percentage of hatch and improved survival.

Perhaps the greatest improvement in the rearing technique has been the adoption of an autoclavable, commercially available membrane to replace Parafilm. Filter membranes composed of Teflon are very thin, acceptable to the larvae for feeding, and the plastic screen which supports these membranes mimics the rough surface of the leaf which appears to be attractive to the larvae as well. We have now adopted these Teflon membranes for all feeders, and the ability to autoclave the entire feeder system has been a major improvement in reducing contamination. These membranes are, however, far more expensive than Parafilm (ca. $2 per membrane), but with care they can be reused at least once.

Fourth instar nymphs (red-eye) were surface sterilized and kept in sterile conditions. When adults emerged, they were moved to feeders under sterile conditions. These adults laid eggs on both Parafilm and Teflon membranes, the eggs hatched in high percentages, and the larvae matured to third and fourth instars on the feeders. These results confirm that eggs laid directly on feeders develop normally, and that materials obtained from plants via the pedicel are not necessary to the hatching and development of the larvae. This technique may prove very useful in establishing larger scale artificial rearing.

Parasitoid wasps including Encarsia formosa and E. pergandiella, but not Eretmocerous mundus., have been reared to adulthood on whitefly larvae on these feeders. Details are presented elsewhere.

Investigator's Name(s): Dan Gerling & Moshe Guershon.

Affiliation & Location: Department of Zoology, the George S. Wise Faculty of Life Sciences, Tel Aviv University, Israel 69978.

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

Dates Covered by the Report: 1998

Whitefly Progeny Production vs. Mortality Factors: Ideas for Future Research

Research on Bemisia populations has been continuing for over 20 years, including about 10 years on B. argentifolii. We examined some of the features of B. argentifolii populations in order to try and point out direction for future research. Recent findings including ours and those of S. Castle showed that SLW may reach 500 or more eggs/female. Such an increase over the 100 or less eggs/female reported elsewhere is overwhelming. Here we deal with it assuming other parameters, such as generation time and adult survival do not change.

In order to maintain a steady population (zero population growth), in a closed system devoid of emigration and immigration, at 100 eggs per female, survival should not exceed 1 female _ or 2% (assuming a sex ratio of 50%); in the case of 300 eggs per female this is 0.7% and for 500 eggs it is 0.4%. Therefore, the difference in percentage mortality required for achieving zero growth between 100 and 500 eggs per female is 1.6%.

Overall immature field mortality of untreated B. argentifolii, according to S. Naranjo and P. Ellsworth's recent findings, is between 91.5 and 99% (differences are in generations of whitefly). This is insufficient for zero population growth even at 100 eggs/female, whereas in cases with 300-500 eggs/female, it will be necessary to obtain additional 1.6% mortality over the 98% if population growth is to be checked. On the other hand, means of reducing oviposition, like adult mortality, could act synergistically with immature mortality factors such as natural enemies, to obtain the necessary control levels. Therefore, the concept should be to develop, introduce and improve a natural enemy complex in order to maintain a high level of immature mortality and augment it, while working on additional means for the reduction in oviposition rates to levels permitting immature mortality to be most effective.

We may exploit the fact that, under certain circumstances, the whiteflies do not cause outbreaks and yet, natural mortality factors do not seem to differ from those under outbreak conditions. A careful life table study leading to the understanding of these cases can enhance our chances to find ways to reduce whitefly populations. Since population buildup is contingent upon oviposition rates (natality) and upon mortality, we could examine the causes for low or high natality such as:

a) Heterogeneity in the female population: There may be "high reproduction females" (females that either live longer or lay more eggs during the same life span) causing outbreaks and "lower reproduction females" that do not cause them. This can be 1). in the same field or 2). in different fields, crops and areas.

Present information indicates that there is little support for 1). but evidence for 2). exists.

b) Plant quality characteristics: The existing evidence concerning differences in oviposition rates all points to plant quality as a determinant in whitefly development differences, but no critical experiments have been conducted to determine the rate of heterogeneity within females, especially in different geographic areas.

Moreover, relevant plant quality traits may change greatly with the season and location as indicated by differences in infestation levels of the same plant species and variety in different fields.

c) Egg quality and Sex ratio differences: This possibility would mean that numerous eggs are deposited but only some would give rise to viable females. So far, observations do not support this idea.

Plans for future action should include:

A. Continued maximal pressure on immatures by natural enemies through their introduction, augmentation and conservation coupled with careful life-table studies.

B. Exploiting and manipulating physiological and behavioral characteristics of the whitefly adults, following research concerning: a. Presence and causes for heterogeneity of adult populations, b. Physiological and behavioral changes in oviposition habits (s.l.) and survivorship as influenced by extrinsic factors and c. Intrinsic influences on whitefly adults such as physiology (e.g. oogenesis, egg fertility and flight), and behavior (e.g. mating host selection and oviposition).

Investigator's Name(s): S. M. Greenbergl, Walker A. Jones2, & W. C. Warfield2.

Affiliation & Location: 1Joint affiliation: Beneficial Insects Research Unit, Kika de la Garza Subtropical Agricultural Research Center, USDA-ARS, and Texas Agricultural Experiment Station, Weslaco, TX; 2Beneficial Insects Research Unit, Kika de la Garza Subtropical Agricultural Research Center, ARS-USDA.

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

Dates Covered by the Report: 1998

Comparative Host Plant Effects on the Biologies of Bemisia and Trialeurodes

Stage-specific development, survival, size, progeny sex ratio, and reproductive potential were measured for Bemisia argentifolii Bellows and Perring and two strains of Trialeurodes vaporariorum (Westwood). One of the two T. vaporariorum cultures (designated here as "A") originated from a greenhouse culture in Ithaca, N. Y.; the other (designated as "B") was colonized from a population collected locally from certain weeds, and believed at first to be a different species due to morphological differences. The B. argentifolii were collected locally. Host plants used were pole beans cv Kentucky Wonder, and cotton cv Sure Grow 125, initially grown in a greenhouse. The whitefly cultures, and the tests, were maintained and cultured on excised leaves placed in floral aquapik tubes filled with hydroponic solution, and maintained in large ventilated Petri dishes in an incubator kept at 25°C, 55% RH, with a 16 : 8 (L : D) h regime. Life table data revealed that host plant species had a significant effect on most biological measurements across each whitefly genotype, and the effects were generally significantly different between whiteflies reared on the same host plant. Total pre-imaginal mortality showed that cotton was a significantly better host for B. argentifolii (35.5% on cotton vs 59.7% on bean), while bean was a significantly better host for typical greenhouse whitefly T. vaporariorum "A" (20.6% on bean vs 54.8% (on cotton). The "wild" T. vaporariorum "B" exhibited a high mortality rate on both host plants, occurring mainly during the early larval stages. Development to adult for the silverleaf whitefly was significantly shorter on cotton (17.5 d) than on bean (22.1d). Conversely, the greenhouse whitefly "A" expressed a shorter develop time on bean (20.5 d) than on cotton (24.6 d). Host plant had no significant difference on percentage female progeny or preoviposition period of the whiteflies tested. Daily egg production was significantly affected. Silverleaf whitefly eggs per day after developing on cotton was 7.6 vs 4.5 on bean. The opposite was recorded for greenhouse whitefly "A"; T. vaporariorum "B" oviposition was very low (< 2 eggs per female) and not significantly different between host plants. Similarly, pupal size corresponded to the other results. These findings clearly demonstrated that green bean is a significantly better host plant than cotton for greenhouse whitefly "A", while cotton a better host for rearing silverleaf whitefly.

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