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

Section A: Part One (1999)

Section A (2000)

Investigator's name(s): L. H. C. Lima; D. Návia; & M. R. V. Oliveira.

Affiliation & Location: Embrapa - Recursos Genéticos e Biotecnologia, Cx. Postal 02372, CEP: 70.849-970, Brasília, DF. BRAZIL. Email:

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

Dates Covered by the Report: January - October 1998

Occurrence and Evaluation of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) Strains in Brazil Using PCR-RAPD

Bemisia tabaci (Gennadius) has become an increasingly important pest of agricultural crops world-wide, causing extensive damage through direct feeding and as a vector of many viruses. Up to 1990, only B. tabaci biotype A was found in Brazilian agroecosystems and it was considered a secondary pest. In 1991, a new biotype, known as the poinsettia strain, or silverleaf whitefly was detected in Brazil causing phytotoxic disorder in cucurbits and attacking weeds. In the last three years this biotype has become a serious pest of various important crops in all five regions of Brazil. In December, 1996, this insect was found present in five states of the country, presently its population spread to 17 states, in just a year and half. The losses caused by the B biotype as vector and/or pest are now over US$ 1 billion. The crops most attacked are tomatoes, watermelons, cotton, benas and soybeans. The taxonomic identity of B. tabaci is problematic as it is highly polymorphic with extreme plasticity in key morphological characters that vary according to the host. We used PCR-RAPD to evaluate the presence of B-biotype and/or others biotypes of B. tabaci that may be present in Brazil. The analysis were realized in 70 samples collected in 30 different localities on 27 different hosts including cultivated and weed plants. It was confirmed the presence of the B-type in 14 states: Alagoas, Bahia, Ceará, Distrito Federal, Goiás, Minas Gerais, Mato Grosso do Sul, Mato Grosso, Paraíba, Pernambuco, Rio de Janeiro, Roraima, São Paulo e Tocantins. In some localities both biotypes A and B were found: Jaboticabal, SP; Rondonópolis and Cuiabá, MT and Goianira, Go. It is necessary to monitor the spread and development of B biotype populations in order to adopt appropriate management strategies to control this whitefly.

Investigator's Name(s): Steven E. Naranjo & Thomas J. Henneberry.

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: January - December 1998

Evaluation of a High-Speed Thermodetector for Estimating Cotton Lint Stickiness

Cotton lint stickiness is problematic at many post-harvest phases of processing including ginning, carding and particularly spinning. The manually-operated sticky cotton thermodetector (SCT) has been a standard research tool for assessing cotton lint stickiness. With the inevitable replacement of the SCT by several competing automated systems (e.g. High Speed Stickiness Detector [H2SD]) it is necessary for us to revisit many of our previous sampling analyses. Here we summarize comparative analysis of field samples assayed by both the SCT and H2SD systems.

Data Collection and Assay: Sample data were collected in a total of 18 sites in 1996 from Maricopa, AZ and Brawley, CA. Five different sample units were examined. After ginning a subsample of lint from all samples was assayed using the H2SD (3 replicate assays per subsample). Approximately one-third of the samples were also assayed using the SCT (2 replicate assays per subsample) to facilitate direct comparisons between systems.

Sampling Distribution: There was a difference in the within-field distribution of thermodetector spots between the SCT and H2SD. Samples assayed by the SCT typically had coefficients of variation (CV) < 1 indicating a Poisson distribution. In contrast, a large fraction of the samples assayed by the H2SD had CV's > 1 indicating an aggregated distribution. The reasons for the striking difference between the SCT and H2SD in our samples is unknown and further detailed investigation is clearly warranted.

Comparison of Systems and Sample Units: Overall our samples did not provide a good range of stickiness levels. Nonetheless, there were clear differences in the total number of thermodetector spots between SCT and H2SD assays, with the latter being consistently lower. The relationship between the SCT and H2SD for our samples differed from those previously reported. In general there were no statistical differences in estimates of stickiness among the various sample units examined nor any clear pattern in levels of variability in relation to size of the sample unit. Thus, it took much longer to collect larger sample units in comparison with smaller sample units, but this extra effort was not offset by less variable estimates. Based on results to date the 1-plant sample unit is most efficient. Additional observations are currently being collected for several boll-based sample units.

Partitioning of Variance Components: Because thermodetector assays are conducted on subsamples from the field sample unit, the process of estimating stickiness is inherently a two-stage sampling problem with two sources of variation. We found that approximately one-third of the variation was attributable to differences among field samples while the remaining variation could be attributed to variability among SCT subsamples. This latter source of variation for the SCT includes variability due to subsampling and the SCT operator. Because the H2SD eliminates operator error we found that approximately 41% of the variability was attributed to subsampling with the H2SD system with the remaining 59% attributable to field-level variability. Taking field and assay costs into consideration our preliminary findings suggest that only one subsample should be assayed per sample unit on either machine and that more time should be devoted collecting additional sample units from the field.

Preliminary Sample Sizes: Taylor's power law and Iwao's patchiness regression were used to generate fixed-precision sample size functions for the SCT and H2SD systems. The much greater sample size requirements for the H2SD reflects the higher levels of between-sample variability observed for lint assayed with this system. For example, to estimate a mean stickiness level of 5 with 25% precision (SE/mean ratio) the SCT would require a sample size of 2-6 whereas 12 samples would be needed using the H2SD system. At high levels of stickiness only 1-2 samples would be required by the SCT; 4-7 samples would be needed for the H2SD.

Theoretically, an automated, high-speed system should provide the most consistent and accurate determination of stickiness, however, there are apparent problems with the H2SD system used in our studies. Further work is needed to identify and correct these problems and to assess the H2SD system on lint samples representing a broader range of stickiness levels.

Investigator's Name(s): Steven E. Naranjo1 & Peter C. Ellsworth2.

Affiliation & Location: 1USDA-ARS, Western Cotton Research Lab, Phoenix, AZ & 2Department of Entomology, University of Arizona, Maricopa Agricultural Center, Maricopa, AZ 85239.

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

Dates Covered by the Report: June - December 1998

Cohort-Based Life Table Studies of Bemisia tabaci in Cotton

Many biotic and abiotic mortality factors impact the population dynamics of Bemisia tabaci (Biotype B) in agricultural ecosystems, yet we have a poor understanding of the rates of these mortality factors and how they may be involved in overall population regulation. We have been using a direct observation technique to construct cohort-based life tables of B. tabaci on cotton in central Arizona over the past three years. These studies have identified, quantified, and compared in situ sources and rates of mortality of immature whitefly stages in untreated cotton plots and in plots under three different insecticide regimes (buprofezin followed by pyriproxyfen, pyriproxyfen followed by buprofezin, and a rotation of conventional materials). Here we summarize our results from a total 10 life tables completed in untreated cotton during 1997 and 1998.

Cohorts of eggs and settled 1st instar nymphs were established from natural populations in each of 4 replicate plots per generation. Four generations were observed from late June through late September in 1997 and six generations were observed from late June through late October in 1998. Each cohorts consisted of approximately 50 individuals of each stage in each plot. The location of individuals on leaves was marked with a non-toxic felt-tip pen. The fate of each individual was then tracked by visual observation with a hand lens every 2-3 days. We attempted to estimate mortality due to predation, parasitism, dislodgment, and inviability (eggs). Mortality that could not be placed into one of these categories was cataloged as unknown.

Combining all immature stages, predation by sucking predators was a large source of mortality, especially during 1997. Observed rates of predation varied from 36-51% in 1997, and 7 to 42% in 1998. A consistently large fraction of immatures were also killed by being dislodged from leaves (29-51% in 1997; 23-43% in 1998). Dislodgment likely resulted from a combination of weather (wind and rain) and chewing predation. Inviability of eggs was a large source of mortality during 3 generations over the two year (30-68%), but was minor in all other generations examined (2-17%). Parasitism by two genera of native parasitoids was a very minor source of overall immature mortality (0-4%). Survivorship from egg to adult ranged from 0.8% to 9.5 % in 1997, and 0-18.2% in 1998 suggesting a large impact of natural forces on whitefly mortality in the field. Partitioning mortality across the five developmental stages, we found that a large portion of immature mortality occurred in the egg stage (42-76% in 1997; 35-97% in 1998). Of the four nymphal stages the largest fraction of mortality consistently occurred during the 4th stadium (7-28% in 1997; 2-23% in 1998). Stage-specific rates of mortality were highest for eggs and 4th instar nymphs, reflecting, in part, the fact that these are the longest developmental stages in the life cycle. Stage-specific rates of mortality rarely exceeded 30% during any of the first three nymphal stadia; stage-specific rates of mortality frequently exceeded 50% for eggs and 60% for 4th stage nymphs. As expected from results of overall immature mortality, predation and dislodgment were consistently the two greatest sources of mortality during each individual developmental stage. The rate of parasitism in the 4th stadium approached 10% in some generations and was consistent with independent evaluations from leaf samples in the same plots. An unusual, but unknown source of mortality affected 4th instar nymphs during the 3rd generation in 1998 and contributed to 0% survivorship in that generation across all treatment plots. The posterior sections of affected nymphs were severely sunken and necrotic areas were sometime visible at the tips of developing wingbuds. Investigations are still underway to define this mortality agent.

To evaluate the relative importance of the various mortality factors we estimated rates of irreplaceable mortality for predation, parasitism, dislodgment and egg inviability. Results showed that overall, relatively little mortality from any source is completely irreplaceable. This indicates that the various mortality factors interact and readily replace one another during the five immature developmental stages. Averaged over 10 generations, 15.5% of mortality from predation, 10.4% of mortality from dislodgment, 2.2% of mortality from inviability, and <1% of mortality from parasitism was irreplaceable. K-factor and density-dependent analyses are underway.

Investigator's Name(s): Dennis R. Nelson, Thomas P. Freeman1, James S. Buckner, Kim Hoelmer2, James Hagler3, & C. Glen Jackson3.

Affiliation & Location: USDA-ARS, Biosciences Research Lab., Fargo, ND; 1Dept. Plant Pathology, North Dakota State University, Fargo, ND; USDA-APHIS, Phoenix Plant Methods Center, Brawley, CA; 3USDA-ARS, Western Cotton Research Lab., Phoenix, AZ.

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

Dates Covered by the Report: 1998

Formation of External Waxy Particles by Adult Bemisia argentifolii and Semidalis flinti

The dustywing, S. flinti Meinander (Neuroptera: Coniopterygidae), a predator in the southwestern USA, is active on noncrop plants (shrubs and trees) in areas surrounding agricultural areas and in urban areas, where whiteflies spend the time between crop seasons. Both larvae and adults feed on Bemisia eggs and nymphs. Larvae ate up to 2000 eggs during development to adults. Starved adults ate 8.5 eggs plus 8.8 nymphs per hour.

Adult whiteflies and dustywings both cover themselves with waxy particles. Whiteflies produce ribbons of waxy material, a mixture of long-chain aldehydes and alcohols, from abdominal wax plates and then use their tibia to break off the extruding ribbons to form the waxy particles which cover the insect as well as its surrounding surfaces. In B. argentifolii and B. tabaci, the major components are 34 carbons in length and in greenhouse whiteflies, Trialeurodes vaporariorum (Westwood), 32 carbons. Also, adult whiteflies cover their cuticular surface with mixtures of lipid classes (as do all insects).

Dustywings were collected from roses near Phoenix, AZ in 1996 and 1997, and cultured on B. argentifolii on cotton leaves. Dustywings also cover themselves with waxy particles from individual wax pores, located around their entire body, each of which produces two waxy ribbons with fluted edges. The end of each waxy ribbon curls back on itself to form a cylinder (particle) about one micrometer in diameter which breaks off. As extrusion continues, additional particles are formed.

Total cuticular surface lipids (including waxy particles) were composed of fatty acids (47%), alcohols (7%), hydrocarbons (20%), putative wax esters (4%) and diacylglycerols (10%); 11% remained at the origin of the TLC plate. Waxy particles alone, were composed of fatty acids (37%), alcohols (9%), hydrocarbons (19%), and diacylglycerols (13%); 20% remained at the origin. No differences in TLC lipid classes were found between males and females. No putative wax esters were detected in particles alone. The similarity in the lipid composition of the waxy particles and the total cuticular surface lipids indicated that waxy particles are the majority of the lipid on the surface of the dustywing.

Analysis by capillary gas chromatography-mass spectrometry did not detect wax esters. The major hydrocarbon was 3,7,11trimethylheptacosane, approximately 70 ng per female and 50 ng per male. The major lipid class (approximately 65%) was the free fatty acids; approximately 1000 ng/female and 1220 ng/male. The major free fatty acid, tetracosanoic acid (24-carbon fatty acid) was 70% of the free fatty acids. Alcohols were only a minor lipid class (6%) on dustywings. The major alcohols were a mixture of 8- and 9-hydroxypentacosanes.

The whiteflies and dustywings had completely different surface chemistry. Adult whiteflies had particles of long-chain aldehydes and alcohols as well as wax esters on the cuticle surface. Dustywings did not have long chain aldehydes and very little alcohol. The major component from the dustywing as well as from the vials in which they were held was free fatty acids. They also had small amounts of methyl-branched hydrocarbons whereas whiteflies had very little hydrocarbon, either branched or straight chain.

Investigator's Name(s): Claudie Pavis & Nathalie Boissot.

Affiliation & Location: INRA, Unit de Recherches en Productions Vegetales, F-97170 Petit-Bourg, Guadeloupe (F.W.I.).

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

Dates Covered by the Report: March 1, 1997 - November 30, 1998

Population Dynamics of Bemisia tabaci B Biotype, on Continuous Pumpkin Culture in Guadeloupe (French West Indies)

In Guadeloupe, as in most of the Caribbean islands, vegetable is being produced all year round. In this region, the silverleaf whitefly was first identified in 1990; it causes direct damage on melon and other Cucurbits and is responsible for PYMV transmission on tomato. We studied the causes of whiteflies outbreaks in order to manage this new situation.

Since population levels depend on the host-plant phenology, we developed a field test with the continuous presence of young, mature and senescent pumpkin plants (Cucurbita moschata, variety Martinica). Pumpkin is a good silverleaf whitefly host, it is easy to grow and is attained by few pests and diseases. Pumpkin plots were planted monthly in two locations of Guadeloupe, under different climatic conditions. Climatic parameters were continuously recorded. Once a week and in each location, we sampled the two most recent plots, each one consisting of 30 plants. Sampling included adult counts under 4 leaves of each plant, and adult counts on yellow sticky traps fixed at the ground level (4 per plot).

Observations during one and a half years showed that the population levels were always very low in the humid area, even outside the rain period. In the dry area, population levels were low when relative humidity was high, or when the rainfalls were frequent. However, outbreaks occurred from May to July, and corresponded to hot periods with little precipitation. These outbreaks came to an end when relative humidity and precipitation increased, although temperature remained high.

We are now focussing on sampling of parasitoids and are developing tests to evaluate the effect of environmental factors (wind, rain, relative humidity) on dispersion of whiteflies on the host-plants. This ecopathological approach is linked with the study of geminivirus transmission on tomato plants.

Investigator's Name(s): Thomas M. Perring1, Judith Brown2, Arthur D. Cooper1, Ian Bedford3, & Peter Markham3.

Affiliation & Location: 1Department of Entomology, University of California, Riverside, CA 92521; 2Department of Plant Sciences, University of Arizona, Tucson, AZ 85721; 3Department of Virus Research, John Innes Centre, Colney Lane, Norich, UK NR4 7UIL

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

Dates Covered by the Report: January 1, 1998 - December 31, 1988

Genetic Analysis of Bemisia (Homoptera: Aleyrodidae) Populations by Isoelectric Focusing Electrophoresis

Twenty two populations of whiteflies in the genus Bemisia were screened for genetic variation at 3 allozyme loci. Ten of these 22 populations were selected for additional analysis in which a minimum of 10 enzymes representing 10 to 14 distinct loci were examined. Calculated allelic frequencies revealed three clusters among the populations examined. The first cluster was comprised of two New World populations, B. tabaci type A from the United States, and a B. tabaci type A-like population from Culiacan, Mexico. A second cluster of seven was formed from 4 Old World populations, and 3 New World populations (a population from Puerto Rico found on Jatropha gossypifolia, and 2 populations of Bemisia argentifolii (=B. tabaci, type B), believed to be old world in origin). A third group contained a single population from Benin that specializes on Asystasia gangetica. While the data support previous work with respect to taxonomic.

Investigator's Name(s): Michael E. Salvucci, Dawn S. Stecher, & Thomas J. Henneberry.

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, 1998

Heat Shock Proteins and Sorbitol Accumulation as Mechanisms for Thermotolerance in Bemisia argentifolii

The silverleaf whitefly (Bemisia argentifolii) thrives in hot, arid regions where daytime temperatures are sufficiently high to damage cellular machinery. Previous studies have shown that whiteflies accumulate sorbitol when exposed to temperatures in excess of about 30ºC. In the present study, experiments were conducted to determine the effectiveness of sorbitol as a thermoprotective agent and to examine the expression of heat shock proteins under conditions conducive to sorbitol accumulation. Turbidity assays and SDS-PAGE showed that sorbitol, at concentrations similar to those present in heat-stressed whiteflies, decreased heat-induced protein aggregation in cell-free extracts of adult whiteflies. Addition of sorbitol to whitefly extracts increased the thermal stability of two soluble whitefly enzymes, hexokinase and sucrase, by increasing the temperature required to inactivate enzyme activity. These results demonstrate that sorbitol is capable of protecting proteins from heat denaturation in vitro, consistent with its proposed role as a thermoprotectant.

Sorbitol levels in whiteflies exposed to cotton leaf temperatures of 38-41ºC (heat-stressed) were 6- to 10-fold higher than in whiteflies maintained at leaf temperatures of 25ºC (control). Northern and Western blot analyses showed that the levels of mRNA for NADPH-dependent ketose reductase (KR), the enzyme which synthesizes sorbitol in whiteflies, were considerably higher in heat-stressed compared with control whiteflies and that the amount of KR protein was slightly greater. Western blot analysis using antibodies against heat shock proteins (Hsps) showed that control and heat-stressed whiteflies contained similar amounts of Hsp70 and Hsp60 protein, and the amount of Hsp90 protein was only slightly greater in heat-stressed whiteflies. In vivo labeling of whitefly proteins with [35S]Met/Cys showed that Hsp70 and Hsp90 were the major labeled polypeptides synthesized in heat-stressed whiteflies, but were only minor components of the labeled polypeptides of control whiteflies. Similar labeling profiles for Hsp70 and Hsp90 were obtained when polyA+ mRNA from control and heat-stressed whiteflies was translated in vitro. The results indicate that Hsp70 and Hsp90 are the major heat shock proteins in whiteflies and that their synthesis in whiteflies increases under the conditions of high temperature that induce sorbitol accumulation. That the increased rate of Hsp70 and Hsp90 synthesis in heat-stressed whiteflies did not lead to a significant increase in accumulation of these proteins suggests that the role of Hsp70 and Hsp90 in thermotolerance involves rapid turnover of these proteins rather than increases in the steady-state amounts.

Investigator's Name(s): Satya Vir.

Affiliation & Location: ARS, Central Arid Zone Research Institute, Jodhpur-342003, INDIA.

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

Dates Covered by the Report: June 1991 - October 1997

Ecology and Population Dynamics of Bemisia tabaci Genn (Aleyrodidae) an Extent of Damage Both by Vector and Yellow Mosaic Virus

Mothbean, Vigna aconitifolia (jacq.) Marechal is a drought hardy crop extensively grown in arid zone of India in an area of 1.24 million hectares with an average production of about 0, 15 million ton's. But the productivity of the crop is extremely poor (125 Kg/ha) due to heavy incidence of yellow mosaic disease and whitefly. Whitefly is by far the most serious insect and it also acts as a vector of yellow mosaic virus (YMV). Study on the biology of Bemisia tabaci was carried out on mothbean crop under laboratory and field conditions. Effect of thermal regime (25-40°C) on development of insect in the incubators and progression of insect under field conditions was compared. Laboratory development of the insect was completed in 28, 42 and 29 days at the three increasing temperatures of 25, 30 and 35°C, respectively. Field development of the insect was the fastest (25-27 days) during July to August and slowest (38-42 days) during December to January.

Effect of sowing dates viz. June 29, July 6, 13 and 20 revealed increase in incidence of both whitefly and YMV with delay in each sowing time. There was significant increase in the development of whitefly population in the mothbean crop sown on July 20 and its direct impact was a sharp decline in the grain yield due to poor growth of the crop. Early sown crop (June to July 6 sowing) had lesser number of whiteflies and low incidence of YMV.

Effect of mixed and intercropping of mothbean, guar and bajra was evaluated for the seasonal incidence of whitefly population and incidence of YMV. The population density of the whitefly during the peak period was 70.5 per plants on the mothbean crop, whereas the density was 44 per plant in the pure guar crop. With pearl millet, the population was 37 per plants for the mothbean and 42 for the guar crop. A similar trend of the lower population existed for the pearl millet intercropped legumes (1:1) i.e. 25 whiteflies on mothbean and 27.5 on guar crop. An incidence of the plant to plant distance from 50cm to 70 cm also reduced the incidence of the whitefly but this maneuver did not have any beneficial effect in terms of the lowering of YMV incidence.

Studies on assessment of yield data revealed that both YMV infestation and whitefly population are responsible for reduction in number of pods per plant and number of seeds per pod. The extend of yield loss due to whitefly and YMV varied from 25.57 to 73.54 in different cultivars of mothbean crop.

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