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

Section A: Part One (2000)

Section A (1999)

Investigator’s Name(s): T. J. Henneberry, L. Forlow Jech, D. L. Hendrix, & T. Steele.

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: 1999-2000

Sticky cotton is a major issue in the textile industry. The problem is widespread and apparent in many parts of the world. A number of contaminants such as crushed seed and seed-coat fragments, pesticides, chemical conditioners used in the cotton gin, oil or grease and immature cotton fiber have been shown to cause sticky cotton. However, the most serious sticky problems have been associated with honeydew contamination as a result of whitefly, Bemisia spp., or cotton aphid, Aphis gossypii Glover, infestations. Research to identify the sugar components of Bemisia honeydew has revealed that, for whiteflies feeding on cotton, two insect-produced sugars, trehalulose and melezitose are the major honeydew components, but a number of other sugars occur in lesser amounts. Trehalulose and melezitose have consistently been shown to be highly correlated to cotton lint stickiness, but as many as 20 individual sugars may occur in whitefly honeydew. Some of the same sugars found in honeydew occur as physiological sugars in the cotton plant (fructose, glucose, and sucrose) and the contributions to lint stickiness of plant physiological and whitefly sugars cannot be separated following extraction from honeydew contaminated lint.

The thermodetector is a readily available device that provides a direct measurement of cotton lint stickiness. It is the internationally accepted method of detecting and quantifying sugar spots on cotton lint. Very simply described, a 2.5 gram sample of cotton lint is spread into a thin mat and layered between two sheets of aluminum foil. The aluminum foil layered lint is heated under pressure. Thereafter, the foil is separated and the number of sticky spots on the foil counted. Studies were conducted with Bemisia argentifolii Bellows and Perring honeydew (to analyze the relationship between honeydew and thermodetector counts): 1) 4 and 6% isopropyl alcohol extracts of honeydew from a contaminated cotton bale, 2) honeydew deposited by B. argentifolii adults on cotton lint in the laboratory, 3) sprays of commercially-procured individual sugars found in honeydew, and 4) micro-pipette applied drops of honeydew. Increased concentrations of either honeydew extracted from lint or individual honeydew sugars from a commercial source resulted in increased thermodetector counts, as did a mixture of sugars simulating honeydew. Thermodetector counts also increased following sprays with increasing concentrations of 4 and 6% isopropyl alcohol extracts of honeydew. These fractions contain carbohydrates that range from tri- to pentasaccharides. Higher thermodetector counts were correlated with an increasing numbers of drops applied to lint as well as with increasing concentrations of honeydew in the drops.

Investigator’s Name(s): T. J. Henneberry, L. Forlow Jech, D. L. Hendrix, & T. Steele.

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: 1999-2000

Silverleaf Whitefly Honeydew Production and Honeydew Sugar Relationships to Sticky Cotton

Bemisia argentifolii Bellows and Perring (= B. tabaci (Gennadius) (Strain B) (SPW) pest status is associated with direct feeding damage and reduced crop yields, virus transmission, plant physiological disorders, and contamination of crops with excreted honeydew that serves as a substrate for several fungal species. Fungal growth on foliage can reduce photosynthesis and causes discoloration on cotton lint. The sugar components of honeydew excretions are major factors in the sticky cotton problem. Honeydew deposits on cotton lint adhere to the working surfaces of cotton processing equipment resulting in reduced ginning rates and stoppages of lint processing machinery at the textile mill. The two most abundant sugars found in SPW honeydew are trehalulose and melezitose. These sugars have been shown to increase on cotton lint in the field in conjunction with increasing SPW populations and to be correlated with elevated cotton lint stickiness. Other honeydew sugars cause sticky cotton, but some of these are also plant physiological sugars and cannot be directly identified with SPW when extracted from honeydew contaminated lint. Laboratory studies were conducted to determine the quantity and quality of the major honeydew sugars produced by SPW adults and nymphs. The effects of temperature, light intensity, and SPW adult density on honeydew production were also determined. In the field, we studied the effects of insecticides on SPW populations and on honeydew production of adults and nymphs collected in treated and untreated cotton plots. In the laboratory, adult females lived longer than males, produced more and larger honeydew drops, and, in most cases, larger amounts of the measured honeydew sugars. Both male and female adults produced more honeydew at 26.7 ± 1º C and 32.2 ± 1º C compared with 21.1 ± 1º C. Under our conditions neither light intensity (30 or 450 µmol/second/m², respectively), nor adult density affected honeydew production. SPW development from egg hatch to adult emergence averaged 12.2 days at 26.7 ± 1^o C. Honeydew production began the first day of nymphal life and peaked on day 3 following emergence. First and second nymphal instars produced more drops than third and fourth nymphal instars. However, honeydew drop size was larger for third and fourth instars compared with first and second instars. The third and fourth instars produced more trehalulose compared with the earlier instars. Adults produced more honeydew than nymphs. The percentage of trehalulose in honeydew was greater for adults as compared to nymphs. Melezitose, as a percentage of all measured sugars, was greater for nymphs as compared with adults.

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

Affiliation & Location: ¹USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ and ²Department of Entomology, University of Arizona, Maricopa Agricultural Center, 37860 W. Smith-Enke Road, Maricopa, AZ 85239.

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

Dates Covered by the Report: June 1999 - December 1999

Life Table Analysis of Bemisia tabaci in Cotton

Bemisia tabaci (Strain B) remains a key pest of cotton and various vegetable crops in the desert southwest. Recent advances in sampling methods and determination of action thresholds have lead to effective management system for this pest in cotton and other crops. However, these systems depend heavily on the efficacy of a few key active ingredients and are not sustainable. Broader-based management systems founded on an understanding of key ecological and biological components of the system are urgently needed in the US and worldwide. Many biotic and abiotic mortality factors impact the population dynamics of B. tabaci 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. For a multivoltine and multi-crop pest like B. tabaci estimating rates of mortality in the field is extremely complex and difficult. There are likely to be large temporal and spatial differences in natural mortality forces within the agroecosystem. The addition of pest management activities provide further sources of mortality that may enhance or disrupt these natural forces. To begin to unravel this complex problem we used a direct observation technique to construct cohort-based life tables of B. tabaci on cotton in central Arizona. These studies have identified, quantified, and compared in situ sources and rates of mortality of immature whitefly stages in unmanaged (unsprayed) cotton fields and fields subject to different insecticide-based management regimes. Here we present results and analyses based on a total 14 life tables completed in unmanaged cotton from 1997 through 1999.

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, six generations were observed from late June through late October in 1998, and 4 generations were observed from late June through late September in 1999. 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.

Survival from egg to adulthood ranged from 0-27% and was < 10% in the majority of generations. Predation by sucking predators and dislodgment were major sources of egg and nymphal mortality in most generations. Egg inviability was moderately high during a few generations and an unexplained factor cause high rates of nymphal mortality during 2 generations in 1998 and 1999. Parasitism by aphelinid wasps was consistently low all years. The bulk of mortality occurred in the egg and 4th nymphal stages. Key-factor analyses identified egg inviability and various forces affecting 4th instar nymphs as the best predictors of total generational mortality. Preliminary analyses suggest that most mortality forces act in a density-independent manner, however some weakly density-dependent effects were found. Further analyses showed that relatively little mortality from any source is completely irreplaceable. This suggests that various mortality factors interact and readily replace one another during the 5 developmental stages. The highest rates of irreplaceable mortality were consistently from predation and dislodgment. Many factors contribute to high levels of mortality in unmanaged populations of B. tabaci in cotton. This understanding will aid on-going efforts to develop more ecologically-based management strategies for this pest in all affected cropping systems.

Investigator’s Name(s): ¹E. T. Natwick,²C. G. Summers, ³C. C. Chu, ²L. D. Godfrey, ¹K. S. Mayberry, ¹C. E. Bell, & ³T. J. Henneberry.

Affiliation & Location: ¹University of California Cooperative Extension, Holtville, CA; ²Department of Entomology, University of California, Davis, CA; ³USDA--ARS, Western Cotton Research Laboratory, Phoenix, AZ.

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

Dates Covered by the Report: 1991 - 1999

Hosts of Silverleaf Whitefly in Imperial and Southern San Joaquin Valleys, California

Plants growing within or around agriculture fields, along fence rows, roadways, waterways, and in urban areas were examined for the presence of silverleaf whiteflies in the Imperial and Southern San Joaquin Valleys, California. A plant was a host when it supported the three life stages (eggs, nymphs and pupal exuvia) of silverleaf whiteflies. When eggs, nymphs and pupal exuvia were not found, the plant species was designated as a non-host species. Leaves from ten plants each species were examined. We have examined 334 plant species since 1991 and found 242 plant species were hosts of silverleaf whiteflies. Numbers of hosts and non-host species, respectively, for each group of plants were: agronomic crops 11 and 7, vegetable crops 47 and 4, ornamental plants, 106 and 72, fruit trees 18 and 5, and weed species 60 and 6. Of the 242 host plant species 149 of them were overwintering hosts. The overwintering hosts included one agronomic crop (alfalfa), 24 vegetable crops, 99 ornamental plant species, 15 fruit tree species, and 20 weed species. This wide host range provides a continuity of year-round plant habitats for B. argentifolii reproduction, survival and overwintering.

Investigator’s Name(s): Dennis R. Nelson¹, Thomas P. Freeman², & James S. Buckner¹.

Affiliation & Location: ¹USDA--ARS, Biosciences Research Lab., Fargo, ND, ²Electron Microscopy Center, Plant Pathology Dept., North Dakota State University, Fargo, ND.

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

Dates Covered by the Report: 1999

Characterization of the External Waxes and Wax Producing Glands of Three Species of Whiteflies

All adult whiteflies studied to date groom and cover themselves, as well as their antennae, with waxy particles, except for their eyes. These particles help to give them a white appearance. Particles shed over an infested leaf surface helps to give it a grayish appearance. Particles are also found sticking to whitefly eggs and on immature stages. These particles are composed of a mixture of long-chain aldehydes and long-chain alcohols in all adult whitefly species examined. One even-numbered chain length of aldehyde and alcohol predominate (about 90%), with the chain length of the dominant component being 34 carbons for B. argentifolii, 32 carbons for T. vaporariorum, and 30 carbons for A. dugesii.

The cuticular surface is also covered by a lipid layer consisting largely of wax esters ranging in chain length, largely even-numbered, from about 32 to 60 carbons. In any given species of adult whitefly, the major wax esters consist of compounds with chain lengths of 40, 42, 44 or 46 carbons. The cuticular surface lipids contain very small amounts of hydrocarbons. No wax esters or hydrocarbons are associated with the waxy particles.

In order to produce the copious amounts of waxy particles, the majority of the abdominal surface is covered with wax plates. These wax plates are composed of many microtrichia which extrude the waxy material. The waxy material is in turn broken off by the tibia to form the particles.

The number of microtrichia were determined per square micrometer. There were 0.49/ m² in the silverleaf whitefly, Bemisia argentifolii, 0.47/ m² in the greenhouse whitefly, Trialeurodes vaporariorum, and 0.43/ m² in the giant whitefly, Aleurodicus dugesii.

The microtrichia are in the shape of a 'T' in which all arms appear to be the same length. The length of an arm was 0.7 for B. argentifolii, 0.8 for T. vaporariorum, and 1.0 for A. dugesii.

The length and width of the waxy particles were: 5.0 by 0.8 for B. argentifolii, 4.7 by 0.9 for T. vaporariorum, and 11.1 by 1.3 for A. dugesii. The particles from B. argentifolii and T. vaporariorum are semicircular and form shapes resembling the letter 'C' or a 'horse collar'. These shapes are effective in enabling the particles to stick to the hairs of the whiteflies. However, the particles from A. dugesii do not curl, they remain as straight fragments on the cuticular surface and do not accumulate to the extent observed for the semicircular particles of the other whiteflies.

Investigator’s Name(s): M. S. Palaniswami, Binu Antony, & Lisha Vijayan.

Affiliation & Location: Division of Crop Protection, Central Tuber Crops Research Institute (Indian Council of Agricultural Research), Trivandrum 695 017, INDIA.

Research & Implementation Area: Section A: Biology, Ecology And Population Dynamics. nt>

Dates Covered by the Report: January - December 1999

Biology, Ecology and Morphometrics of Bemisia Tabaci Genn. on Cassava, Sweet Potato, Brinjal, Cotton and Tobacco

Investigation carried out on comparative biology of Bemisia tabaci in five major crops are covered in the paper. Developmental duration, fecundity, longevity and sex-ratio of B. tabaci was studied on cassava, sweet potato and cotton, under natural conditions (Tem. 29.76 degree celsius, RH 79.29%). Total life cycle of the whitefly was 23.5, 19.5 and 20.0 days on cassava, sweet potato and cotton respectively. Mean total fecundity on cassava, sweet potato and cotton was 45.0, 41.67 and 41.3 respectively. Female longevity was greater on cassava (17.8 days) than on sweet potato (16.33 days) and cotton (13 days). Sex ratio (Male: Female) of B. tabaci was 1:1.8 on cassava; 1:1.59 on sweet potato and 1:1.2 on cotton.

It was observed that the cassava whitefly was found to breed only on cassava, eggplant and tobacco, while sweet potato whitefly was breeding only on sweet potato, eggplant, tobacco and cotton. Total life cycle of cassava whitefly on tobacco and egg plant ranged from 16.0-18.0 and 15.3-17.3 respectively; sweet potato whitefly on cotton, egg plant and tobacco ranged from 17.0-23.0, 18.0-23.0, 17.0-21.0 days respectively. Morphometric studies of fourth instar (pupae) on all five plants showed clear sexual dimorphism. The implications of these findings are very significant in further investigation of Biotype identification, through crossing/breeding and isozyme studies.

Investigator’s Name(s): C. H. Pickett & D. Overholt¹.

Affiliation & Location: California Department of Food & Agriculture, Biological Control Program, Sacramento, CA; ¹Pink Bollworm Program, CDFA, Visalia, CA.

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

Dates Covered by the Report: August 1997 - November 1998

Survivorship of Silverleaf Whitefly Overwintering in Citrus in the San Joaquin Valley

The survivorship of silverleaf whitefly Bemisia argentifolii has been monitored in citrus throughout the year as part of an effort to establish new, exotic parasites of this pest. Four study sites have been used, one each in Fresno, Tulare, and two in Kern Counties. Sites consisted of citrus and cotton acreage managed by the same owner. Cotton is grown directly adjacent to the citrus, and growers have had a history of silverleaf whitefly problems. The whitefly population on citrus leaves was monitored by counting, depending on time of year, once every 1 to 4 weeks. Leaves were removed from trees, shipped to our laboratory, and examined for the number of parasitized pupae, whitefly eggs, early instar nymphs, and late instar nymphs using a dissecting microscope.

I summarized whitefly density counts using a multicohort stage frequency analysis method to measure the survivorship of silverleaf whitefly (Manly, B. 1990. Stage-Structured Populations, Sampling, Analysis and Simulation). We wanted to measure the number of eggs laid on citrus leaves in fall that successfully developed to adults the following spring. Few whitefly reproduce in citrus during summer months. Large numbers of adults, however, migrate into orchards shortly or during cotton defoliation, i.e., September - November. Two estimates were used to measure the likelihood of eggs maturing to adults over this period of time when whiteflies were continuously present in the orchards: survivorship from egg to adult, and from egg to late nymph, the former being a far more conservative measurement (successful adult maturation was measured by the presence of an exit hole in the late stage exuviae, which can fall off within two weeks of adult eclosion, i.e. some were likely missing). The actual value is probably somewhere in between. On average from 1997 to 1998, 0.073% to 3.73% of eggs survived to adults. The following winter fewer whiteflies survived, 0.036% to 0.38%, an entire order of magnitude less. Summer – fall, 1998 was cooler than the former, which may explain part of the drop in survivorship. The peak in egg production came almost two months later in 1998 (November vs. September), increasing the exposure of eggs to lower temperatures. Also, the actual number of eggs oviposited was about half in 1998 than in 1997: 12.5 vs. 7.6 (not including the new site in Kern County). Another trend is the drop in egg survivorship as one moves from the southern end of the San Joaquin Valley to the central region. This was not true for other instars suggesting egg survivorship is more sensitive to lower temperatures than nymphal stages.

The delay in cotton maturity, as a result of the cool spring, undoubtedly played a role in forestalling the migration of whiteflies from this preferred host plant. Parasitism of silverleaf whitefly on citrus was always quite low, even in the second year of releases, rarely exceeding 10%, despite the massive releases of parasites into these trees (see Pickett et al., this volume). Survivorship data shows that anywhere from 49% to 100% late instar whiteflies die before maturing to adults. Most likely many young parasites never fully develop because they die within these hosts. Under optimal conditions, i.e. inside a heated greenhouse, we have found up to 72% of nymphs dying in the absence of any predation or parasitism. Citrus appears to be a poor host for silverleaf whitefly.

Investigator’s Name(s): Michael E. Salvucci & Steven J. Crafts-Brandner.

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 1999 - December 1999

Effects of Temperature and Dietary Sucrose Concentration on Respiration in the Silverleaf Whitefly, Bemisia argentifolii

A system was developed to measure homopteran respiration during feeding. Insects were placed in a flow-through respiration chamber that was specifically designed to provide access to an artificial diet. The respiration chamber was connected to a commercial infra-red gas analyzing system that continually monitored and recorded respiratory CO₂ evolution during feeding. Using this system, respiration rates of 240 and 251 µ mol CO₂ h^-1 g^-1 were determined for whiteflies and cotton aphids, respectively, at 30ºC on diets containing 15% sucrose. Whitefly respiration increased with temperature over the range of 25 to 46ºC with a Q₁₀ of about 2.12 on diets containing 15% sucrose. Respiration rates were similar throughout this temperature range on diets containing 10, 15 and 20% sucrose, but were considerably lower at all temperatures on diets containing 2.5% sucrose and at temperatures greater than 35ºC on diets containing 5% sucrose. Respiration rates decreased following the addition of sodium azide to the diet or upon extended exposure to 47ºC. The rate at which respiration decreased at 47ºC was inversely related to the concentration of sucrose in the diet over the range of sucrose concentrations from 2.5 to 15% sucrose. The results indicate that whiteflies require a sucrose concentration of between 5 and 10% (i.e., 0.15 and 0.3 M) for maximum basal metabolism. Higher concentrations of sucrose in the diet delayed high temperature mortality, possibly a reflection of the high sucrose requirement for sorbitol synthesis in these insects.

Investigator’s Name(s): Alvin M. Simmons ¹, Gloria S. McCutcheon², Richard J. Dufault², Richard L. Hassell², & James W. Rushing².

Affiliation & Location: ¹USDA-ARS, U.S. Vegetable Laboratory, Charleston, SC; ²CREC, Clemson University, Charleston, SC.

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

Dates Covered by the Report: 1998-1999.

Bemisia and Associated Parasitoids on Species of Medicinal Herbal Plants

Most whitefly related research has focused on row, horticultural, and ornamental crops. In addition, weed and some other species have been examined as whitefly hosts because of associated plant pathogens and other aspects of whitefly ecology. Consumers have expressed much interest in medicinal herbals over the past decade, and industry has been trying to respond to the demand. A multi-study research project was recently initiated on the production potential of selected medicinal herbal plant species as new crops suitable for cultivation in South Carolina. One phase of the research examined the vulnerability of 5 medicinal herbal species to Bemisia argentifolii Bellows and Perring. In an experimental production field, natural populations of adult and immature B. argentifolii infested the 5 perennial species of medicinal herbal plants (feverfew, Tanacetum parthenium (L.) Schultz-Bipontinus; St. John’s wort, Hypericum perforatum L.; purple coneflower, Echinacea pallida (Nuttall) Nuttall and E. purpurea (L.) Moench; and common valerian, Valeriana officinalis L.). Whiteflies have not been previously reported in the literature on these plant species. Capture of adult whiteflies on yellow sticky cards agreed with the relative density of immatures among the plant species. The density of whiteflies was greater on some of the species, such as E. purpurea, than on others. Similarly, adult capture on sticky cards was high in plots of E. purpurea compared with plots of the other 4 species, and adult counts were elevated in the highest (440 kg N/ha) of 3 fertility rates in E. purpurea. Likewise, laboratory choice and no-choice tests agreed with the observation of a higher population of B. argentifolii on E. purpurea compared with the other 4 plant species. The whitefly completed development on all 5 plant species, and whitefly associated parasitoids emerged from field leaf samples of each plant species. The impact of the whitefly was not determined, but the data indicate potential problems from this pest. While all 5 species in this study supported feeding and development by Bemisia, the commercial use of these plants dictates restrictions on the use of traditional insecticides.