National Silverleaf Whitefly Progress Review 
National Research, Action, and Technology Transfer Plan,
1997-2002: Fifth Annual Review of the Second 5-Year Plan
February 10-12, 2002

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Text of second 5-year plan (1997-2001)

 

Text of first 5-year plan (1992-1996)

 

Technology Transfer (1992-1997)

 

Bibliography of Bemisia tabaci (Gennadius) and Bemisia argentifolii
Bellows & Perring

NATIONAL RESEARCH, ACTION, AND TECHNOLOGY TRANSFER PLAN, 1997-2001

Introduction
Objectives
Technology Transfer
Section A: Biology, Ecology, and Population Dynamics
Current Research Activities
Knowledge Gaps
Section B: Viruses, Epidemiology, and Virus-Vector Interactions
Current Research Activities
Knowledge Gaps
Section C: Chemical Control, Biopesticides, Resistance Management, and Application Methods
Current Research Activities
Knowledge Gaps
Section D: Natural Enemy Ecology and Biological Control
Current Research Activities
Knowledge Gaps
Section E: Host Plant Resistance, Physiological Disorders, and Host-Plant Interactions
Current Research Activities
Knowledge Gaps
Section F: Integrated and Areawide Pest Management Approaches, and Crop Management Systems
Current Research Activities
Knowledge Gaps

INTRODUCTION

The overall orientation of the new research and technology transfer plan is for development of integrated pest management strategies for incorporation into existing community action plans and for the development of new areawide and IPM efforts. The research community, within the framework of the new plan, is charged with the responsibility of providing new knowledge and developing innovative, novel, adaptable approaches for silverleaf whitefly areawide and integrated pest management programs. The basis for community-based pest management programs resides in a comprehensive knowledge of natural mortality regulating population factors; impact of natural enemies, and potential for augmentation, and conservation and introduction of new natural enemies, and development of other effective control strategies where applicable. The new plan strongly supports biological, biologically-based control approaches, host plant resistance, and their implementation. More than 30 natural enemies, including fungi, are in various stages of colonization and evaluation to determine their potential as control agents. Cultural control, biologically-based strategies, and host-plant resistance offer the most socially and environmentally acceptable technologies for use in integrated management programs. The plan stresses research to fill gaps in our knowledge base that prevent our full understanding and potential manipulation of the whitefly complex including: environmental response and plant interactions in cultivated crops, urban and nursery plantings, and weed ecosystems. Fundamental research in genetics, biochemistry, molecular biology, virus and virus transmission, and behavior is needed to provide a research base upon which to sustain long-term integrated management programs.

A major effort in technology transfer under the new plan will be directed to development of area-wide and IPM community-involved programs that will take advantage of compatible and effective suppression components acceptable to the environmentally concerned, and the farming community. The major responsibility for this effort will be absorbed by federal and state action and regulatory agencies, university cooperative extension services, and the leaders within the agricultural commodity associations. Growers must be full partners in all aspects of the programs. Such programs will also rely heavily on expanded efforts of education-extension to develop far reaching technology transfer and provide the necessary communication to our clientele on progress and products available for implementation. The new plan consists of six priority research sections, and provisions for annual reporting on outcomes, impacts, and products delivered or under development.


OBJECTIVES

The new plan provides a transition from the completed "5-Year National Research and Action Plan for Management and Control of the Silverleaf Whitefly, Bemisia argentifolii" to a new research and technology transfer plan for the years 1997 through 2001. It provides a continued basis for communication, coordination, and cooperation, and for generating ideas, identifying knowledge gaps and research needs, setting priorities, and facilitating the transfer of information and technology for management and control.

The specific objectives of the new research and technology transfer plan are to:
Provide a forum for discussing and evaluating SLW research program progress, and for describing delivered end products.
Enhance team-based communication, coordination, and cooperation between research, methods development, extension, and education components in order to address SLW management issues.
Identify knowledge gaps and clarify research needs, opportunities, and strategies for use in integrating interdisciplinary, interagency, and industry resources in the management of silverleaf whitefly.
Serve as a decision base for progressive reassessing of priorities as the need arises.
Describe new technologies, and transfer deliverable technologies to clientele.
Facilitate community-based partnerships for implementation of areawide SLW management programs.


TECHNOLOGY TRANSFER

Technology transfer will be discussed at each progress review and updated as necessary. The tables will include delivered products, products being developed, and the impacts and outcomes of the technology transfer. Technology transfer will be accomplished through meetings, workshops, training sessions, cooperative research and development agreements, agricultural experiment station mailings, and USDA and state experiment station networking systems.


A. Biology, Ecology, and Population Dynamics

Current Research Activities
Generalized silverleaf whitefly host plant sequences on cultivated crops, urban plantings, weeds, and native plants from spring through winter have been identified and are being studied in relation to population dynamics. Seasonal population trend studies have shown differences in whitefly magnitude on various host species and among locations and years; however, the overall trends in hosts utilized and sequence have been similar among locations and years. The mediating influence of the environment has been the overriding influence on whitefly density. Initial population dynamics modeling efforts have been made and serve to identify needed research. Initial modeling studies in multiple crop systems indicate that population simulations compare reasonably with trends observed in sampling data, and were highly sensitive to natural enemy parameters. LandSat data obtained for the Imperial Valley and San Joaquin Valley of California are being ground-truthed. Appropriate simulations may eventually be used to study the influence of areawide management practices, and host plant interactions on SLW populations. This will involve extensive study and validation.

Considerable research progress has been made in developing and delivering sampling plans and in relating SLW densities with cantaloupe and cotton yields and cotton lint stickiness in relation to action thresholds. These plans provide a base for growers, pest control operators and researchers in development and implementation of action thresholds. The cotton sampling plan was validated in more than 8000 acres of commercial production. The cotton and melon sampling plans have been implemented in IPM programs in the United States and internationally. Sampling plans and decision-making tools are a necessity for all cultivated hosts and should be continually refined, improved and updated in light of newly developed control options. In cotton and cantaloupe, key components that influence economic injury and economically based thresholds have been suggested. Action thresholds should greatly benefit growers in making management decisions and are essential for other crops. SLW sampling plans and action thresholds are being developed for cole crops, tomatoes, and alfalfa. Whitefly action thresholds for lint stickiness and quality losses in other crops are less well-defined. Initial studies correlating cotton photosynthesis (and resulting yield) with SLW densities have been accomplished but results need to be quantified, and related to other plant measurements. The technique will help to further refine the threshold and to better understand the insect-plant interaction and may provide a linkage with whitefly-induced plant disorders and whitefly-vectored virus stresses on crop plants.

SLW dispersal abilities are being quantified in relation to environmental conditions (wind) and agronomic factors (water stress). Knowledge of SLW dispersal and movement influencing factors will be important considerations for designing efficient management schemes. This will be a key research area as areawide management schemes are designed and implemented.

Behavioral studies of whitefly feeding have revealed information that may provide clues to methods of manipulating these vital feeding functions in plants through conventional plant breeding or molecular approaches to incorporating resistant characteristics into acceptable agronomic crop types. Feeding behavior of first-instar immatures of several whitefly species show activities related to host plant structure and effects on morphology of later instars. Results from adult behavioral studies have suggested a potential new trap design which is badly needed for survey and monitoring since sticky traps are difficult to use, time consuming to count and can only be used for very short-term exposures.

Histological study of stylet penetration has correlated with host plant microstructure and plant morphological characteristics. Surface and internal structures of leaves from different host plant species and cultivars have been shown to affect immature whitefly morphology and ability to colonize (density of immatures) and feed. The latter is probably due to variation in density and proximity of available vascular bundles within leaves. Serine proteinase activities were undetectable in whole extracts of whiteflies, but other enzymes, especially those secreted into the plant, may affect host plant responses significantly.

Whiteflies showed differences in mating behaviors among species. If extended to studies of B. tabaci populations, this may have relevance to the reproductive isolation observed between biotypes. The absence of clear morphological differences between B. tabaci Strain B and B. argentifolii, and differences of opinion as to the validity of molecular genetics approaches for making species level determinations remains unresolved. Key morphological characters by which the A, B, and other biotypes can be distinguished are still lacking. Morphological plasticity and data from several molecular studies indicate the most likely classification for the group is as a B. tabaci species complex, with the common name sweet potato whitefly. Some morphological plasticity stems from stimuli received by crawlers based on plant geography, hairiness, and crowded conditions on the leaf. Clearly, this insect is more complex than previously realized.

Carbohydrate analyses of whitefly honeydew has revealed several new oligosaccharides, and more clearly defined the composition of honeydew. The role of these materials as by-products of biochemical mechanisms in the whitefly is unknown. Continuing studies with endosymbionts, enzymes and physiological characterization need to be done to identify potential weak links in biological systems that might be manipulated between the insect and host as a possible control approach. A possible solution might be the use of specifically constructed enzymes that would hydrolyze Bemisia sugars on cotton lint. Some work has been initiated but it is preliminary in nature and needs expanded objectives. Several artificial whitefly feeding systems have been developed. These systems have been used to rear whiteflies from egg to adult for the first time. Fundamental biological studies have been conducted to evaluate the effects of insecticidal compounds and antibiotics on whiteflies (presumably, antibiotics affect primary endosymbionts), and to study the transmission pathway of gemini viruses in adult whiteflies in relation to feeding behavior. Feeding chambers have also been utilized in the identification of sugars that contribute to stickiness due to whitefly honeydew. Recent development of artificial feeding systems and diets may allow mass rearing of natural enemies. Whiteflies were shown to be unsuitable prey for some predators, due to nutritional deficiencies. The availability of the artificial feeding systems opens the door to numerous studies on nutrition, digestion, metabolism, biochemistry and physiological relationships.

 

Knowledge Gaps
  1. Knowledge of key factors in over wintering mortality and environmental and biological parameters stimulating population increases is limited and life table analysis is needed.
  2. Identification of life cycle vulnerabilities that may be manipulated for population management have not been found.
  3. Economic thresholds, except for cotton and cantaloupe, have not been addressed.
  4. Sampling methods and action thresholds have been developed for only a few major crops.
  5. Extensive refinement of existing models needs to be made, to interface with multiple host plant phenology, biological, environmental and other potential control inputs.
  6. Sampling for cotton lint stickiness is complex and field sampling needs to be extended; other whitefly induced crop quality impacts need to be addressed.
  7. The time of whitefly dispersal, environmental and biological factors affecting dispersal, the proportion of the population that disperses and reproductive efficiency need to be carefully defined to determine the impact of dispersing populations on population dynamics at the source and the end of the dispersal cycle.
  8. Whitefly mating behavior needs to be better understood in relationship to reproductive isolation. The marked differences between agricultural effects of biotypes or species of Bemisia is not reflected in the current understanding of systematic relationships, although reproductive isolation is often an assumed mechanism. Knowledge of mating behavior may allow prediction and action against population increases with changes in genetic composition.
  9. Taxonomic and nomenclatural issues must be clarified to expedite international scientific communication.
  10. Endosymbionts are crucial to whitefly survival and reproduction. Enhanced knowledge of endosymbiont biology and function may lead directly to novel control technologies that incorporate endosymbiont disruption, e.g. with antibiotics, and subsequent reduction of whitefly populations.
  11. Whitefly artificial feeding and rearing systems are now showing promise and must be developed further. These systems will provide valuable tools for efforts in whitefly control as diverse as mass production of natural enemies, development of new biorational insecticides, discovery of secretions that are central to host plant disorders, and disruption of endosymbionts.


B. Viruses, Epidemiology, and Virus-Vector Interactions

Current Research Activities
The whitefly-transmitted geminiviruses (Subgroup III Geminiviridae) pose an extremely serious threat to agricultural systems within the habitat range of Bemisia, in the U.S. and in adjacent regions of Mexico, as well as the Caribbean Basin. These viruses were recognized as a new group of virus pathogens in 1978, and during the last decade have become recognized as globally occurring in a variety of food and fiber crops. At present, whitefly transmitted (WFT) gemini viruses have made serious inroads into agricultural systems in Arizona, California, Florida, and Texas of the U.S. and in adjacent regions in the subtropical agroecosystems throughout the Americas. Hence, the potential for the occurrence of new diseases and epidemics caused by geminiviruses is now greater than ever. Geminiviruses that predominate as plant pathogens in the U.S. include: cotton leaf crumple geminivirus in Arizona and California cotton crops, squash leaf curl geminivirus throughout Arizona in melons, squash, and watermelon plantings, and in watermelon in the Rio Grande Valley, Texas.

In addition, numerous uncharacterized whitefly-transmitted geminiviruses have been detected in crop and weed species in Arizona and in the Rio Grande Valley of Texas, including new geminiviruses of cotton, kenaf, okra, and tomatillo. In Florida, bean golden mosaic, tomato mottle geminivirus, and cabbage mottle geminiviruses are economically important in bean, tomato, and cabbage crops throughout the state. Two whitefly-transmitted viruses of tomato and lettuce have been described and appear to be distinct from geminiviruses. Most of these viruses remain unstudied or poorly understood, despite some effort to investigate the respective diseases.

Considerable progress has been made in developing polymerase chain reaction (PCR) assays to specifically and universally detect subgroup III geminiviruses. PCR primers target the core or central fragment of the coat protein (CP) gene and yield a 550 bp diagnostic product. The products are cloned and sequenced, and incorporated into the geminivirus CP gene sequence database. This approach has greatly facilitated geminivirus identification and is an essential tool for molecular epidemiological studies.

The relationships between WFT viruses and their whitefly vectors are characterized as either persistent and circulative for geminviruses, and semi-persistent or non-persistent for the few representative non-geminiviruses known to be vectored by whiteflies. The geminiviruses predominate worldwide among WFT-transmittted viruses as important pathogens in agroecosystems. Very little is known about the mechanisms involved in this process. Preliminary studies have been conducted that will lead to elucidating the underlying mechanisms involved in the process, and to understand the basis for geminivirus-whitefly specificity in transmission of geminiviruses. Several laboratories have begun to establish experimental systems by which to study these important interactions.

A novel micro-feeding chamber has been developed and used to examine several accessible steps in the transmission pathway between the vector whitefly, Bemisia, the non-vector, greenhouse whitefly, and the squash leaf curl geminivirus. PCR was used to amplify a diagnostic fragment of SqLCV coat protein to demonstrate presence of the virus. In time-course studies, virus was detected in whole body extracts, honeydew, and saliva of the vector, whereas, in the non-vector, virus was detectable in body extracts and honeydew, but never in saliva. This result provides the first evidence of geminivirus in whitefly vector saliva and paves the way for additional evaluation of the transmission process using this novel approach. Several laboratories have developed assay systems that utilized PCR to detect geminivirus DNA in individual whiteflies given sufficient acquisition access periods on infected plants.

A method for preparing whole whiteflies for light and electron microscopy examination, and for immunocytochemistry has been developed. Adult whitefly anatomy was studied using this method of preparation, and a detailed functional anatomical interpretation has been put forth. Immunocytochemistry studies using antibodies to the whitefly-transmitted squash leaf curl virus was applied for locating the virus and to elucidate specific aspects of geminivirus-whitefly vector interactions. Labeled antibody was detected in most organs and fluids of adult whiteflies, and several hypotheses about virus-vector specificity, with respect to postulated anatomy were also put forth.

The utility of key morphological characters for differentiation of subtaxa within the Bemisia tabaci species complex was revisited. Results of an extensive, high resolution TEM study in which key characters of a suite of B. tabaci populations examined indicated no significance differences in any character or character combination that permitted separation of populations intosubtaxa, including distinct species. The ASMS4 setae, has been proposed as a diagnostic character, and universally absent or present, to differentiate A and B biotypes (also B. argentifolii). In this study, ASMS4 setae were neither consistently absent in populations from the Eastern Hemisphere or from the Old World B biotype, nor consistently present in all New World populations. However, the ASMS4 setal pair was more often absent in Old World populations, and when, present, it was most often associated with a New World whitefly, indicating that the trait is clearly not as definitive as originally proposed. In addition, other characters used to identify B. tabaci to genus and species, varied for different host-plant species, and/or with the absence of, or degree of leaf pubescence, and with degree of crowdedness.

Genetic analysis by RAPD-PCR and a mitochondrial 16S DNA marker postulated to distinguish between subtaxa or lower, indicate a high degree of polymorphism between distinct populations, and extend similar conclusions drawn from general esterase electromorph patterns. Further, using both methods, most populations examined clustered by either a New or Old World origin, but several interesting collections are either split, or form a continuous distribution, depending on type of analysis applied to data sets. Thus, it still appears that there is inadequate data upon which to base taxonomic decisions about the B. tabaci species complex at subspecies levels, or with regard to universal acceptance of the proposed species novae, B. argentifolii, for a population also known as B biotype.


Knowledge Gaps
  1. Identification and characterization of virus-vector relationships of economically important whitefly-transmitted viruses, with an emphasis on geminiviruses, presently recognized as the most important emerging group of plant viruses in US crops.
  2. Molecular epidemiological information: geographic virus distribution, identification of economically important and predominant viruses, specific plant hosts and natural sources in agroecosystems, virus reservoirs, and alternate virus hosts.
  3. Factors affecting whitefly-mediated virus transmission, including a possible role for endosymbionts and their relation to the transmission process, the pathways traversed by specific viruses in the vector, the basis for virus-vector specificities, and their impact on disease epidemiology.
  4. Management strategies to reduce the spread of whitefly-vectored viruses and/or the impact on disease incidence. Strategies may involve interference with whitefly-mediated transmission processes, development of resistant or tolerant germplasm through classical breeding approaches and/or development of transgenic plants with anti-viral capabilities, and management of diseases through a practical knowledge of virus identity, distribution, virulence, host range, molecular epidemiology of important diseases, and the potential for economic damage.
  5. Definition and assessment of the impact of genetic and biological variability between whitefly vector types or Bemisia species on whitefly-mediated transmission/disease spread; genetic based methods are needed to identify and differentiate economically important whitefly biological types and/or Bemisia species (within the proposed B. tabaci species complex).
  6. Molecular engineering approaches to develop plant resistance to whitefly-transmitted viruses.


C. Chemical Control, Biopesticides, Resistance Management, and Application Methods

Current Research Activities
Chemical control approaches can be valuable IPM tools that help maintain or improve crop yields. Yield responses to chemical control have been demonstrated in virtually all host crops for a variety of insecticides. The ability of chemical control to maintain SLW populations below economic thresholds has been demonstrated in cotton and melons, and chemical control of geminivirus epizootics has been achieved in tomatoes. Action thresholds help limit indiscriminate use of insecticide, and have been established for cotton (5-10, adults per leaf) and cantaloupe (3 per leaf). These values need to be constantly refined and updated in response to changing conditions of crop ecology, control efficacy and market economics.

A major goal now is to preserve and improve the efficacy of insecticidal tools and enhance integration with other management strategies. Genetic, biochemical and ecological mechanisms of insecticide resistance need to be better elucidated to achieve successful resistance management. Wide spread monitoring for resistance has underscored the fact that problems are most likely to occur in closed systems like greenhouses and desert agriculture with no refuge for susceptible populations. Although rotation of available chemistries is undoubtedly useful in slowing selection for resistance, rotation alone did not reverse trends expressed by whitefly populations in a large scale experiment conducted in Maricopa county in 1995-96 More selective chemistries, including natural products and other biopesticides, could fit into insecticide rotations to better manage resistance. Experimentation with hydraulic and air assisted ground applications or electrostatic charging of spray material applied by ground or air has not resulted clear cut advantages between methods. However, underleaf coverage can always be improved by optimizing pump pressure, spray volume, and nozzle configuration of conventional hydraulic systems, thereby enhancing efficacy of selective contact materials such as detergents and myco insecticides.

Where available, alternatives to broad-spectrum insecticides should be encouraged to conserve natural enemies and advance the integration of biological and chemical control. Responses of parasitoids and predators to specific chemicals and biopesticides need to be constantly evaluated, and compatibility tested in the field. The potential for synthesis of these compounds in transgenic plants needs to be evaluated through extensive biochemical and genetic analysis.


Knowledge Gaps
  1. Information is needed on more efficacious, diverse, and selective chemistries and natural products to control whiteflies.
  2. Action thresholds for most cultivated crops have not been developed and current developed thresholds need to be related to specific insecticide usage patterns.
  3. More efficacious method of applying insecticides are needed to improve coverage of selective, inexpensive and environmentally safe contact materials.
  4. Methods to preserve the efficacy of existing and new chemicals must be developed. Resistance mechanisms need to be identified and resistance management strategies developed with biochemical, genetic, and ecological resistance factors considered.
  5. Timing, rates, placement, and insecticide selectivity must be improved to conserve natural enemies and provide opportunities for biological control. Guidelines for integration of chemical and biological control need to be developed and tested.


D. Natural Enemy Ecology and Biological Control

Current Research Activities
Biological control approaches remain the fundamental base for developing long-term integrated whitefly management. Research in this area has resulted in several biological control products that have become commercially available. Natural enemies studied in both agricultural and urban systems include parasitoids, predators, and fungal pathogens. The contributions of natural control, natural enemy conservation, augmentation and importation have been significant but varied. The majority of field studies have been conducted in California, Arizona, Texas, and Florida, while the majority of greenhouse studies have been conducted in California, Florida and the Northeast.

Surveys of indigenous natural enemies (esp. parasitoids) have been conducted for most whitefly infested areas of the U.S. Experimental data evaluating the impact of indigenous natural enemies on whitefly in the absence of insecticide pressure is generally lacking. Survey data document varying levels of parasitism across a wide range of habitats and individual host plant species, but the manipulative experiments necessary to measure whitefly suppression have not been conducted.

Methods for conserving natural enemies have received relatively minor attention compared to the other forms of biological control. Progress has been made to determine the potential of natural enemy refugia to sustain natural enemies in the field. Research conducted in California has shown that annual plant refuges may lead to increased levels of parasitism by Eretmocerussp. in adjacent crop plants. However, these annual plant refuges may not be practical for the desert Southwest because of poor sustainability and high costs. New studies with perennial plantings will be initiated and need to be intensified.

Identification of physiologically or ecologically selective insecticides has received only minimal attention. Effort should be exerted to quantify the effects of insecticide chemistries and application methodologies on beneficial arthropods, and to select the materials and methods that are least harmful to nontarget species. Little effort has been made to identify variation among natural enemy populations based on their susceptibility to insecticide practices, and no effort has been made to develop insecticide-resistant natural enemies.

Foreign explorations have been extremely successful in terms of making available a wide array of natural enemy species or strains exotic to the U.S. Quarantine procedures for most of these species are completed, and permits have been issued for release of biological control agents for research. California, Arizona, and Texas have initiated release programs to evaluate several of the parasitoid strains. Evaluation is time consuming and several years of study are required before establishment and impact can be assessed with any degree of confidence.

Molecular methodology for classification of Eretmocerus and Encarsiahas been completed and publications with keys to identify the various parasitoid species are in various stages of preparation for publication. New concerns have arisen regarding the systematics of Delphastus. Continued support for natural enemy systematics should accompany the field research associated with importation and augmentation programs, and additional support should be made available to resolve taxonomic issues of the key natural enemies.

Mass-rearing procedures have been developed for some natural enemy species with the potential to produce millions of parasitic wasps for augmentation and release. Artificial diets are being investigated to improve cost-effectiveness of mass-produced predators. A major increase in this area for potentially useful predators and parasitoids must be made if augmentation approaches are to be fully investigated and implemented.

A number of fungi associated with whiteflies have been collected. Several fungi have shown exceptional potential. In field trials, however, results indicate that formulation and application technology need improvement. An effective microbial control component for incorporation in whitefly management systems that could substitute for synthetic chemicals would be a major breakthrough that would greatly improve the probability of success for parasitoid-predator augmentation approaches.

Opportunities for advancement of the science of biological control remain plentiful. The mechanisms leading to the success or failure of biological control should be studied within the context of developing a solution to this important pest. Research on tritrophic interactions among host plants, whitefly, and their natural enemies may be useful in configuring optimal refuges for natural enemy conservation and prioritizing target cropping systems for augmentation or importation biological control. The diverse array of biologies represented by the numerous natural enemy species that may be released during biological control programs provide a unique opportunity to assess the role of interspecific interactions, the development and utility of pre-release evaluations, the identification of species-level attributes, and the value of various release strategies that may facilitate or hinder biological control. These represent only a selection of the numerous opportunities. Given the magnitude of the program in terms of research dollars, facilities, and personnel, the opportunity for advancing the science while solving an agricultural problem should be seized.


Knowledge Gaps
Natural Control and Conservation
  1. Quantify the effects of crop management practices, including pesticide use, on the ability of natural enemies to suppress whitefly populations.
  2. Develop techniques to enhance natural enemy abundance and effectiveness in urban and agricultural systems.
Augmentation of Natural Enemies
  1. Gather information on nutrition, physiology, and ecology of parasitoids, predators, and pathogens essential to cost-effective mass-rearing procedures.
  2. Evaluate the efficacy of natural enemies in the greenhouse and field, develop implementation technologies, and assess the economics of biological control methods.
  3. Develop storage, shipping, release and quality control technologies to maximize the effectiveness of mass-reared natural enemies in the field.
Importation Biological Control
  1. Evaluate establishment and long-term control effects of exotic natural enemies in the field. Systematics, Ecology and Population Dynamics of Natural Enemies
  2. Conduct systematics research of natural enemies present in target release sites and of imported and mass-reared natural enemies.
  3. Examine the effects of host plants and plant community structure on natural enemy performance.
  4. Identify the attributes of natural enemy biology and population level interactions contributing to biological control success or failure.


E. Host Plant Resistance, Physiological Disorders, and Host-Plant Interactions

Current Research Activities
Some degree of plant resistance to whiteflies has been reported for nearly every major crop studied. Limited reports have been made on progress in developing virus disease resistance. Significant progress for incorporating resistance into acceptable agronomic crops is dependent on identifying resistance mechanisms. Some factors influencing host selection have been suggested for cotton and other crops. Leaf thickness was shown to be highly correlated with the depth of vascular bundles in cotton, suggesting a more tedious access for whitefly feeding. Work on other potential resistance mechanisms in cotton identified reduced trichome numbers, okra-leaf shape, and red plant color. A glabrous cotton type with whitefly resistance, good yield, and other desirable qualities, was released to industry and is now commercially marketed as Texas 121. Progress with alfalfa germplasm with whitefly resistance and other good qualities suggested germplasm should soon (ca. 1998) be available for seed increases by industry. For collards, two hybrids had fewer whiteflies than open-pollinated cultivars, but three glossy leaf genotypes were the most resistant. Two zucchini squash lines (Sunseed 3 and A21-7) have been identified with resistance to silverleaf symptoms, but not to whitefly infestations. Resistance sources have not been located in tomato, and foliar pubescence had a variable influence on whitefly populations. In contrast, pubescence was a factor in host oviposition selection and vertical distribution of oviposition on soybean, but whitefly survival and development were not affected.

Information is needed on the concentrations of dietary components in the phloem tissue of various host plants to fully understand host plant preference by the whitefly. It is also important to determine how dietary components change in response to specific changes in plant physiology (i.e., water stress, nitrogen nutrition) that affect whitefly populations. Knowledge of the uptake and metabolism must be acquired to develop strategies for inhibiting feeding functions. Feeding behavior of first-instar immatures of several whitefly species shows activities related to host plant structure and affects on morphology of later instars.

Some biochemical relationships are being revealed that may explain physiological syndromes associated with silverleaf whitefly infestations. Expression pathogenesis related proteins in tomatoes resulted from feeding by whiteflies. Duration of feeding and distance from fruiting structures affected the severity of tomato irregular ripening. a -Tocopherol, squalene, and linolenic acid increased in squash (pumpkin) plants prior to expression of squash silverleaf; these compounds may serve nutritional and/or structural needs of the host plant or the insect.

The universal acceptability of plant resistance as an economic, environmentally desirable solution to pest problems provides justification for expansion of our efforts in this direction. Although, in the past, the time required to develop resistant, agronomically acceptable crops has been long, new genetic and molecular biology methods in conjunction with conventional breeding approaches may significantly expedite the process. Emphasis on the physiology, biochemistry and molecular biology of the host-plant interaction is required to identify key aspects of whitefly metabolism that can be targeted for inhibition by plant manipulation.


Knowledge Gaps
  1. Mechanisms of resistance and modes of action for whitefly and disease resistance.
  2. Sources of germplasm for use in selection for whitefly resistance, disease resistance and associated physiological traits.
  3. Molecular biology approaches for developing whitefly and disease resistance.
  4. Definition of whitefly-plant interactions and identification of plant characteristics that may be useful in developing plant resistance approaches to control.
  5. Characterization of new plant physiological syndromes that have appeared with the new biotype or with the increase in whitefly populations.


F. Integrated and Areawide Pest Management Approaches, and Crop Management Systems

Current Research Activities
Many of the cultural and chemical control research achievements have been implemented at some level in one or more farm communities. Integrated areawide approaches are a goal for the future. The wide host range of cultivated, nursery, and ornamental crops and inter an intra crop movement suggests that areawide, community-involved strategies have the best potential for whitefly management. The role of natural enemies in whitefly population regulation and potential augmentation candidates are being identified. Some information is available regarding chemical control and natural enemy interactions to develop potential conservation strategies. Additionally, natural enemy refugia are being investigated. Sampling protocols and action thresholds for cotton and cantaloupe have been developed and are continually being refined. Resistance monitoring techniques have been developed and implemented and new insecticide chemistry developed that is helping reduce insecticide use and resistance development. Other management approaches with potential as components for IPM systems include cultural practices such as destruction of host crops after harvest, timely irrigation to avoid water stress, and close attention to planting dates and harvest for sequentially planted susceptible crops. These strategies provide a strong base for successful efforts to develop community involved areawide management systems.

To facilitate areawide management approaches, aerial crop mapping of California's San Joaquin Valley, California, is being developed to build better cropping systems. Ground surveys are essential to verify aerial technology analytical potential. Spatial models of cropping systems are being assembled and possible interlinking with precision farming may enhance the research approaches on an areawide basis. Multicrop modeling work has been initiated in Florida and site specific models for examining the impact of multiple control tactics are being developed in Arizona. As chemical, cultural control , biological control and sampling technology becomes refined there remains a vital need for state and federal regulatory and action agencies and university Cooperative Extension to provide implementation pathways, communication and educational systems for the consumer in development of community action plans.

Crop production inputs and management techniques have been found to have substantial influence on silverleaf whitefly population dynamics. The reasons why this occurs are not known, and must be determined if crop management systems are to be developed for optimum yield and minimum whitefly problems. Reduced water stress has been studied extensively in cotton but not in other crop systems. Fertilizer inputs also appear to influence whitefly population development, but again no explanations are evident and results are variable. Little or no information exists for cotton crop production methodology such as plant density, planting dates, harvest dates, and in the case of cotton, such activities as defoliant and desiccant applications. Generally, planting considerations include crop sequencing to extend cultivated host-free periods as long as possible and to identify spatial considerations with respect to existing crops. Current guidelines are vague and need to be defined.

Large scale cropping patterns in relation to reproduction and movement of silverleaf whitefly are being studied in some areas. These studies are utilizing LandSat data to map cropping patterns for modeling the spatiotemporal dynamics of silverleaf whitefly in large cropping systems. Localized migrations by whitefly adults are being studied in cropping systems in western Arizona. These studies have focused on wind patterns, distance adults migrate, distribution of migratory whitefly from a point source, and the ability of whiteflies to reproduce in relation to distance of migration from a point source. Studies on host adaptation show that host source can affect the quality of whitefly populations. These studies are in their infancy, and extensive studies are needed to define areawide cropping systems, impact of diverse ecosystems, and different environmental parameters.


Knowledge Gaps
  1. Community and areawide management systems implementation protocols.
  2. Understanding of mechanisms involved in irrigation and fertilization crop production inputs on whitefly population dynamics. Lack of information on relationships of other crop production and cultural practices to whitefly population dynamics.
  3. Guidelines for multicrop planting sequences in relation to whitefly population development.
  4. Information on efficiency of mulches, row covers, and exclusion techniques on crop production and whitefly populations.
  5. Improve sampling methodology for decision making in areawide management programs.
  6. Mechanisms to guide the transition from local, farm-by-farm focus to community and areawide cooperative efforts.
 
 

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