NATIONAL RESEARCH, ACTION, AND TECHNOLOGY TRANSFER PLAN, 1997-2001
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.
The specific objectives of the new research and technology transfer plan are to:
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.
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.
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.
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.
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.
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.