1a. Objectives (from AD-416)
1. Prevent invasions of chilli thrips and whiteflies by developing genetic tools to track/predict/monitor and ultimately aid in controlling movement from likely sources of infestation and suppress established populations with biologically-based and environmentally-sound techniques. 1a. Determine the global sources of invasive chilli thrips, Scirtothrips dorsalis and describe the global genetic diversity of this pest species so that effective international and national barriers can be erected to further introductions. 1b. Investigate biological control and ecological interactions of chilli thrips and whiteflies with their natural enemies in order to promote their environmentally sound control in vegetable and ornamental crops 2. Investigate structural, physiological, molecular and chemical aspects of the whitefly feeding process and identify inhibitor strategies/molecules that can be used in the development of novel interdiction strategies envisioned to work either through production of transgenic plants or application of chemical treatments that block feeding. 2a. Characterize the method of polymerization that occurs when whitefly saliva is secreted and solidifies during insect probing for the plant vascular tissue. 2b. Determine the structural composition of salivary sheaths and the source of the precursors. 2c. Identify small molecule inhibitors of the whitefly feeding process using the artificial diet system as a bioassay.
1b. Approach (from AD-416)
Research will focus on establishing a DNA barcoding reference data set for S. dorsalis that will allow construction of a robust global molecular phylogeny for this species using both mitochondrial cytochrome oxidase I (mtCOI) and nuclear genes to be identified. A commercially available predatory mite (Amblyseius swirskii) will be evaluated for sustainable control of chilli thrips, whiteflies and other key pests in ornamental and vegetable crops using banker plants and ‘predator-in-first’ tactics. Molecular and biochemical methods will be used to develop a model describing whitefly salivary sheath formation and to identify inhibitors of this process that can be evaluated for use in control strategies.
3. Progress Report
Morphometric traits used for taxonomy of chilli thrips from five different geographical regions including Florida were compared. No differences were found for 14 morphological characters between Israel, India, St. Vincent, and Florida; however, these populations were different from Japan which agrees with our deoxyribonucleic acid (DNA) barcoding results suggesting that chilli thrips has distinct types and research is required to understand the biological significance of this variation. Further DNA barcoding research showed that typical sequences used for species verification of many eukaryotes will not work for chilli thrips. Intragenomic variation (variation within the genomes of different cells from the same individual) exists with the commonly used intragenic ribonucleic acid (RNA) gene sequence that is greater than what could be observed between different species. Traditional taxonomy using scanning electron microscopy was coupled with molecular techniques for deoxyribonucleic acid (DNA) barcoding to develop a novel diagnostic method for chilli thrips identification where a single specimen could be first used for identification and then analyzed molecularly. In the past, thrips had to be slide mounted for morphological identification and were destroyed or rendered unusable for molecular analysis. Predatory mite reproduction, development, adult longevity and survival (egg to adult) on leaf discs of ornamental pepper varieties were compared to two vegetables (tomato, green bean). The mite had similar reproduction, immature development time, adult longevity and survival rate on the ornamental peppers but laid few eggs and did not survive on either vegetable host with adult longevity similar to the filter paper control. Population dynamics of the predatory mite on ornamental peppers and two vegetables was evaluated in the greenhouse over time. Mites thrived on three varieties of ornamental pepper plants for long periods (60 days or more) without prey while mites died on both vegetables after 10 days. Population peaks coincided with blooming periods and mites preferred the top canopy of the plant possibly due to more leaf domatia and flowering activity. Predation studies using ornamental pepper as banker plants for dispersal of the predatory mite against whiteflies and thrips infested green bean demonstrated the mite’s ability to survive, reproduce, and provide excellent control on prey infested green bean. Predatory mites provided 94 and 99% control of thrips and whitefly, respectively. This banker plant system has the potential to control multiple pest species in commercial vegetable greenhouses. The feeding process of the whitefly requires the synthesis of a tube through which the piercing mouthparts probe into the host plant tissue while the insect searches for phloem cells on which to feed. This tube is called a salivary sheath and we show that it can be produced solely from secretions from the stylets of the insect and does not require plant components. We have also developed an artificial chamber in which these sheaths can be harvested and used these chambers to identify two inhibitors of salivary sheath formation.
1. Papaya banker plant system for biological control of whitefly has broad application in protected horticultural crops. ARS entomologist at Fort Pierce, Florida collaborated with the University of Florida to develop a novel banker plant system for the management of Bemisia tabaci, a whitefly pest and virus vector of vegetable and ornamental crops worldwide. To combat this pest, papaya was used as a non-crop banker plant particularly preferred by the papaya whitefly, Trialeurodes variabilis, which acts as an alternative host for rearing and dispersal of the parasitoid wasp, Encarsia. sophia that is also a natural enemy of the target pest, B. tabaci. By introducing papaya banker plants loaded with wasps into the greenhouse before any pest whiteflies are detected, the wasps act as sentries and attack any whiteflies that might become established in the tomato crop. Whiteflies can be biologically controlled with this system resulting in successful greenhouse tomato production without the use of pesticides. This system has broad application in protected horticultural crops and has also been used successfully in commercial herb, cucumber, eggplant, lettuce and poinsettia greenhouses in Florida.
2. Intragenomic DNA sequence variation invalidates standard phylogenetic markers. Precise taxonomic identification of invasive species such as the chilli thrips is an important part of developing monitoring and control methods to minimize damage to U.S. agriculture. ARS Researchers at Fort Pierce, Florida discovered greater than 10% intragenomic variation among the cells of a single organism in a genetic sequence commonly used for taxonomic genotype barcoding in chilli thrips. This calls into question previously published methods used to provide genotype distinctions in this highly damaging polyphagous plant pest. These findings have significant impact in regulatory monitoring and development of control strategies for this pest. They also indicate the need for new methods of genotyping this thrips and possibly other thrips pest species.
3. Identification of salivary sheath formation inhibitors opens the door for new insect control strategies. Hempiteran piercing-sucking insects are major limiters of food and fiber production worldwide and current control practices that work are a serious economic drain on crop production and in some cases, there is no practical control method. One behavior that these insects that feed on plant vascular tissue share is the synthesis of a secreted salivary sheath that forms a tube through which their stylets move as the insect probes for the plant sap containg cells. ARS researchers in Fort Pierce, Florida have initiated a research project targeting this feeding process as a point of interdiction that would have no collateral damaging effects on beneficial insects. As a result they have characterization salivary sheath composition and the mechanism of synthesis. This work has led to the identification of inhibitors of Bemisia tabaci salivary sheath biosynthesis in artificial diet systems. These finding are being adapted to determine if transgenic plants can be produced that contain such inhibitors in their extracellular spaces, thus preventing successful B. tabaci feeding.
4. Insect mini-vac saves researchers time, increases efficiency in the laboratory and field. A modified mini-aspirator, powered with a vacuum pump and fitted with a specially adapted (removable) collecting vial to reduce the handling time was developed and tested on chilli thrips, a predatory mite and minute pirate bugs. The use of the mini-aspirator for moving and infesting both pest and predators resulted in up to a 15-fold time savings. Additionally, the mini-aspirator can be fitted with a battery powered mini-vac, making it portable for field applications, such as pesticide resistance screening of field populations.