Location: Screwworm Research2011 Annual Report
1a. Objectives (from AD-416)
Objective 1: Determine the ability of screwworm flies to cross major geographic barriers. Objective 2: Develop a strain of screwworm fly that produces only males. Sub-objective 2a: Develop a genetic sexing system in the horn fly, a species closely related to screwworm, as a model system to be applied to screwworms. Sub-objective 2b: Using information and techniques developed with horn flies, develop a genetic sexing, males-only strain of screwworms. Objective 3: Identify chemical oviposition attractants that can be used in attract-and-kill and for improved rearing. Sub-objective 3a: Identify the natural volatiles from bacteria-inoculated and incubated bovine blood that are responsible for attracting gravid screwworm flies to oviposit. Sub-objective 3b: Identify synthetic compounds with equivalent attraction to gravid screwworm flies for depositing eggs as natural volatiles from bacteria-inoculated and incubated bovine blood. Objective 4: Develop new surveillance methods based on oviposition attractants.
1b. Approach (from AD-416)
For our first objective we will identify four islands, in the Republic of Panama, where sterile screwworm flies marked with fluorescent powder will be released. Traps, placed on the mainland, will be used to detect if the screwworms crossed the water 'barrier'. The second objective, development of a male-only strain of screwworm, will be approached by an ARS collaborator first developing transgenic techniques in a related species, the horn fly. Once genetic transformation is accomplished in horn flies the techniques will be transferred to the screwworm. This two-step approach is necessary because 1) the screwworm is a quarantined pest and adequate facilities for its study are not present in the U.S., and 2) the collaborator has extensive genetic information/experience with horn flies that will directly transfer to screwworms. Volatiles from bacteria-infested wounds that previously were shown to attract female screwworms will be collected and identified with standard gas chromatograph and mass spectrometry to approach our third objective. Existing 'synthetic' chemicals that are equivalent to the 'natural' volatiles identified as most attractive will then be evaluated for attractiveness to female screwworms using standard bio-assay techniques. Our approach to the fourth objective will be a combination of evaluating the attractiveness of trap design (color, shape, and size) along with the use of volatiles, natural or synthetic as determined from objective 3, that are attractive to female screwworms under field conditions.
3. Progress Report
Lack of personnel, because of greater than expected work-load involved in Objectives 2 and 3, and difficulty in receiving permission to work in selected areas has lead to curtailment of work in Objective 1. Furthermore, after meetings with our main customer (COPEG), the NP-104 Program Leader, and the Unit, it has been determined that higher priority be given to work relative to our contingency of "Comparison of DNA 'fingerprints' of fertile fly samples". To that end, contact is being initiated with potential collaborators in Caribbean and South American locations to provide screwworm samples for genetic analyses. Substantial progress, accelerated from the originally projected milestones, has been achieved in Objective 2. Microinjection was successfully applied to screwworm eggs; electroporation was also successfully applied for 'injecting' DNA into screwworm eggs. To our knowledge this is the first time electroporation has been successful with higher flies; we are currently screening dozens of putative transgenic lines of screwworm. Four different genetic 'constructs' have been inserted into screwworm embryos; all putative lines are 'marked' with genes for fluorescent proteins. The most recent genetic 'constructs' were designed directly from analysis of a screwworm 'genetic library'. Objective 3 has progressed on schedule. Volatiles were identified from the blend of bacteria, as isolated from animal wounds, that was inoculated to fresh bovine blood; assays are underway with individual, synthesized chemicals and chemical blends. Volatiles were also identified from decomposing animal liver, which has been shown to be attractive to wild females in previous field trials, and successfully assayed with the secondary screwworm as the model insect; assays will next be done with screwworm. A trap design used with a species of blow fly similar to screwworm was assayed with screwworm to determine its usefulness as part of the work associated with Objective 4. Results were promising but modifications are necessary to improve effectiveness with screwworms; further trap designs will be evaluated in the upcoming year.
1. Ground releases of sterile screwworms successfully eradicate an infestation in Aruba. The small (about 70 square miles) island nation of Aruba, located in the Caribbean about 17 miles north of Venezuela, relies heavily on the tourism industry. After positive verification of a suspected infestation of screwworms (verified by a Unit scientist) and mindful of the potential negative impact on tourism, the Aruban Government requested assistance from the ARS Screwworm Research Unit (SRU, headquartered at the Knipling-Bushland U.S. Livestock Insects Research Laboratory in Kerrville, TX, with a satellite in Pacora, Panama) and the Panama – U.S. Commission for Eradication of Screwworms (COPEG, located in Pacora, Panama) to eradicate the screwworm infestation. Scientists of the SRU developed the strategy for eradication through the release of sterile screwworms from strategically located ground release chambers, COPEG provided the sterile screwworms, and Arubans implemented the successful program. Verification of the screwworm infection occurred in early March of 2011; four months later, cases of screwworms have been eliminated. The timely, efficient response by all parties has successfully eliminated screwworms from Aruba and protected the tourism industry, the Aruban human population, and the animal population from this insidious pest.
Pitti, A., Skoda, S.R., Kneeland, K.M., Berkebile, D.R., Molina-Ochoa, J., Chaudhury, M.F., Youm, O., Foster, J.E. 2011. Effect of adult screwworm male size on mating competence. Southwestern Entomologist. 36(1):47-60.