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United States Department of Agriculture

Agricultural Research Service


Location: Screwworm Research

2010 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
During FY10, the first year of project 6205-32000-35-00D as the replacement and continuation of project 6205-32000-030-00D in the National Program 104, progress was made in the three relevant objectives and their sub-objectives. For Objective 1, we have identified islands in Lake Gatun with similar habitats but located at different distances from the mainland; in Lake Bayano, we have identified islands with different habitats but located at similar distances from the mainland. Both lakes are in protected areas of Panama. We are developing protocols and schedules with our collaborators and negotiating with Panamanian Government Officials for permission to gain access, and we foresee no problems. Also, we have established contact with scientists in Brazil to arrange for the collection of screwworm samples for use in genetic analysis to determine if there are genetic differences in screwworms separated by geographic barriers. In Objective 2, the Sub-objective 2a was modified since horn flies were not amenable to injection of DNA and, thus, could not serve as model organisms for developing techniques in screwworms. The cDNA library of screwworms has been completed. Further components of the library may be investigated for utility in the project. Techniques for injection of DNA to eggs of the screwworm have been developed and are being optimized. Also, appropriate levels of the antibiotic tetracycline in the rearing media of screwworms have been determined as part of Sub-objective 2b. Techniques have been developed to capture volatiles emitted from bacteria that had been collected from screwworm-infested wounds and artificially propagated as the initial step to fulfilling Sub-objective 3a. Permits have been issued to move bacteria and appropriate organs of screwworms to the ARS Unit in Lincoln, NE, where the work required for developing bioassays will be accomplished in collaboration with the ARS Unit in Lincoln, as the necessary facilities do not exist in Panama. As progress continues in Sub-objective 3a, work on Sub-objective 3b and Objective 4 will be initiated.

4. Accomplishments
1. A new strategy for releasing sterile screwworms in the barrier zone. Eradication of screwworms from mainland North America using the sterile insect technique has been successful. Release rates for barrier maintenance in the Darien Province of Panama will differ from those necessary for a full eradication program. ARS researchers of the Screwworm Research Unit in Kerrville, Texas, and Panama reviewed and analyzed release technologies to improve screwworm fly dispersal relative to where, when, and how many sterile flies are released in the barrier zone. Recommendations with supporting data presented to the Panama – U.S. Commission for Eradication of Screwworm (COPEG) would (1) result in updated equipment on-board dispersal aircraft, (2) strategically reduce the number of flies dispersed, and (3) save up to $1,000,000 annually. Recommended improvements to the navigational software and equipment are currently being implemented by USDA-APHIS-IS and COPEG, the eradication action agencies.

2. An alternative for spray-dried blood for mass rearing screwworms. Successful eradication of screwworms from North America necessitates that a barrier against reinvasion of this horrific pest be maintained in the Darien Province of Panama. Mass rearing screwworms for sterilization and release at the barrier is a very expensive but necessary undertaking. Often there is a threat to disruption of mass rearing due to inadequate ingredients, particularly the spray-dried blood. Research was completed showing that defibrinated (mechanically removed 'blood clots'), fresh bovine blood-based diets were equivalent to the current standard, spray-dried blood-based diet. We also learned that the use of sodium citrate, commonly used as an anticoagulant was detrimental to developing screwworms (this could be important not only in fresh blood but also spray-dried blood). Therefore, defibrinated, fresh bovine blood may serve as a substitute for spray-dried blood if needed. This provides an alternative for a critical component of the diet of screwworms and is valuable information for the mass rearing facility of the Panama-U.S. Commission for Eradication of Screwworm.

Review Publications
Angel-Sahagun, C.A., Lezama-Gutierrez, R., Molina-Ochoa, J., Pescador-Rubio, A., Skoda, S.R., Cruz-Vazquez, C., Lorenzoni, A.G., Galindo-Velasco, E., Fragoso-Sanchez, H., Foster, J.E. 2010. Virulence of Mexican isolates of entomopathogenic fungi (Hypocreales: Clavicipitaceae) upon Rhipicephalus-Boophilus microplus (Acari: Ixodidae)larvae and the efficacy of conidia formulations to reduce larval tick density under field conditions. Veterinary Parasitology. 170(3-4):278-86.

Chaudhury, M.F., Skoda, S.R., Sagel, A., Welch, J.B. 2010. Volatiles emitted from eight wound-isolated bacteria differentially attract and stimulate gravid screwworm flies (Diptera: Calliphoridae) to oviposit. Journal of Medical Entomology. 47(3):349-354.

Chaudhury, M.F. 2009. Insect diet, feeding and nutrition. In: Schneider, J.C., editor. Principles and Procedures for Rearing High Quality Insects. Mississippi State, MS: Mississippi State University. p. 121-143.

Last Modified: 06/21/2017
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