Location: Screwworm Research2013 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:
As reported in FY10, Objective 1 was eliminated and replaced with determining the genetic variation of screwworms. As part of a Specific Cooperative Agreement with the University of Nebraska the molecular technique random amplified polymorphic DNA – polymerase chain reaction (RAPD-PCR) was successfully used to identify screwworms from other flies that may be infesting wounds and showed promise for use to determine genetic variability in screwworms. The post-doctoral investigator working with our Unit, through the Specific Cooperative Agreement with North Carolina State University, has used transgenic techniques to develop one line of screwworms that expresses genetic sexing characteristics (nearly 100% males), which addresses Sub-objective 2b. Work is progressing in evaluation of several more putative transgenic lines. Once several lines are available, quality parameters will be evaluated so that the 'best' lines move forward in testing; the goal for Objective 2 is to deliver high quality genetic sexing lines to the Panama – U.S. Commission for Eradication of Screwworms. A 5-chemical synthetic blend of volatiles was developed, to address Sub-objective 3b, that was effective in attracting gravid females to oviposit. Work is progressing to determine the relative amount and the ratio of these chemicals to achieve maximum attraction and oviposition; this work addresses both Sub-objective 3b and Objective 4.
1. Improved diet for adult screwworms. Screwworms, economically devastating pests of livestock and a human health menace, were eradicated from North America using the sterile insect technique (SIT). For SIT, competitive screwworms are mass produced, sterilized, and released to the environment so that no fertile eggs are produced. Maintaining a barrier against the reintroduction of screwworms also relies on SIT. The large number of fertile adults required for mass production requires high quality yet economical food. A gelled diet was developed that, when compared to the previously used dry diet, showed no difference in the quantity of eggs, improved egg hatching, and wasted less diet. The gelled diet has been adopted by the Panama – U.S. Commission for Eradication and Prevention of Screwworms, will save about $3000 per year, and greatly improved the environmental conditions in the adult colony room of the sterile fly mass production facility.
Skoda, S.R., Figarola, J.L., Pornkulwat, S., Foster, J.E. 2013. Inter- and Intraspecific identification of the New World screwworm using random amplified polymorphic DNA-polymerase chain reaction. Journal of Insect Science. 13:76.