Rapid response research by USDA-ARS for the ongoing New World screwworm outbreak

Authors: Arp, Alex

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 7/7/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: The sterile insect technique (SIT) has been crucial in eradicating screwworm from North and Central America. By regularly releasing sterile insects along the Panama-Colombia border, reinfestation has been prevented since 2006. However, screwworm cases resurfaced in Panama in 2022 and are now spreading into Nicaragua. The screwworm flies used in sterile releases have met or exceeded quality control parameters during this outbreak, so the cause for the outbreak is unknown. To investigate, we compared wild screwworms from the outbreak with the flies produced by the mass-rearing plant and used in sterile releases. We tested their quality and developed new methods to measure their fitness and ability to mate. We found that while the typical quality checks didn't show big differences, mating tests revealed significant variations. Wild screwworms were much better at mating throughout the day compared to the ones we produce. These findings show that we need new ways to measure the effectiveness of future screwworm strains we're developing for mass-rearing. Further mating evaluation of the newly developed strain and the previous production strain did not show significant differences. Flies were also evaluated for flight capability using flight mills, revealing improved flight capabilities of the new production strain.

Technical Abstract: The development and first application of the sterile insect (SIT) technique was instrumental at eliminating screwworm, Cochliomyia hominivorax, from North and Central America. A permanent control zone was established along the Panama-Colombia border in 2006, with constant sterile releases preventing incursion since its implementation. However, screwworm cases in Panama began increasing in 2022 and have since spread west into Nicaragua. The direct cause of the increase in cases is unknown as mass-reared screwworm flies consistently met or exceeded quality control parameters. To evaluate possible causes for the outbreak, wild lines sourced from outbreak cases were reared in parallel with the production strain and quality parameters were evaluated. Behavior differences were observed with updated mating competition assays and the adaptation of flight mills and locomotor activity monitors to screwworm. Few significant differences were observed in quality control metrics but the wild line, Yaviza, had significantly larger wings. Competitive mating assays revealed that the wild line mated nearly twice as many females as the production strain and did not perform differently throughout the day. Both lines of males mated more in early morning and decreased activity through the afternoon. Flight mills were constructed to evaluated fine scale differences in flight ability of screwworm lines. Current production strain PanCR-24 flew significantly farther than the previously used strain.