SEASONALITY OF ORMIA-DEPLETA AND LIMITS TO ITS SPREAD

Ormia depleta (Wiedemann) (Diptera: Tachinidae), a South American fly that homes on the calling songs of its hosts, was brought from Piracic aba, Brazil; colonized; and released in Florida for the biological con trol of Scapteriscus spp. mole crickets. It became established at some release sites and rapidly spread through most of peninsular Florida. The seasonal distributions of phonotactic females in Florida differed from the single summer peak recorded at Rio Claro and Piracicaba, Braz il (23 degrees S). Near Bradenton (27 degrees N), spring and fall peak s with a summer hiatus were quickly established and remain evident. At two sites near Gainesville (30 degrees N), a strong fall peak and a m odest-to-none spring peak developed. Peaks of phonotactic females seem to follow times of peak host availability and sometimes coincide with minima of host availability. Limits to geographical expansion of O. d epleta are poorly understood. At Gainesville, the site of earliest int roduction, establishment was rapid and sound-trap catches increased fo r the first 3 years, exceeding 1000 per year at one site. Catches then declined for 3 years, almost reaching zero in 1994. On the other hand , annual sound-trap catches of O. depleta at Bradenton exceeded 1000 i n <2 years and have remained above that level for 5 years. Winters in Gainesville are more severe than those in Bradenton, but minimum winte r temperatures in Gainesville were substantially lower during O. deple ta's increase than during its decline. Perhaps mild winters cause the flies to become active early-only to starve for lack of adequate winte r nectar sources. O. depleta may yet spread to the northern limits of its Scapteriscus hosts as present populations adapt to new environment s. Or farther spread may require that new strains be introduced from m ore southerly sites in South America. (C) 1996 Academic Press, Inc.