Responses of Screwworm (Diptera: Calliphoridae) Populations to Sterile Male Challenge in Veracruz

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Abstract
An effort was made to test the hypothesis that increasing the probability of placing sterile screwworm (Cochliomyia hominivorax) males into actual or potential breeding arenas will increase sterile mating frequencies among the target populations. Sterile fly releases were made in a 5180-km2 area of northern Veracruz, Mexico, during seasonally comparable 11-wk periods of 1974 and 1975. Sterile fly “doses” aerially released were ca 386/km2/wk but the release indices themselves were varied. There was a mean distance between flight lanes of 4 km in 1974 and 3.2 km in 1975. The number of flies per release carton was ca 1000 in 1974 and ca 333 in 1975. The average number of release units per unit area was 0.39/km2 in 1974 and 1.16/km2 in 1975, a 3-fold increase. The native screwworm population densities and frequencies of sterile mating were estimated by the recovery of ovipositions from artificially inflicted wounds on sentinel sheep. Average frequencies of sterile matings were 10% in 1974 and 23.3% in 1975, the difference being highly significant. The magnitudes of the screwworm populations, however, were approximately the same between years and therefore do not explain the disparity in sterile mating frequencies. Liver-baited trap captures of mostly released females were significantly greater in 1975 than in 1974, evidence that sterile fly placement was more effective in 1975. All the foregoing data are consistent with the hypothesis that increasing the probability of sterile male placement into mating locations (instead of depending upon males to arrive there by their own efforts) results in greater sterile mating frequencies among the target populations.