FIELD-TESTS OF STERILE SCREWWORM FLIES, COCHLIOMYIA-HOMINIVORAX (DIPTERA, CALLIPHORIDAE), AGAINST NATURAL-POPULATIONS IN 3 COASTAL AREAS OF MEXICO

The dynamics of screwworm (Cochliomyia hominivorax) populations were investigated from May-October 1974 in three 5180 km2 plots in Mexico. Two plots were in the barrier zone of semiarid Tamaulipas. The 3rd plot was well to the south of the barrier zone, in equable, humid northern Veracruz. Responses of native screwworm populations to sterile male challenge were compared. THe sterile fly treatments were a uniform 386 flies/km2/wk. The then currently mass-produced strain, Tex Mex, was applied to the northernmost and the Veracruz test plots. A newly synthesized strain, FF8, was applied to the southern Tamaulipas testing site. The number and fertility of screwworm ovipositions, collected daily from the artificially inflicted wounds of sentinel sheep, provided estimates of population magnitudes and frequencies of sterile matings. Maximum screwworm populations were observed when conditions were moist, minimum populations during periods of drought. High variance-mean ratios and rapid onset and decline in ovipositions per sampling site demonstrated that screwworm populations were highly aggregated and transient. Spatial and temporal heterogeneity was most pronounced in the barrier zone, suggesting that populations were unstable. Veracruz populations were relatively homogeneous and stable. Factors governing screwworm population magnitude and dynamics are discussed. Frequencies of sterile mating were less among the previously unchallenged screwworm populations of Veracruz than among populations in the barrier zone. These data do not support the hypothesis that assortative mating has evolved in the barrier zone. No significant difference in the frequencies of sterile mating was observed between native populations challenged with the newly synthesized FF8 strain or the older, standard production strain, Tex Mex.