AGRICULTURAL PRACTICES AFFECT BOTFLY (CUTEREBRIDAE) PREVALENCE IN GRAYTAILED VOLES (MICROTUS-CANICAUDUS)

We examined the effects of an organophosphorous insecticide (azinphos- methyl) and mowing of alfalfa on larval botfly (Cuterebridae) prevalen ce in gray-tailed voles (Microtus canicaudus) in enclosed populations. Twelve of 23 enclosures were mowed, and three concentrations of an in secticide (Guthion 2S) were applied three weeks after mowing. We captu red 3,529 voles between 17 July and 27 August 1993, and 88 had at leas t one botfly parasite (overall prevalence = 2.5%). We found the preval ence of botflies in male voles (3.25%) to be significantly greater tha n in females (1.82%; X(2) = 6.06, P < 0.015). The interaction of mowin g and insecticide application was a significant factor in determining botfly prevalence (X? = 5.88, P < 0.055); prevalence of botfly infecti on was lower in mowed than in unmowed enclosures except at the highest insecticide concentration, which reduced the number of infections in both mowed and unmowed enclosures. We suggest the reduction in botfly prevalence resulted from the direct effect of both mowing and insectic ide application on adult botflies and botfly eggs, whereas differences in prevalence between genders was related to the voles' activity patt erns.