With the recent decline of honey bees, Apis mellifera (L.) (Hymenoptera: Apidae), there is a need for alternative or supplemental crop pollinators, such as Osmia cornifrons (Radoszkowski) (Hymenoptera: Megachilidae). However, O. cornifrons propagation could be impeded by a cleptoparasitic mite, Chaetodactylus krombeini Baker. We investigated the effects of formic acid and wintergreen oil on mortality of C. krombeini hypopi and O. cornifrons adults by determining the lethal concentration of each compound on each species. On average, >4.8 and >1.8 h were required to cause mortality in O. cornifrons adults when <2,473.5 ppm of formic acid and wintergreen oil was applied as a fumigant, respectively. When the two chemicals were directly applied to the exoskeleton of O. cornifrons adults, 353.4 ppm of wintergreen oil caused bee mortality within 10 min; however, no mortality was found with any formic acid application attempted. Mortality of C. krombeini hypopi occurred 5 and 10 min after application of >176.7 ppm of formic acid and wintergreen oil, respectively. Estimates of LC50 for C. krombeini hypopi treated with formic acid and wintergreen oil were 54.3 and 271.3 ppm, respectively. This study showed that C. krombeini could be controlled effectively without inducing O. cornifrons adult mortality based on concentration and duration of fumigation.