THE TOXICITY OF BRAN BAITS, FORMULATED WITH CARBARYL, CHLORPYRIFOS AND DIMETHOATE, ON YELLOW MEALWORMS (TENEBRIO-MOLITOR L)

Laboratory feeding trials were conducted to assess the comparative imp act of registered and new unregistered insecticide-treated bran baits on nontarget epigeal arthropods. Yellow mealworm beetles and larvae (T enebrio molitor, L.) (a surrogate, surface-dwelling tenebrionid) were individually housed in containers and exposed to five different dosage s (quantities) of bran bait treated with three different insecticides. Response to the baits (mortality in beetles and moribundity in larvae ) was greatest in the chlorpyrifos bait treatment, and progressively l ower in the dimethoate and carbaryl treatments. Mealworm beetles and l arvae showed a probit response to increasing dosages of carbaryl and d imethoate bran bait as evidenced by the sigmoidal shape of their morta lity curves (i.e., increasing dosages of bran bait corresponded to inc reased responses). The mortality curves for yellow mealworms exposed t o these same dosages of chlorpyrifos bait lacked the characteristic si gmoidal shape and were low-sloped. Feeding trials with chlorpyrifos ba it at low enough dosages to produce a sigmoid response were not conduc ted. It is postulated that field populations of tenebrionids could be negatively affected by the use of chlorpyrifos bait to control grassho pper populations.