Effects of an Ultra-low-Volume Application of Etofenprox for Mosquito Management on Megachile rotundata (Hymenoptera: Megachilidae) Larvae and Adults in an Agricultural Setting

Abstract

The alfalfa leafcutting bee, Megachile rotundata F. (Hymenoptera: Megachilidae), is one of the most intensively managed solitary bees and greatly contributes to alfalfa production in both the United States and Canada. Although production of certain commodities, especially alfalfa seed, has become increasingly dependent on this species\' pollination proficiency, little information is known about how M. rotundata is affected by insecticide exposure. To better understand the risk posed to M. rotundata by the increasing use of insecticides to manage mosquitoes, we conducted field experiments that directly exposed M. rotundata nests, adults, and larvae to a pyrethroid insecticide via a ground-based ultra-low-volume (ULV) aerosol generator. We directly targeted nest shelters with Zenivex E20 (etofenprox) at a half-maximum rate of 0.0032 kg/ha at dusk and then observed larval mortality, adult mortality, and the total number of completed nests for both the treated and control groups. There was no significant difference in the proportion of dead (P = 0.99) and alive (P = 0.23) larvae when the control group was compared with the treated group. We also did not observe a significant difference in the number of emerged adults reared from the treated shelters (P = 0.22 and 0.50 for females and males, respectively), and the number of completed cells after exposure to the insecticides continued to increase throughout the summer, indicating that provisioning adults were not affected by the insecticide treatment. The results from this study suggest that the amount of insecticide reaching nest shelters may not be sufficient to cause significant mortality.