Non-target effects of methoprene and larvicidal surface films on invertebrate predators of mosquito larvae

Abstract

Mosquito larvicides are used across a variety of aquatic habitats, although when applied they likely affect other aquatic organisms. The removal or impairment of top insect predators via larvicides could be beneficial to mosquitoes by allowing their populations to rebound once pesticide levels dissipate. Our goal was to determine if two larvicide types, growth regulators (IGRs) and surface films (SFs), harm non-target aquatic insect communities, and if these chemicals influence the ability of predatory aquatic insects to regulate mosquitoes. We surveyed aquatic sites before and after IGR and SF-application, then compared changes in insect community structure. Evenness was lower in SF treated habitats, and when analyzing prey/ controphic taxa only, evenness and diversity changed in untreated reference areas, suggesting that differences measured were due to other environmental factors, not larvicide presence. A field experiment was then conducted by exposing specific predatory aquatic insects to varying doses of IGRs and SFs and then placing them in mesocosms containing mosquito larvae. Surface films were directly lethal to adult dytiscids at recommended and high concentrations. Although we found no significant differences in mosquito emergence among all treatment levels, there was a trend of negative controls (no predator mesocosms) and SF-treated predators allowing the most mosquitoes to emerge compared to positive controls (predators not exposed to larvicides) and IGRtreated predators. Thus, these larvicides may have minimal effects on mosquito larvae predators, but the direct effects of surface films on insects that interact with the water's surface require further investigation.