Aquatic mesocosm assessments of a neem (azadirachtin) insecticide at environmentally realistic concentrations - 2: Zooplankton community responses and recovery

A neem-based insecticide, Neemix 4.5, was applied to forest pond enclosures at environmentally realistic concentrations (i.e., below the worst-case expected environmental concentration of 35μgL^-1). Crustacean zooplankton communities were examined by multivariate ordination (nonmetric multidimensional scaling) and time-course analyses of population trends among indicator taxa over two field seasons to determine application effects on community structure and recovery patterns. Significant effects on zooplankton community structure were detected at all main test concentrations (n=5) of 28, 17, and 10μgL^-1 azadirachtin. There was also evidence of adverse effects on zooplankton communities at an auxiliary test concentration (n=2) of 5μgL^-1 azadirachtin. Community-level effects resulted primarily from reductions in adult copepods with short-term, reciprocal increases in cladocerans. Copepod nauplii were not significantly affected. Response patterns suggested that the reductions in adult copepods resulted from growth-regulating effects of the active ingredient azadirachtin, or other neem compounds, and not from formulation ingredients. There was no evidence of recovery among adult copepods within the season of application. At the beginning of the second sampling season, there were apparent carryover effects similar to the community responses in the previous year. By the end of the second season, there was evidence indicating recovery of community structure at the two lower test concentrations of 10 and 17μgL^-1, but not at 28μgL ^-1. The selective toxicity to adult copepods is problematic in that this group has a relatively long life cycle (1 year), contributes a major component of zooplankton biomass and respiration, and occupies critical functional guilds within zooplankton food web structures. Mitigation measures such as reductions in application rates and efforts to avoid deposition of sprayed materials on water bodies will be required to reduce the risk of harmful effects on zooplankton communities of forest ponds and other shallow, standing-water bodies.