D. Kreutzweiser, T. Sutton, R. Back, K. Pangle and D. Thompson
SERG Project # 2001/02
1. The applications of Neemix 4.5 to forest pond enclosures resulted in mean initial concentrations of total azadirachtin of 28, 17 and 10 µg L-1 (ppb) among main test concentrations (n=5), and 59 and 5 µg L-1 in auxiliary enclosures (n=2).
2. Following Neemix 4.5 applications, azadirachtin concentrations in water were moderately persistent. Fifty percent dissipation of total azadirachtin occurred in about 30 d, with 90% dissipation estimated at about 66 days. The AZA-B isomer of azadirachtin was slightly more persistent in water than the AZA-A isomer.
3. There were significant concentration-dependent declines in the abundance of adult copepods at all main test concentrations, including 10 µg L-1, which is about 1/3 the regulatory expected environmental concentration of 35 µg L-1, assuming complete deposit of a 50 g ha-1 application. There was also evidence of a decline in the abundance of adult copepods at the 5 µg L-1 auxiliary test concentration.
4. There was no evidence of recovery among adult copepods within the sampling season. Sampling into the following year will be continued to determine rates and processes of recovery.
5. There were no significant adverse effects on juvenile copepods or cladocerans at any test concentration. The persistence of juvenile copepods through the exposure period may facilitate recovery of copepod populations by the following year. Results suggested slight increases in cladocerans of treated enclosures that may have been responses to release from competition with herbivorous copepods. This was demonstrated in a previous enclosure study with Neemix 4.5.
6. The significant adverse effects on adult copepods at all test concentrations were sufficient to induce measurable disturbances to community and ecosystem characteristics and functions. The selective toxicity to adult copepods at these test concentrations resulted in delays in community recovery (at least to the end of the season), reduced total plankton biomass, declines in community respiration, and disruptions to crustacean zooplankton food web stability.
7. The results of the study indicate that crustacean zooplankton communities of small, standing water bodies are at risk of harmful effects from applications of Neemix 4.5. The study found significant adverse effects at 10 µg L-1 azadirachtin, and indications of adverse effects at 5 µg L-1. Under current regulatory guidelines, the expected environmental concentration based on a 50 g ha-1 application rate is about 35 µg L-1 azadirachtin. Reductions in application rates and efforts to avoid contamination of productive standing water are likely to reduce the risk to zooplankton communities of forest ponds and other shallow water bodies