The potential for effects of lambda-cyhalothrin on aquatic organisms under field conditions was studied using 16 mesocosms, each being a pond 15 × 30 m with a maximum depth of 2 m. Sets of four replicate mesocosms were treated with three rates of the pyrethroid insecticide and control. The product was applied to the total water surface as multiple applications of simulated spray drift (12 ×; weekly) and runoff (6 ×; biweekly), having commenced June 1986. The middle of the three treatment rates was designed to represent the typical maximum estimated entry rate (MEER) into natural aquatic environments, and each mesocosm was dosed for spray drift at 0.5% of the field rate and for runoff at 1.5% of the field rate. The high and low rates were 10 × and 0.1 × the middle rate, respectively.

A wide range of parameters was studied, pre-, during, and postapplication, over a 24-week period. Residues in water and hydrosoil were determined and physicochemical characteristics monitored. The biology of the various trophic levels was studied: microbes, algae, macrophytes, zooplankton, macroinvertebrates, and fish.

Residues of lambda-cyhalothrin in the water column declined rapidly after applications, with less than 20% of the applied dose remaining after 3 days. Physicochemical characteristics of the system, such as temperature, dissolved oxygen, pH, conductivity, turbidity, and alkalinity, were not altered by the test chemical. Neither were there significant effects at any of the treatment rates on hydrosoil microbes, phytoplankton, periphyton, filamentous algae, and macrophytes.

Zooplankton populations were only affected at the highest rate, with a reduction in copepod numbers. A substantial proportion of the macroinvertebrate families were also impacted by 404the high rate of lambda-cyhalothrin. Some changes in macroinvertebrate populations occurred at the middle rate, but there was a negligible effect at the low rate.

By the end of the study period, bluegill sunfish numbers had increased and biomass reduced at all three treatment rates, with no dose-related response. However, the data shows this was probably not due to lambda-cyhalothrin, but possibly the result of fish overcrowding and competition with tadpoles for food or habitat.

The mesocosm test is a severe test of exposure, and thus may substantially overestimate typical field risk. It is concluded that the study shows that lambda-cyhalothrin is unlikely to cause adverse effects on overall populations or productivity of aquatic ecosystems, although some minor or transient effects may occasionally occur.