ABSTRACT
The potential for effects of lambda-cyhalothrin on aquatic organisms under field conditions was studied using 16 mesocosms, each 450 m3. Sets of four replicates were treated with three rates of the pyrethroid insecticide, by multiple applications of simulated spray-drift (12×; weekly) and run-off (6×; bi-weekly); commencing June 1986. The mid rate mesocosms were dosed on each occasion for spray-drift at 0.38% of the lambda-cyhalothrin current maximum field rate for USA cotton and for run-off (with a soil-water slurry) at 1.1% of the field rate. The high and low rates were 10× and 0.1× the mid-rate, respectively. Four control mesocosms were included.
Residues of lambda-cyhalothrin in the water column declined rapidly after each application, with less than 20% of the applied dose remaining after 3 days. The physicochemical characteristics of 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 was also impacted by the 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 had increased, from the 25 adults introduced, to approximately 20,000 fish per mesocosm. Numbers of young fish were greatest and biomass lowest at the three pyrethroid treatment rates, although there was no dose-related response. Field and laboratory data show that this was probably not due to lambda-cyhalothrin.
It was concluded that the agricultural use of lambda-cyhalothrin is unlikely to cause adverse effects on overall populations or productivity of aquatic ecosystems; although some minor or transient effects on invertebrates may occasionally occur.
