The effects of phorate, an organophosphorus insecticide, on aquatic ma
croinvertebrates was studied in littoral mesocosms in South Dakota wet
lands. In 1991 and 1992, four mesocosms were constructed in each of th
ree wetlands. In each wetland, one mesocosm was a reference and phorat
e concentrations of 1.2, 2.4, and 4.8 kg/ha were applied to treatment
mesocosms. Phorate caused mortality to amphipods and chironomids throu
ghout the study. Leeches and snails were resistant to phorate. Few liv
ing macroinvertebrates were present in mesocosms after phorate treatme
nt and populations recovered only slightly after 1 month. Macroinverte
brate taxa that were sensitive to phorate included: Odonata, Hemiptera
, Culicidae, Heliidae, Ephemeroptera, Acarina, Coleoptera, Stratiomyid
ae, and Hydracarina. Taxa that were tolerant to phorate included: Hiru
dinea, Gastropoda, Oligochaeta, and Ostracoda.