Impact of Altosid on Selected Members of an Aquatic Ecosystem

Repeated treatments of the insect growth regulator (IGR), Altosid EC Sub(4) (isopropyl 11-methoxy-3, 7, 11-trimethyl-2, 4- dodecadienoate), at the rate of 0.1 ppm (0.27 lb/acre) to experimental ponds reduced abundance of several arthropod prey and predator spp. Because of the size differential among affected spp, the altered community biomass reflected the impact of Altosid more profoundly than abundance. Pupae of the mosquito, Culiseta inornata, were devastated by the IGR, representing large potential losses of biomass during the height of the mosquito breeding season. Chironomid larvae were reduced 2-fold by the treatment, and their importance to the biomass increased with rising spring water temp. Preliminary lab and pond assays determined that Altosid induced mortality in early and late instars of the mayfly, Callibaetis pacificus. During colder winter months mayflies were eliminated from ponds under repeated treatment but populations in check ponds remained low and contribution to total biomass was slight. Rising water temps reduced IGR impact on the mayfly population. The ostracod, Cyprinotus sp, was a major prey component and was not affected by treatment. A major predator, the larval dytiscid beetle, Laccophilus sp, was eliminated from the treated ponds. This loss represented 84% of the predator biomass during 1 period. Odonata naiads comprised the 2nd major group of predators throughout the study. These naiads preyed heavily on mosquitoes and ostracods and were not affected by the IGR.