Impact of the insect growth regulator diflubenzuron on invertebrates in a residential-recreational lake.

The effects of the insect growth regulator (IGR) diflubenzuron [1-(4-chlorophenyl)-3(2,6-difluorobenzoyl)-urea] on planktonic, nektonic, and benthic invertebrates in a residential-recreational lake were studied during the period of April to October 1977. A 25% WP formulation of the IGR was applied at 156 g AI/ha (0.012 ppm) to the entire lake surface, once on April 26, and again on August 24. The April treatment severely affected populations of the cladoceransDaphnia laevis Birge,Ceriodaphnia sp.,Bosmina longirostris (O. F. Muller), and the copepodsCyclops spp. andDiaptomus sp., completely eliminating them within one week posttreatment.Daphnia andCeriodaphnia did not recover even after 6 months of treatment butB. longirostris reappeared 11 weeks posttreatment.Diaptomus was depleted for four months, whileCyclops recovered within six to seven weeks after treatment. The amphipodHyalella azteca (Saussure) was completely eliminated within four weeks remaining absent for more than six months after the treatment. The prolonged absence of these nontarget organisms following treatment is probably due to the seasonal cycle of the species involved and the inability of some of the taxa to rapidly recolonize the treated habitat. Nymphs of the mayfly,Caenis sp., were reduced 99%, recovering in six weeks posttreatment. The IGR had no adverse effects on oligochaetes (Naididae and Tubificidae), Hydrachnellid mites, the gastropodPhysa sp., and the ostracodCypridopsis sp. The second application in August caused complete elimination ofCyclops andB. longirostris within one to two weeks, the former recovering in four weeks and the latter in eight to nine weeks after the treatment.Caenis, also was adversely affected, recoveringin four to five weeks posttreatment. No significant reduction in Cypridopsid, Hydrachnellid, Physid, or oligochaete populations was noted.