FECUNDITY AND EGG VIABILITY OF NORTHERN AND WESTERN CORN-ROOTWORM (COLEOPTERA, CHRYSOMELIDAE) BEETLES SURVIVING LABELED AND REDUCED SOIL INSECTICIDE APPLICATIONS

The influence of soil insecticide application rates on the reproductiv e biologies of held-collected northern and western corn rootworms, Dia brotica barberi Smith & Lawrence, and D. virgifera virgifera LeConte, respectively, was investigated under laboratory conditions. Comparison s were made on beetles obtained from plots treated with planting-time applications of labeled (1X) and reduced (0.5 and 0.75X) rates of chlo rethoxyfos, tefluthrin, and terbufos, and from untreated control plots . Captured insects were pooled by species and insecticide treatment in to laboratory rearing cages and allowed to mate. Gravid females were h eld individually in oviposition chambers to assess fecundity. Egg viab ility was measured on subsamples of eggs from all treatments. No signi ficant differences (P > 0.05) were detected in fecundity, fertility, o r percentage of nonviable eggs of D. barberi or D. virgifera with any insecticide or application rate within insecticide tested. However, re ductions in total eggs produced and hatch were 31.2 and 53.7%, respect ively, for D. virgifera that survived chlorethoxyfos treatment in comp arison with beetles that emerged from untreated plots. Also, the perce ntage of nonviable eggs produced by D. virgifera emerging from chloret hoxyfos-treated plots was 2-fold of that in beetles captured from untr eated corn plots. Whereas, D. barberi females from tefluthrin-treated plots experienced a 44% increase in fecundity and 49.2% more egg hatch than those from the untreated controls. Additionally, total eggs and hatch were increased by 32.1 and 33.7%, respectively, in D. barberi th at emerged from terbufos-treated corn plots when compared with their c ounterparts from untreated control plots. The numerical disparities th at we observed appeared to be species specific rather than associated with insecticide class or application rate. Our evaluations indicate t hat using reduced (0.5 and 0.75X) application rates of these organopho sphate and pyrethroid insecticides will not likely have major effects on the reproductive capacities of D. barberi or D. virgifera. Thus, co rn rootworm management programs should include options for using the l owest efficacious application rates of these insecticides.