Reduced Insecticide Rates and Strip-Harvesting Effects on Alfalfa Weevil (Coleoptera: Curculionidae) Larval Populations and Prevalence of Zoophthora phytonomi (Entomophthorales: Entomophthoraceae)

In 1994 and 1995, the effects of reduced insecticide rates and harvesting on Hypera postica (Gyllenhal) larval populations and prevalence of the fungal entomopathogen Zoophthora phytonomi Arthur were evaluated in alfalfa strips. Insecticide and harvest treatments were timed with predicted 1st occurrence of Z. phytonomi to maximize combined mortality. In Chariton, IA (1995), Lorsban 4 E (chlorpyrofos) applied at 0.140, 0.280, 0.560, and 1.120 kg (AI)/ha and Pounce 3.2 EC (permethrin) applied at 0.112 and 0.224 kg (AI)/ha reduced peak H. postica densities by >41% compared with control plots during an epizootic (>50% infection) of Z. phytonomi. Combined H. postica mortality was additive (insecticide and Z. phytonomi) in plots treated with Lorsban and Pounce and was 7.5-25.3% higher than in control plots. In Ames, IA (1994 and 1995), Lorsban, Pounce, and harvesting (1994) reduced H. postica densities during an enzootic (<50% infection) of Z. phytonomi; however, because of low levels of Z. phytonomi, combined mortality with treatments was negligible. This study demonstrated effective integrated management of H. postica by combining predictable mortality from Z. phytonomi and reduced rates of insecticides. The critical components of this integration were predictable levels of H. postica mortality by Z. phytonomi, and predictable mortality of weevil populations from reduced insecticide rates and strip-harvesting.