Entomopathogenic Nematodes and Other Natural Enemies as Mortality Factors for Larvae of Diaprepes abbreviatus (Coleoptera: Curculionidae)

Parasitism and persistence of three species of entomopathogenic nematodes, Steinernema riobrave, Heterorhabditis bacteriophora, and H. indica, were evaluated as biopesticides against larvae of Diaprepes abbreviatus in a mature citrus grove using different soil sampling methods. In three separate tests, commercial formulations of different nematode species were applied with herbicide delivery equipment at rates from 11 to 216 infective juveniles (IJs)/cm2 to the soil beneath the tree. The prevalence of parasitism and/or predation by either commercially applied nematodes or indigenous natural enemies associated with weevil larvae in the soil was measured using larvae-baited screened cages placed in the soil before and after nematode application. The results showed that ant predation and nematode parasitism were the dominant mortality factors of caged, 6th instar D. abbreviatus during 7 days exposure to field soil. Entomopathogenic fungi and bacteria were incidental. Inundative applications of different rates of entomopathogenic nematodes showed a positive quadratic relationship between number of nematodes applied per given area (log scale) and parasitism of caged larvae. Higher rates (>54 IJs/cm2) were required to achieve levels of parasitism significantly greater than that in the controls. Nematode parasitism of D. abbreviatus larvae was similar in caged versus no-cage comparisons conducted in the greenhouse. Nematode numbers in the soil declined over time, reaching pretreatment levels at 14 days posttreatment based on a modified Baermann sampling procedure.