Effects of culture method and formulation on the virulence of Steinernema riobrave (Rhabditida : Steinernematidae) to Diaprepes abbreviatus (Coleoptera : Curculionidae)

The Diaprepes root weevil, Diaprepes abbreviatus, is a pest of vegetables, ornamental plants, sugarcane, and citrus in Florida and the Caribbean. The entomopathogenic nematode, Steinernema riobrave, can reduce larval populati ons of D. abbreviatus substantially. Efficacy of entomopathogenic nematodes , however, may be affected by culture method and formulation. Using D. abbr eviatus as the host, we compared the efficacy of two commercial S. riobrave formulations, a liquid and a water-dispersible granule (WDG), with each ot her and with in vivo produced S, riobrave. Nematodes in the commercial form ulations were produced in vitro through liquid fermentation; the in vivo ne matodes were cultured in Galleria mellonella and applied in aqueous suspens ion. Laboratory experiments measured nematode virulence in plastic cups con taining soil and seventh-eighth instar D. abbreviatus. One laboratory exper iment was conducted using only fresh nematodes (less than 5 days old); anot her experiment included WDG nematodes that were stored for 25 days at 10 de greesC. Two field experiments were conducted in which nematodes were applie d either to potted citrus (containing D. abbreviatus larvae) placed beneath mature citrus trees or to soil directly beneath the tree. In the latter ex periment, efficacy was determined by measuring mortality of caged D. abbrev iatus larvae that were buried beneath the soil surface prior to application . Mortality of D. abbreviatus treated with nematodes ranged from 80-98% and 50-75% in laboratory and field experiments, respectively. In all experimen ts, we did not detect any significant effects of culture method or formulat ion.