Comparative efficacy of Bacillus thuringiensis var. israelensis and Steinernema feltiae for controlling fungus gnats (Diptera: Sciaridae) infesting oyster mushrooms

Feeding by fungus gnat (Diptera: Sciaridae) larvae reduces the production of gourmet oyster mushrooms (Pleurotus spp.). To identify alternatives to conventional insecticides, the objectives of this study were to assess the efficacy of Bacillus thuringiensis var. israelensis (Bti) and Steinernema feltiae against fungus gnat larvae. A container bioassay was developed, whereby pasteurized straw was inoculated with P. columbinus mycelium and treated with (1) Bti (Gnatrol®), (2) S. feltiae (Nemashield), (3) Bti and S. feltiae, or (4) water. Gravid fungus gnats (Lycoriella sp.) were introduced into each container for ovipositing onto the straw pre-colonized by P. columbinus, and the resulting larvae were thereby exposed to the aforementioned treatments. Sticky cards affixed within the bioassay containers entrapped fungus gnats emerging from the substrate as an indirect measurement of larval survivorship. During three independent bioassays, Bti reduced adult emergence by 40−57% compared to 2-27% for S. feltiae when compared to the water control. Subsequent bioassays used straw pre-colonized by Pleurotus ostreatus, and treatments were applied 3 days after infestation to ensure there was not a deterrent effect of the Bti or S. feltiae treatment on the egg-laying adults. During three independent bioassays, Bti reduced adult emergence by 53−89% compared to 53-57% for S. feltiae, and 46-95% for Bti and S. feltiae, compared to the water control. Incorporating Bti into straw-based substrate is a promising approach for managing fungus gnat larvae infesting oyster mushroom, but additional studies are warranted to determine optimal rates.