ARS | Publication request: Attraction and oviposition responses of the fungus gnat Bradysia impatiens to microbes and microbe-inoculated seedlings in laboratory bioassays

Submitted to: Entomologia Experimentalis et Applicata
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 29, 2012
Publication Date: N/A

Interpretive Summary: Large populations of fungus gnats are frequently associated with Pythium root rot disease outbreaks in greenhouse crops, and the prevailing belief for many years has been that these pests play a major role in creation of root-disease problems by acting as vectors. Our recent studies have demonstrated, however, that fungus gnats are not significant vectors of Pythium, and an alternative explanation for the observed association is simply that fungus gnats are attracted to diseased/decaying plants. In laboratory assays we investigated the attractiveness of geranium seedlings inoculated with a broad range of pathogenic and non-pathogenic microbes to larvae and adult females of the fungus gnat Bradysia impatiens. Seedlings infected with Pythium and Thielaviopsis root rot and Xanthomonas blight pathogens were all highly attractive to fungus gnat larvae and/or egg-laying adults. Pure-culture preparations of Pythium, were also attractive, but inactivation via freezing or drying rendered them unattractive. Seedlings inoculated with the beneficial fungi Beauveria bassiana and Trichoderma harzianum currently marketed for greenhouse pest and disease management also attracted fungus gnat egg laying. Our findings suggest that the commonly observed association between fungus gnats and diseased plants is primarily the result of fungus gnat attraction to microbial activity and will occur even in the absence of pathogen transmission by fungus gnats. These results underscore the importance of basic sanitation in fungus gnat management programs and point also to a need for additional studies under crop-production conditions to assess potential impacts of beneficial-microbe applications on fungus gnat populations.

Technical Abstract: Laboratory tests were conducted to examine preferences of Bradysia impatiens Johannsen (Diptera: Sciaridae) larvae and adults for various microbes associated with greenhouse crops. Fungus gnat larvae and adults exhibited a preference for cultures of Pythium spp. over the medium used to grow the pathogens. Larvae also exhibited a preference for geranium seedlings infected with pathogenic Pythium spp. (P. aphanidermatum, P. ultimum, and P. irregulare) over non-inoculated plants. Adult fungus gnats exhibited a strong ovipositional preference for the aforementioned Pythium spp. as well as a variety of other microorganisms, including the pathogenic fungus Thielaviopsis basicola, the geranium-infecting bacterium Xanthomonas campestris pv. pelargonii, the non-pathogenic species P. torulosum and P. graminicola, the pathogen-suppressive fungus Trichoderma harzianum, and the insect pathogenic fungus Beauveria bassiana. Our study demonstrates that fungus gnats are attracted to and/or stimulated to oviposit by a wide array of living microorganisms both in pure culture and in association with plant seedlings. These findings have important implications with respect to the role of fungus gnats in plant disease epidemics.