Attraction of female fungus gnats, Lycoriella ingenua (Diptera: Sciaridae) to mushroom-growing substrates and the green mold, Trichoderma aggressivum

Using a static-flow two choice olfactometer system, we previously showed that gravid female fungus gnats, Lycoriella ingenua (Diptera: Sciaridae), one of the most severe insect pests of the white button mushroom, Agaricus bisporus, are attracted to the compost in which mushrooms grow and not to the mushrooms themselves. We also showed that these flies are attracted to the mycoparasitic green mold, Trichoderma aggressivum, a severe pest of mushrooms. In the present study, in an effort to understand what these flies are attracted to in the mushroom-growing compost, we isolated and cultured several fungal species from adult and larval flies that had been reared on this compost. We then performed two-choice assays in our static-flow olfactometer examining the attraction of gravid female L. ingenua flies to these isolated fungal cultures versus pure T. aggressivum cultures. We also performed two-choice oviposition assays in small glass Petri dishes with gravid females using these same cultures versus T. aggressivum. Lastly, we performed no-choice oviposition assays with these cultures using individual, gravid, one-to-seven-day-old female L. ingenua flies. We found that females were as attracted to two different Penicillium species isolated from adult flies as they were to T. aggressivum, and laid an equivalent number of eggs on these two PenIcillium species as they did on T. aggressivum. We also found that females were less attracted to an Aspergillus species isolated from larval frass than to T. aggressivum, but would lay more eggs on this Aspergillus species than on T. aggressivum. In no-choice oviposition assays, gravid females refused to oviposit on some fungal isolates no matter how close to death they were.