The abundance of fungus gnats in the greenhouse is typically monitored by counting the numbers of adults attracted to yellow sticky cards. Sticky cards indirectly estimate the abundance of damaging fungus gnat larvae in the plant pots. This method has traditionally been used because removing the soil from pots to extract larvae is impractical. Placing freshly cut slices of potato on the soil surface attracts larvae to the surface and allows estimation of larval abundance in a plant pot without destroying the plant. The numbers of larvae attracted to potato slices gives a much more accurate estimate of larval populations than do adult numbers on sticky cards.

Using this monitoring method, researchers have found that Steinernema feltiae (SN strain) provides excellent control of fungus gnats. Plants treated with this nematode had significantly fewer larvae than plants treated with kinoprene (EnstarII), Gnatrol (Bacillus thuringiensis israeliensis), Steinernema carpocapsae (All strain), or a water control. There were also fewer larvae than in pots treated with diazinon (Knox-Out), although the difference was not statistically significant.

Larval stages of fungus gnats were more susceptible than pupae to infection by S. feltiae.

Source:

Harris, M. A., R. D. Oetting and W. A. Gardner. 1995. Use of entomopathogenic nematodes and a new monitoring technique for control of fungus gnats, Bradysia coprophila (Diptera: Sciaridae), in floriculture. Biological Control 5: 412-418.