Nursery, Greenhouse, and Landscape News

Monitoring Fungus Gnats in Greenhouses and Control With Different Nematodes

Fungus gnats are small flies that can be important pests of container-grown plants. The larvae feed on roots, as well as fungi, and can weaken plants and pass on plant diseases. Flourishing in moist soil conditions, they are excellent candidates for control with entomophagous nematodes. Research conducted on infested poinsettia has revealed differences in the control potential of nematode species and a new technique for monitoring this pest.

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.


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