After mating, the female fly deposits egg clusters in the top ½” of soil. The larvae hatch out and feed on soil fungus and small plant roots. In severe infestations larvae can even bore out a plant’s crown tissue. Within a couple of weeks the larvae will pupate and emerge from the soil and the cycle begins again. Adult fungus gnats are not very swift fliers and will often run or jump across the soil surface when disturbed rather than try and fly away. Fungus gnats also help to spread disease pathogens from host to host making them even more problematic for growers.

Fungus gnat larvae, adult

There are some chemical control options available including Botaniguard, Flagship and Azatin, but most need to be applied often, making them costly. Beneficial nematodes work extremely well against larvae. These living organisms are produced by Becker-Underwood and Bioworks and are drenched onto the soil surface. The worm-like organisms move through the soil and seek out the fungus gnat larvae. The nematode enters the larvae through holes and pores in its skin, and begins to excrete a bacteria which quickly kills the fungus gnat larvae. The nematode lives in the corpse and actually reproduces inside its hosts. The larvae will swell up and eventually burst releasing new nematodes back into the soil… pretty cool huh? IPM programs that employ these beneficials along with a responsible spray program seem to be offer the best control.

Article by Marshall Horsman and Matt Shultz