Greenhouse - Biological Control of Fungus Gnats

The following is information on the biological control of fungus gnats in the greenhouse.

Insect parasitic nematodes (Steinernema feltiae) are primarily used against fungus gnat larvae and thrips pupae in the growing media. The nematodes enter the insect host through body openings. They multiply within the host and release a symbiotic bacterium whose toxin kills the fungus gnat larvae. The larvae may be killed in 1 to 2 days by blood poisoning.

Preventative soil drenches to moist growing media work best. Like any biological control agent, they are best used preventatively before an outbreak occurs. Because the nematodes are living organisms, check for viability before application (which can vary depending upon shipment and company). Place a small amount of the product in a small container, add 1 to 2 drops of room temperature water, wait a few minutes and look for actively moving nematodes. (A dark background helps.) Treat as soon as possible after sticking cuttings, planting seeds or plugs. Optimum media temperatures are between 60 and 70°F. Apply in the evening or at dusk or on a cloudy, overcast day (Nematodes are very sensitive to UV light and desiccation). They are compatible with a number of different pesticides (consult with your supplier for more information or see web sites below). Repeated applications are often needed. The symbiotic bacteria break down the insect cuticle so the larvae may rapidly disappear. Infected fungus gnat larvae are often opaque-white to light yellow in color.

Native, soil dwelling predatory mites (sold under the name of Hypoaspis miles) feed upon small, first instar fungus gnat larvae as well as thrips pupae. If soil dwelling prey insects are scarce, this predatory mite can survive by scavenging upon plant debris and algae. Hypoaspis are best used preventatively, at planting time. Apply to moist media, with media temperatures of at least 60°F. To evaluate quality before release, place a small sample of mites on a sheet of white paper. The Hypoaspis mites are tan and move quickly. Food source mites are translucent, white and move slowly.

Rove beetles are generalist predators that may feed upon shore flies, as well as fungus gnats and thrips. Both adults and larvae are generally found in the growing media, hiding in cracks and crevices. Once established in a greenhouse, they will be there year round, but population levels vary depending upon prey populations.

Bacillus thuringiensis subsp. israelensis (Gnatrol) is most effective against the young first instar larvae. The younger instars do not have to consume as much material for it to be effective. Because larger instars are less susceptible, repeated applications at high rates may be needed to provide effective control.

Information from Leanne Pundt, University of Connecticut Cooperative Extension, in the October, 2007 edition of the New England Greenhouse Update.