Garden Gnats

Description

• Fungus gnats are very small sized flies with a mature length of 1/8 to 1/10th of an inch. The black or gray pests have long antennae and legs. A distinguishing feature is a Y-shaped vein at the tips of the wings. Despite the wings, fungus gnats don't fly very well. The insects lay the tiny 1/100 inch eggs in large clusters. The black headed, white maggots appear within four to six days and are ¼ inch long when mature.

Favorable Growth Conditions

• Garden soils with high levels of organic matter or those that are excessively moist support the optimal growth of fungus gnats. It is common to see a peak in population during temperatures of 65 to 75 degrees Fahrenheit. Houseplants become prone to infestation when taken out for airing during the summer, spreading the insects to other plants once indoors. Keep the landscape clear of large amounts of decomposing grass clippings, compost and organic mulches to help reduce garden gnat problems. Avoid excessive use of manure or bloodmeal as garden fertilizer.

Damage

• The larvae of fungus gnats burrow right into the soil after hatching and start feeding on the feeder roots and root hairs of infested plants, creating tunnels with their feeding. The pests also eat the foliage growing low on the soil. Early symptoms of damage include gradually declining plant health and yellowing foliage. Infested roots develop scars on the surface. Controlling fungus gnat adults and their larvae is important because the pests are also likely vectors of pathogens including Botrytis, Verticillium, Phythium, Fusarium and Phoma.

Control

• Control the adult gnats with yellow sticky traps. Biological control options include the use of Bacillus thuringiensis subsp. Israelensis or Steinernema feltiae nematodes. Other chemicals that provide effective control of fungus gnats include bifenthrin and pyrethrins. Using soil drenches containing organophosphates such as malathion or acephate are also highly effective for killing larvae. Use chemicals sparingly as these are harmful to many beneficial soil organisms.