Tiny Insects in the Windowsill

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Fungus gnats are often seen swarming in windowsills.

Homeowners often find tiny insects hovering in their windowsills. Many species of gnats and flies are attracted to light, making windowsills a favorite place to swarm. Fungus gnats and vinegar flies are two small insects that are often seen indoors near natural light. While neither of these insects bite, they often become nuisance pests when they multiply rapidly.



Fungus gnats are 1/10 to 1/8 inch in length and look like a long-legged mosquito. These gnats are attracted to light and are often seen flying in windows inside homes. Fungus gnats are gray, black, yellow or orange and lay eggs in the potting soil of indoor plants. The larvae are white, legless worms that have black heads and reach 1/4 inch in length. Vinegar flies are also common indoor pests that are attracted to natural light. These flies are 1/8 to 1/5 inch long and are brown or black in color. Vinegar flies are attracted to overly ripe fruit, garbage cans and fermented drinks such as beer and wine.



Fungus gnats multiply rapidly in organic matter such as potting soil. When potted plants are bumped or moved, these gnats swarm. Fungus gnats do not bite, sting or transmit diseases to humans. These gnats are usually seen where moisture is present, such as kitchen windowsills, bathrooms and laundry rooms. Vinegar flies do not bite or sting humans, but can transmit diseases such as intestinal myiasis which causes severe diarrhea in humans. These flies are especially attracted to light immediately after hatching and can reproduce two days after emergence.


Cultural Control

Fungus gnats develop as a result of excess moisture in the potting soil of indoor plants. To reduce fungus gnat development, avoid overwatering your indoor plants and use sterile potting soil. Repot your indoor plants every few months to avoid the breakdown of organic material in the potting soil. Sticky traps can catch fungus gnats as they swarm in windowsills. Keeping your kitchen free from fermenting fruit can help reduce vinegar fly infestations. Avoid placing fermented or rotten food in your garbage and place 16 mesh window screens in your windows, as vinegar flies can slip through ordinary window screens because they are so small.


Chemical Control

If fungus gnats are swarming inside your home, eliminate them with an insecticidal spray labeled for use on flying insects. Treat your indoor plants with fungus gnat larvae with soil drenches to keep larvae from becoming adult gnats. Vinegar flies are also susceptible to aerosol sprays labeled for flying pests, but the sprays will not kill the larvae.