Tiny Fungus Gnat "Outbreak" in Iowa

Callers the past week have described swarms of millions to billions of small black gnats on the side of the house, crawling on the compost pile or wood chip mulch or hanging around the landscape. A few are wandering indoors as accidental invaders. 

The tiny insects are called fungus gnats and recent phone calls and emails indicate the "outbreak" is nearly state-wide. Fungus gnats are an equal opportunity offender.

Fungus gnats are tiny flies only 2 to 3 mm in length (one-eighth inch or less). They have a dark, slender body and compared to more typical flies such as house flies, fungus gnats have long, dark antennae. The wings are dark, to light gray in color, giving one common and abundant group the common name "dark-winged fungus gnats."

As the name implies, fungus gnats live and breed in damp areas where the larvae feed on fungi and decaying organic matter. The larvae live and grow in soil, decaying vegetation such as leaf litter, rotten wood and under the loose bark of dead trees – all damp locations where decaying organic matter will be found.

Fungus gnats are harmless; they can’t bite, can’t sting and do not carry diseases. They do not attack crops, garden plants, trees, shrubs or flowers, and they do not attack the house, its contents or occupants. They are annoying in large numbers but otherwise harmless and ecologically beneficial as “recyclers.”

Why so many fungus gnats??

To quote Phil Nixon at University of Illinois: "We have fungus gnats every spring but usually not so many at one time.  Usually they come out in smaller numbers over a span of several weeks. I think with the sudden warm up with temperatures in the 80s for most of last week, what happened is that the number of fungus gnats that should have come out in a month-long period came out in one week. That's why people saw so many of them and why they are so obvious."

The “cure” for the problem is to wait and they will all disappear. No treatment is necessary or advised. Vacuum or sweep up those that have invaded the house and discard. Exterior insecticide applications will have little if any benefit.

Why so many on the compost pile?

It is unlikely the fungus gnats have been breeding and growing in compost piles or in wood chip mulch where people report seeing the adults. Instead, it is our best guess that compost and wood chips are attractive to fungus gnats because of the moisture they hold, and the “fungusy” smell they produce. The fungus gnats cannot reproduce in wood chips or compost unless they are way too wet, such as when water is standing underneath for long periods of time. Most likely, fungus gnats originating from nearby damp soil or leaf litter are congregating on the mulch or compost because of the smell.


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