Any successful gardener knows that mulch can be an asset to practically any gardening project. Mulches are sometimes used to increase the aesthetics of a growing area but are also used to control the flow of moisture and nutrients to plants' root systems. Of course, the organic activity that makes mulches so beneficial can also attract unwelcome guests such as flies and other insects. Understanding why the flies are attracted to mulch in the first place can help you decide if you need to try to get rid of them, and how to do so if you do.
Flies and Mulch Piles
Because mulches are most often composed primarily of organic matter, microorganisms such as bacteria and fungi are attracted to mulch piles, since these microorganisms feed on decaying organic matter and help break it down. The breakdown of organic matter in turn attracts insects that can sometimes be a nuisance, including flies, worms, centipedes, sowbugs and others. Their being a nuisance notwithstanding, the presence of these insects is usually considered positive, since it indicates that organic matter is being broken down in a way that will increase the effectiveness of the mulch.
There are some exceptions to the usually beneficial nature of mulch pile insects, however. Common species of flies attracted to decaying organic matter can be a nuisance at best by affecting outdoor activities near the mulch pile, and they are a sanitation concern at worst. One particular species of fly known to inhabit mulch piles is the fungus gnat, a pest of many different kinds of plants that travels from mulch into the soil beneath the plants and infects the plants from there.
To Remove or Not to Remove
Because of their typically beneficial nature, insects on mulch piles should often be tolerated whenever possible. An important exception to this rule has already been mentioned: the fungus gnat. Because of its classification as a damaging pest to many plants, fungus gnats should be removed from your mulch piles whenever they are discovered. Fungus gnats can also be a nuisance indoors, should they travel from a mulch pile to indoor parts of your home.
Stinkhorns, Mulch and Flies
As noted, flies are very often attracted to mulch piles by fungi and the bacteria that break down the organic matter in the mulch. However, fungi that produce fruiting bodies known as stinkhorns are also frequently attracted to mulch piles; as their name implies, these stinkhorns produce a foul odor similar to rotting meat that attracts flies. Therefore, if you decide to remove a fly population that has been attracted to your mulch pile by a stinkhorn fungus, your approach to removal should be centered around removing the stinkhorn fungi that attracted the flies in the first place.