The presence of small, dark flies hovering over plants and yard can quickly become frustrating. These tiny insects, often broadly referred to as gnats, are common nuisance pests in landscaping. A large swarm indicates that your yard provides ideal conditions for their life cycle, and this article will help identify the specific culprit and the environmental factors attracting them.
Identifying the Type of Small Fly
The term “gnat” is non-specific, grouping several types of small flies, but the most common yard pest is the Fungus Gnat (Bradysia species). These flies are slender, dark, and measure about one-eighth of an inch long, giving them a delicate, mosquito-like appearance. They are weak, erratic fliers that often prefer to run across the soil surface. The larvae are translucent white with a shiny black head, dwelling just beneath the soil surface where they feed on organic matter and plant roots.
Other similar-looking flies exist, making location important for identification. Drain flies, for instance, are found near plumbing fixtures, while shore flies are associated with heavy algae growth or decomposing matter. Correct identification, primarily distinguishing the soil-dwelling Fungus Gnat from these other flies, is the first step toward effective control.
Environmental Factors Attracting Gnats
Gnats thrive in your yard due to the availability of constant moisture and decaying organic material, which are necessary for the larval stage to survive. Fungus gnat larvae require perpetually damp conditions to complete their development, making overwatered lawns and garden beds prime breeding grounds. This saturation allows the larvae to consume the fungi and decaying matter they rely on for food.
Poor drainage in the soil composition exacerbates the problem by preventing the top layer of earth from drying out between watering cycles. Heavy, compacted soils or those rich in moisture-retaining materials like uncomposted peat moss will hold water for too long. These excessively damp conditions not only attract the adults to lay eggs but also create an environment where the larvae can feed continuously before pupating.
Excessive organic matter acts as both a food source and a sheltered location for egg-laying females. Thick layers of mulch, especially wood chips or bark, create a dark, humid barrier that traps moisture near the soil surface. Compost piles, decaying grass clippings, or organic fertilizers like manure also provide a feast for gnat larvae. Shaded areas that remain consistently cool and moist are particularly susceptible to supporting large populations.
Strategies for Eliminating the Problem
Addressing a gnat infestation requires breaking the cycle by removing the attractants and targeting the soil-dwelling larvae. The most effective cultural control is immediately adjusting your irrigation schedule to allow the top layer of soil to dry out completely between waterings. For garden beds, aim to let the top one or two inches of soil become dry to the touch, which eliminates the moist environment the larvae need to survive. Improving drainage in problem areas, perhaps by incorporating sand or perlite into the soil, will help prevent water from pooling or stagnating.
You can also reduce the food source by minimizing the amount of decaying organic material near susceptible plants. This may involve reducing the depth of mulch layers around plants or promptly clearing away any dead leaves and plant debris from the garden floor. Avoiding the excessive use of highly organic fertilizers that break down slowly will further limit the available food for the developing larvae. Removing these attractants makes the yard significantly less hospitable to the breeding adults.
Biological and Physical Controls
Targeting the larvae directly in the soil is accomplished through the use of biological controls. Beneficial nematodes, specifically the species Steinernema feltiae, are microscopic roundworms that actively hunt and infect gnat larvae in the soil. Another option is the biological insecticide Bacillus thuringiensis israelensis (Bti), a naturally occurring bacterium that is toxic only to the larvae of certain flies when ingested. Physical traps, such as yellow sticky cards placed horizontally near the soil surface, are an excellent non-toxic way to monitor the population and capture adult gnats before they can lay more eggs.