The appearance of flies hovering over or emerging from a lawn, accompanied by dead or thinning patches of grass, is a common frustration for homeowners. These “flies in the grass” are typically not harmless nuisance insects but are the adult or larval stages of specific turf pests. Effective control and the recovery of a healthy lawn depend entirely on correctly identifying the organism responsible and applying targeted management strategies.
Pinpointing the Culprit: Identifying Turf-Damaging Pests
The two most frequent culprits are the larvae of Crane Flies and Fungus Gnats, each causing distinct damage. Adult crane flies are large, fragile insects, often mistaken for giant mosquitoes, but their larvae, known as leatherjackets, destroy the turf. These larvae are grayish-brown, legless, tough-skinned creatures up to an inch and a half long that feed voraciously on grass roots and crowns, causing irregular brown patches that lift easily like a carpet due to severed roots.
Fungus gnats are tiny, dark flies less than one-eighth of an inch long that swarm low over the soil surface. Their larvae are small, translucent, worm-like organisms with distinct shiny black heads, thriving in overly saturated soil and decaying organic matter. While adult gnats are a nuisance, the larvae feed on fine root hairs, causing plants to wilt and display stunted growth, often mimicking drought or disease symptoms.
To confirm a leatherjacket infestation, a homeowner can use the soap flush test. This involves mixing a tablespoon of liquid dish soap with two gallons of water and pouring it over a square foot of damaged lawn. The solution forces the larvae to the surface within minutes, allowing for accurate identification.
Non-Chemical Solutions: Modifying the Lawn Environment
Managing the lawn’s environment is the first and most sustainable line of defense against these pests, as many thrive in damp conditions. Overwatering creates the ideal breeding ground for both crane fly and fungus gnat larvae. Shifting irrigation practices to water deeply but infrequently allows the top inch or two of soil to dry out completely between sessions, which is lethal to the larvae.
Physical modification involves addressing the layers of organic material that accumulate between the soil and the grass blades. Dethatching removes this thick layer, which serves as a protective habitat and food source for the larvae. Aeration, the process of removing small plugs of soil, reduces compaction and improves soil drainage, moving water away from the root zone and strengthening the grass’s ability to tolerate minor root feeding.
Adjusting the mower height also contributes to a less favorable environment. Maintaining a taller grass height, typically between 3.5 to 4 inches, encourages the development of a deeper, more robust root system. Healthier roots are less susceptible to the feeding damage caused by leatherjackets and fungus gnat larvae.
Applying Targeted Pest Controls
When cultural controls are insufficient or the infestation is severe, a targeted application of controls becomes necessary, starting with biological agents. The insect-parasitic nematode Steinernema feltiae is an effective, natural option for controlling both leatherjackets and fungus gnat larvae. These microscopic roundworms seek out the pest larvae in the soil, enter their bodies, and release symbiotic bacteria that kill the host within days.
Successful nematode application depends on specific environmental conditions. They must be applied to soil that is moist but not waterlogged, and application should occur during the evening or on a cloudy day to protect the organisms from ultraviolet light. Soil temperatures must be within the range of 50°F to 86°F for the nematodes to remain active, with 57°F to 79°F being optimal for parasitic activity.
If chemical intervention is required, timing is paramount for maximum efficacy. For crane fly larvae, insecticides are most effective when applied in late summer or early fall when the larvae are young and near the soil surface, or in the winter months before spring damage occurs. Active ingredients like chlorantraniliprole offer a lower-risk option, while broad-spectrum products containing bifenthrin are used.
Long-Term Prevention Strategies
Moving beyond immediate eradication, a proactive strategy focuses on maintaining long-term turf health to build resilience against future infestations. The first step involves a comprehensive soil test to determine nutrient and pH levels. This analysis allows for a balanced fertilization program, promoting deep root growth and dense turf that can easily outgrow minor pest damage.
The proper soil pH, ideally between 6.0 and 7.0 for most turfgrasses, maximizes the availability of nutrients like nitrogen and potassium, which are essential for stress tolerance and disease resistance. Applying fertilizer based on a soil test prevents the overapplication of nutrients that can lead to lush, soft growth, which is often more attractive to pests.
Another preventive measure is selecting pest-resistant grass varieties. Certain cultivars of perennial ryegrass, tall fescue, and fine fescues contain beneficial fungal endophytes. These naturally occurring fungi live within the grass and produce chemicals toxic to various leaf-feeding insects and some root feeders. Selecting these endophyte-enhanced varieties reduces the likelihood of pest problems.