Crane flies, often mistaken for oversized mosquitoes, are a common presence in lawns and gardens, but the adult insects are ultimately harmless. The real threat to turf and plant health comes from their soil-dwelling larval stage, commonly known as the leatherjacket. These grubs feed voraciously on plant roots, causing significant damage that manifests as yellowing patches and thinning grass. Effectively managing this pest requires a two-pronged approach: making the environment unattractive for egg-laying and actively eliminating existing infestations. This strategy ensures long-term prevention while addressing immediate problems.
Understanding the Crane Fly Life Cycle
Adult crane flies typically emerge from the soil in late summer and early autumn, primarily between August and October, with the goal of mating and laying eggs. The female deposits up to 300 eggs directly into the turfgrass or moist soil within 24 hours of emergence. These eggs hatch into the leatherjacket larvae within one to two weeks.
The larvae immediately begin feeding on grass roots through the autumn before overwintering deep within the soil. They resume feeding aggressively in the spring when the soil begins to warm, which is when the most visible damage appears on lawns. These larvae grow to about one inch in length and possess a tough, leathery skin, the source of their common name. They remain in the soil until mid-to-late summer when they pupate just below the surface, completing the cycle as new adults emerge.
Cultural Practices for Environmental Repellence
Adult females prefer to lay their eggs in areas with high soil moisture, so reducing surface dampness during the egg-laying period can cause the eggs to dry out before they hatch. This means limiting irrigation or watering to a minimum during the late summer and early fall months. If watering is necessary, deep, infrequent applications are better than daily, shallow sprinklings, as this encourages deeper root growth and keeps the surface drier.
Optimizing soil drainage is also important because crane flies are attracted to naturally wet or poorly draining areas. Incorporating organic matter can help improve the soil structure, while regular aeration alleviates compaction, allowing water to pass through more freely. Dethatching eliminates a protective, moist environment where young larvae can hide and feed immediately after hatching.
Adjusting the mowing height in late summer also aids prevention. Crane flies favor laying eggs in areas of tussocky or longer grass, which offers protection for the emerging larvae. Keeping the turf tightly mown during the adult flight period can disrupt the egg-laying process and expose the eggs to greater environmental stress. For small, localized infestations, covering the affected area with a sheet of black plastic or polythene overnight can force the larvae to the soil surface, where they can be collected and removed by hand or become prey for birds.
Targeted Control Methods for Leatherjackets
Active intervention focuses on eliminating the existing leatherjacket population. The most accessible method for homeowners is the use of biological control agents, specifically beneficial nematodes. These microscopic roundworms, such as Steinernema feltiae, are natural parasites of the larvae. Once applied to the soil, the nematodes actively seek out the leatherjackets, enter their bodies, and release a bacterium that kills the pest.
Timing is crucial for nematode application, with the optimal window being late summer to early autumn (late August through October). This period targets the larvae when they are newly hatched and small, making them most vulnerable to the nematodes. The soil temperature must be consistently above 10 to 12 degrees Celsius for the nematodes to remain active. Application requires the soil to be thoroughly moist before and for up to two weeks after treatment, as nematodes travel through the soil’s water film to reach the larvae.
Chemical control options are restricted and often require professional application due to regulatory constraints. Certain modern insecticides, such as those containing chlorantraniliprole, may be available to lawn care professionals for targeted control. These products are typically applied when the larvae are young, similar to the timing for nematodes, to maximize effectiveness. Homeowners should check local regulations and strictly follow all label instructions, recognizing that deeply burrowed leatherjackets are often less susceptible to surface applications.