Crane fly larvae in a lawn often lead to patches of brown, dying grass, signaling an infestation beneath the surface. These larvae, commonly known as leatherjackets due to their tough, leathery skin, feed voraciously on the roots and crowns of turfgrass, leading to thinning and eventual turf loss. The damage is often compounded when animals like birds, skunks, and raccoons tear up the turf while foraging for the larvae. Successfully managing these pests requires understanding their life cycle and timing control methods to target the most vulnerable stages. This guide provides actionable solutions, from cultural practices to chemical treatments, to help restore a healthy lawn.
Identifying the Pest and Its Lifecycle
The adult crane fly is often mistaken for a giant mosquito, though it is harmless and does not bite. These adults have long, spindly legs and a slender, brownish body, emerging from the soil in late summer, typically from August through September. Females quickly mate after emerging and deposit hundreds of tiny, dark eggs just beneath the soil surface, especially in moist conditions.
The eggs hatch within about two weeks into the leatherjacket larvae, which are legless, grey-brown, and cylindrical, growing up to one and a half inches long. These larvae begin feeding on grass roots in the fall, surviving the winter underground, and becoming most active and causing the greatest damage in the early spring as temperatures rise. The larvae then retreat to pupate in the soil during the summer before the cycle begins again. The most effective treatment window is the late summer or early fall when the larvae are small and near the surface, or in the early spring before the most severe damage occurs.
Cultural and Biological Control Methods
Adult crane flies are strongly attracted to moist soil for egg-laying, so reducing irrigation in late summer around the time of adult emergence can significantly decrease the number of eggs laid. Maintaining a healthy, dense lawn through proper fertilization, aeration, and mowing also helps the grass tolerate and recover from root damage. Lawns with excessive thatch buildup should be addressed, as the larvae often prefer to stay in this layer of dead organic matter.
For a targeted non-chemical approach, entomopathogenic nematodes are an effective biological control method. These are microscopic roundworms, specifically species like Steinernema feltiae or Heterorhabditis bacteriophora, that seek out and parasitize the leatherjacket larvae in the soil. The nematodes enter the larvae and release symbiotic bacteria, which multiply and kill the host within a few days.
Successful application depends on environmental conditions, as nematodes are living organisms. The soil must be moist, not saturated, for the nematodes to move, and they should be applied in the evening or on a cloudy day to protect them from ultraviolet light. Soil temperatures should be between 55°F and 85°F, with S. feltiae being effective even at cooler temperatures. The most vulnerable stage to target is the small, newly hatched larvae, making the late summer or early fall application window the most effective.
Chemical Treatment Options
Chemical control is reserved for severe infestations where the population exceeds 25 to 30 larvae per square foot, or when non-chemical methods have failed. Active ingredients used to control crane fly larvae include synthetic pyrethroids or newer compounds like chlorantraniliprole, depending on local regulations and product availability. These products are formulated as granules or liquids that are watered into the turf to reach the larvae in the root zone.
The timing of chemical treatment is critical to maximize effectiveness against the small, vulnerable larvae. Applications must be timed to coincide with the newly hatched larvae in the fall, generally two weeks after adults have been seen laying eggs, or in the early spring before the larvae grow to their full size. Applying insecticides when the larvae are large in late spring is less effective, as much of the damage has already occurred and the larvae are harder to kill.
When using chemical control, follow all label instructions concerning application rates, personal protective equipment, and environmental precautions. Many insecticides, especially pyrethroids, can be toxic to beneficial insects like bees and aquatic life. Care must be taken to avoid runoff into waterways and to apply outside of peak pollinator activity. Consulting local extension services is recommended to confirm the best timing for treatment in your region.