Crane flies are a common late-summer sight that can signal future lawn damage. While the adults are harmless, their offspring cause significant destruction to turfgrass roots and crowns. Managing these pests requires a targeted approach that disrupts their life cycle, focusing heavily on non-chemical methods to protect the environment and maintain a healthy lawn.
Identifying the Pest and Its Vulnerable Stages
The adult crane fly is a slender, tan-to-gray insect with an elongated body and notably long, fragile legs, typically emerging in late summer or early fall. These adults live for only a few days, primarily to mate and lay eggs directly into the soil. Their appearance marks the beginning of the destructive phase.
The true culprits are the larvae, commonly known as leatherjackets, which hatch shortly after the eggs are laid in the fall. These larvae are legless, gray-brown, and worm-like with a tough, leathery skin, growing up to one and a half inches long. They feed on the roots and crowns of turfgrass throughout the fall and winter, causing the most noticeable damage in the early spring before they pupate.
This life cycle provides two distinct windows for effective control: late summer when the eggs are newly laid or the larvae are very small, and early spring before the larvae grow to their maximum size. Targeting the small larvae in the fall is generally the most effective way to prevent spring damage.
Essential Cultural Practices for Prevention
The most sustainable way to deter crane flies is by making the lawn an undesirable place for the adults to lay their eggs. Crane fly eggs and young larvae require consistently moist soil to survive and develop. A primary preventative measure involves reducing or stopping irrigation in late summer and early fall, particularly around the time adults are active.
Allowing the soil surface to dry out during this egg-laying period significantly reduces the survival rate of the newly laid eggs. Improving soil drainage is also important, as areas with poor drainage become ideal breeding grounds. Consider deep-tine aeration to improve water penetration and reduce surface moisture.
Maintaining a healthy, vigorous turf helps the grass tolerate any feeding that does occur. Practices like dethatching and regular aeration reduce the dense, moist organic layer that leatherjackets prefer to inhabit. Mowing your grass at a slightly higher setting promotes deeper root growth, allowing the lawn to withstand minor root damage without showing patches of thinning or dead turf.
Utilizing Nematodes for Larval Control
For existing infestations, the application of beneficial entomopathogenic nematodes offers an effective, non-toxic treatment option. Specifically, species like Steinernema feltiae are microscopic roundworms that actively seek out and parasitizes the leatherjacket larvae in the soil. Once inside the host, the nematodes release a symbiotic bacterium that quickly kills the crane fly larva.
The timing of application is crucial for success, as nematodes are most effective against small larvae near the soil surface. This generally means applying them in late August through October, shortly after the eggs have hatched, or as a secondary treatment in the spring when soil temperatures reach about 55 degrees Fahrenheit. The soil must be thoroughly moist before and for two weeks following application to allow the nematodes to move freely and survive.
To prevent the nematodes from drying out or being killed by ultraviolet light, they should be applied during cooler parts of the day, such as early morning or evening, or on an overcast day. It is necessary to irrigate immediately after application to wash the nematodes off the grass blades and down into the root zone where the larvae reside. This biological control method targets the pest without harming beneficial insects, pets, or people.
Deterring Adult Crane Flies
While the adult flies do not feed or cause lawn damage, deterring them prevents the next generation of destructive larvae from being laid. Adult crane flies are strongly attracted to light, which can draw them toward your home and lawn at night. Switching exterior lights to yellow-spectrum bulbs or motion-activated lights can reduce this attraction, particularly in the late summer months when females are searching for egg-laying sites.
If the adults are becoming a nuisance by entering the home, focus on physical exclusion measures. Inspect and seal any gaps or cracks around windows, doors, and utility entry points. Ensuring that window screens are intact will prevent the adults from making their way indoors. Addressing the outdoor environment with cultural controls remains the most effective long-term deterrent.