The large, long-legged insects often seen clumsily flying near lights are crane flies, frequently mistaken for oversized mosquitoes. These adult flies are harmless, but the true concern lies beneath the turf in their larval stage, which causes considerable lawn damage. The larvae, commonly known as leatherjackets, are the primary reason homeowners seek effective methods for control and prevention. Successfully managing a crane fly problem requires understanding their life cycle and implementing strategies that target the destructive stage in the soil.
Identifying the Pest and Damage
The destructive stage is the larvae, called leatherjackets because of their tough, grey-brown, tubular skin. These legless larvae can grow up to 1.5 inches long, resembling a headless, legless caterpillar.
Leatherjackets reside just beneath the soil surface, feeding voraciously on the roots, crowns, and sometimes the blades of grass. This root-chewing results in noticeable brown, dying patches of turf that often feel spongy underfoot. An infestation also attracts secondary predators like birds, skunks, and raccoons, which tear up the lawn while trying to dig up the grubs for food.
Habitat Modification for Prevention
The most effective long-term strategy against crane flies is altering the environment to make it unsuitable for egg-laying and larval survival. Crane flies prefer moist, waterlogged soil for their eggs to hatch and for the larvae to thrive. Reducing excessive irrigation is a powerful preventative measure.
Adjusting watering habits, especially in late summer and early fall when adult crane flies emerge to lay eggs, is highly beneficial. Watering deeply but less frequently will help keep the top layer of soil drier, which increases egg and young larval mortality. Improving soil aeration and drainage further denies the larvae their preferred damp habitat.
Maintaining a higher grass height, typically between 3 to 4 inches, helps promote a healthier lawn that can better withstand and recover from minor root damage. Adult crane flies are strongly attracted to exterior lighting, especially at night during the mating season. Reducing or redirecting bright outdoor lights near the lawn can discourage females from laying eggs in the immediate area.
Immediate Repellent and Control Methods
Targeting the leatherjacket larvae in the soil is the main goal for immediate control. Natural deterrents, while not effective for eliminating an established infestation, can help repel adult flies and discourage egg-laying. Essential oils such as peppermint, lavender, or cedar, when mixed with water and sprayed, can create a barrier that confuses and deters the adults.
A highly effective and environmentally conscious control method for active infestations involves the application of beneficial nematodes. These microscopic, parasitic roundworms are watered into the lawn, where they actively seek out and infect the leatherjacket larvae in the soil. The nematodes release bacteria that kill the pest within two to three days, offering a natural and safe alternative to chemical treatments.
Timing is paramount for successful nematode application, as they must be applied when the larvae are small and active near the soil surface. The optimal time is typically in the late summer or early fall, from August through October, shortly after the eggs hatch. A second application can be done in the spring, from March to May, if the larvae are still present and soil temperatures are above 50°F (10°C).
For chemical control, several active ingredients are available to target the larvae. Insecticides containing products like bifenthrin, carbaryl, or chlorantraniliprole are used to treat the soil. These products are most effective when applied in the fall to target the newly hatched larvae or in the spring to eliminate the overwintering grubs before they pupate.