When large, awkward insects suddenly appear in great numbers, often bumping clumsily against windows and light fixtures, they are usually crane flies. They are frequently mistaken for giant, non-biting mosquitoes due to their slender bodies and extremely long, fragile legs. While the adults are mostly a harmless nuisance, a sudden population boom signals ideal conditions for their larval stage, which can affect lawns and turf. Understanding their biology explains why they become abundant in a specific area.
Understanding the Crane Fly: Biology and Stages
The adult crane fly is a delicate insect with two functional wings and a pair of specialized, smaller wings. Their primary purpose during their short lifespan is reproduction, and they do not possess the mouthparts necessary to bite or sting humans or animals. After mating, females deposit eggs directly into moist soil, often laying up to 300 eggs.
These eggs hatch into the larval stage, commonly known as leatherjackets, a nickname earned from their tough, leathery skin. The larvae are legless, worm-like, and can grow up to four centimeters long. They feed on organic matter and grass roots just below the soil surface. The larval stage typically overwinters, feeding in the fall and again in the spring before pupating in the soil. The cycle concludes when adults emerge, sometimes leaving behind their empty pupal cases sticking out of the ground.
Environmental Factors Driving High Populations
The number of emerging adult crane flies is related to the success of the leatherjacket larvae in the preceding months. Larvae thrive in environments with consistently high soil moisture and poor drainage. Areas that experience wet winters or heavy irrigation become ideal breeding grounds because the larvae survive best in saturated soil conditions.
Adult females actively seek out these damp locations, such as low-lying sections of a lawn, to lay their eggs in late summer or early fall. A thick layer of thatch—organic debris between the grass and the soil—also provides an excellent protective habitat and food source for the developing larvae. Furthermore, a lack of natural predators, such as certain birds, ground beetles, and entomopathogenic nematodes, allows populations to grow unchecked. When these factors align, it creates a scenario for a mass emergence of adult crane flies during their seasonal window, usually in spring or late summer depending on the species.
Assessing Damage and Practical Management Strategies
A large swarm of adult crane flies is generally only an annoyance, but it signals the presence of a high population of damaging larvae in the turf beneath. Larval feeding on grass roots and crowns causes noticeable damage, which often appears in spring as irregular, thinned, or brown patches of turf that do not respond to watering. Another telltale sign of a high infestation is increased activity from birds, like starlings, or other foraging animals digging in the lawn to feed on the leatherjackets.
Managing Adult Crane Flies
For managing the adult insects, simple exclusion methods are most effective. This includes ensuring windows and doors have fine mesh screens to prevent them from entering structures. Since adults are attracted to light, managing exterior lighting at night can also reduce their presence near the home.
Long-Term Larval Management
The most effective long-term management focuses on making the environment less hospitable for the larvae. Reducing soil moisture is a primary strategy, which involves adjusting irrigation schedules to water deeply but less frequently, especially during the late summer egg-laying period. Improving soil drainage is also beneficial, and techniques like core aeration can help break up compaction, reduce thatch, and dry out the upper soil layer where larvae reside. Healthy, well-maintained turfgrass is more resilient and can tolerate higher larval populations. In cases of extreme infestation, professional consultation for targeted application of biological controls, like beneficial nematodes, or chemical treatments may be considered.