Gardening: Diagnosing and dealing with crane fly infestations
by Mary Robson, WSU Extension Agent
Many western Washington lawns have suffered from winter rains and soggy soils and may look scruffy and tattered this time of year as they come back into active growth. Symptoms of spotty growth, yellowed areas, and generally poor growth may be related to the presence of European crane flies, a soil pest that affects lawns by chewing and feeding at the root crowns of lawn grasses. During some years, the European crane fly is more numerous than in others. Home lawns in one part of western Washington may experience noticeable damage while other areas don't have many of these insects.
How can the gardener determine whether European crane flies are present and feeding on grass crowns? The crown is the area of the turf just above the roots, where new shoots emerge. Healthy crowns make healthy turf. If infestation of crane fly is suspected from the symptoms of yellowed grass patches and irregular growth, check by digging out a sample of the grass. Pick a piece of turf 12 inches by 12 inches, in the apparently affected area. Dig it out to a depth of 2 inches, down at the crown area, and flip it over. Look for the presence of grayish-white or grayish-brown larval forms of the European crane fly, called "leatherjackets," because they have a distinctly touch-looking outer skin. They are about 1/2-inch long, narrow and wormlike, but do not taper to a narrow end as earthworms do, and are a distinctly lighter color. If you pull on them, holding them at both ends, they stretch out and it's easy to examine the tough skin which keeps them from being pulled apart easily. If the affected sample turf that's been drug out contains more than about 25-30 larvae per square foot, treatment should be considered.
During mid to late-April, depending on the weather, crane fly larva damage to turf usually becomes noticeable if they are present. If the lawn is in generally good health and a survey reveals fewer than 25-30 per square foot, the lawn can usually outgrow the damage without any chemical treatment. Birds will peck at the ground and eat the larvae helping to control infestations. (Starlings eat crane fly larvae, so if you observe them or other birds on the grass pecking at the crowns of the turf, recognize that they are acting as beneficial predators of lawn-dwelling grubs and larvae. This may be one of the few useful reasons for the existence of starlings.) If you don't find significant crane fly larvae infesting the ground, don't reach for a chemical treatment. Support the grass with good basic care.
Understanding the life cycle of this odd pest will help with comprehension of when and how damage may occur. The adult crane fly looks like a large, poorly-constructed mosquito with very long legs and a body length of about an inch not counting the legs. They do not bite or bother people or structures. They emerge from the soil of lawns, pastures, and other grass areas between late August and mid-September. They may congregate on house walls or even be found inside houses flying around lights. They mate and the females lay eggs in the soil at the grass crowns within 24 hours; the adults then die without doing any damage. One gardener suggests stomping as many of these as possible when they emerge--it can't hurt!
The crane fly females seem to prefer healthy, well-watered turf for laying eggs, so a desiccated lawn area that receives no water in late summer may not be bothered. Unfortunately, it's well-cared-for turf that they often choose. The eggs hatch during late fall into larvae that begin feeding at the roots and crowns of the turf. They overwinter as "leatherjackets" and continue to feed in spring. Damage to turf may be most noticeable in March, April, and mid-May. In about mid-May, depending on weather, they stop feeding and pupate, staying underground in a non-feeding stage until they wiggle to the surface and come out as adults in August and September, to start the cycle all over again.
A fall infestation may not lead to spring damage, because birds and other pests, as well as the variations of winter weather conditions, may kill off many of the leatherjackets before they can harm the turf. If, however, a serious infestation is seen in March and April, consider chemical treatment, only after counting the larvae and determining that treatment is really necessary. An isolated spot on the lawn may not require chemical treatment of the whole lawn.
One newer treatment for crane fly larvae is a biological agent, a predatory nematode sold as "BioSafe" or "Scan-Mask" or "Exhibit." This is a living predator that is mixed with water and poured on the affected areas. The nematode invades and kills the crane fly larvae. Tests done with predatory nematodes at the Washington State University Center in Puyallup showed that about 40% of the existing larvae were eliminated by the biological control.
Chemical treatments for crane fly control do exist, and can be applied during mid to late April on home turf if it's certain there is a definite infestation. The chemical registered for home use on crane fly is chlorpyrifos, sold as "Dursban." If used, follow label instructions exactly. Larvae may appear on the surface after treatment and may not die immediately. Don't take this as an indication that more chemical needs to be applied. This chemical, often sold in a granular form, is toxic to humans, pets, and birds, and should be used as a last resort when insects are present in damaging numbers, not as a regular application for crane fly prevention. Take care when spreading to avoid scattering granules on walks or areas where they can be encountered by children or pets.
On the whole, in dealing with crane flies, it's preferable to rely on natural controls such as birds and on keeping the lawn in a healthy condition so it can fight infestations.
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