Right on schedule , European craneflies are being reported in prolific numbers throughout our coastal area. Blame it on our early fall weather, or short summer-whatever the reason, populations this year appear to be much larger than normal. Dozens of reports have come into the WSU Extension office in Grays Harbor about hundreds if not thousands of these notorious lawn insects being found on porches and house walls.
European crane flies are a common insect pest that infests both lawns and pastures. Sometimes called “mosquito hawks”, adult crane flies look like large mosquitoes with bodies about an inch long, not including the legs. Adult crane flies do not bite or sting, nor do they cause damage to lawns.
Adult crane flies emerge form the soil in mid to late September, with the females mating and laying eggs within 24 hours. The eggs develop into worm-like larvae known as “leather jackets”, nicknamed for their tough skin. The leatherjacket stays underground during most of the day. However, it is not unusual for them to surface and feed on above ground parts of plants on a warm night. As larvae, the crane fly feeds on grass roots during the fall and throughout winter when conditions are favorable. Lawn damage caused by leatherjackets is noticeable in March and April. Feeding usually stops in mid-May and early June. High numbers of larvae can cause dead patches in lawns.
The first step to managing crane flies is to determine if they’re a problem by scouting your lawn. If you’re in doubt as to whether your lawn is infested with the European Crane fly, simply take a flashlight and examine the lawn at night or take a soil sample. Select three or four random areas on the lawn. Measure a 12 inch square. Cut three sides of the square and peel it back. Dig up the top layer of soil to a depth of 1 to 2 inches and count the larvae. The larvae are usually found at the base of the vegetative layer (thatch) or very shallow in the soil. In areas of heavy infestation where the lawn is quite bare, the larvae will be found deeper in the soil than in spots where some green grass remains.
Fortunately birds have discovered European craneflies are good eating and as a result most leatherjacket infestations are quickly consumed. WSU entomologists recommend insecticide treatments containing imidacloprid or cyfluthrin only when cranefly larvae populations exceed 25 per square foot and birds do not appear. Healthy lawns that have had best management practices applied have been known to have 40 larvae per square foot and still not show any damage
Lawns severely damaged by cranefly feeding can be renovated by power raking to remove the thatch and over-seeding with a mixture of 50% fine fescue and 50% ryegrass. Use 3 to 4 pounds per one thousand square feet of lawn area. In addition, power raking or aerating the lawn cuts and destroys leatherjackets, further eliminating the need for spraying.