Crane fly larvae, commonly called leather jackets, are small, grey worms that eat the roots of grass and clover. Larvae hatch in the fall, but their damage often goes undetected until patches of dead grass appear in spring. While adult crane flies are harmless, the larvae can damage lawns and pastures.
Keeping grass healthy and well-fertilised is the best preventive to crane fly larva damage. Strong, vigorous grass usually recovers without chemical treatment. In addition, birds such as robins and crows provide natural pest control by eating crane fly larvae.
If chemical treatment is necessary, Oregon State University recommends applying insecticide in October or November before significant damage occurs. Insecticide can also be applied in the spring, but this will not prevent lawn damage. Use pesticides carefully and sparingly to protect birds and other non-target species.
Certain nematodes are natural enemies of crane fly larvae and can be used to control infestations. Beneficial nematodes, which are microscopic worms, are available at garden centres and should be applied to the lawn in March or April. According to Thurston County Public Health and Social Services, correct use of nematodes can reduce crane fly larvae by 50 per cent.