Controlling Crane Flies

Q: I'm confused about control of crane flies. When do I get after the critters?

A: We asked two Washington State University Cooperative Extension specialists - Art Antonelli, extension entomologist, and Gwen Stahnke, extension turfgrass specialist - to address this important and common question.

They recommend control measures in early or mid-April and discourage winter pesticide applications for several reasons:

-- Crane fly larvae do little damage to turfgrass in fall and winter.

-- Early researchers found that more than 50 percent of over-wintering larval populations succumb to diseases and other problems and do not survive until spring, when most of the damage due to feeding is done.

-- There are no data that show winter control measures are effective; fall control measures, however, are effective.

-- Insect-eating birds that remain in our area all winter have learned that crane-fly larvae are an easy, concentrated winter food source. Many birds have died from poisoning while dining on them after a winter insecticide application.

There can be selected situations where fall or even winter applications might be acceptable and even recommended. Fall applications for control of young larvae on golf greens is expedient, since even small populations can interfere with putting-green quality. Since there is only one generation per year, there would be no need to treat the following spring.

This is probably the only time we would recommend fall treatment unless lawns were badly damaged during the growing season (usually in areas of our recent crane-fly invasion) and homeowners realized too late what was happening and failed to treat in the spring. Under these circumstances, an application in early or mid-October may make sense if crane-fly populations were very high going into the fall.

Crane flies overwinter as larvae in the ground at fairly shallow depths with a weak hibernation that is temperature-driven. Western Washington can have mild spells in winter. If these last several days, the larvae wake up and feed. If populations are high, 30 or more larvae per square foot, serious damage could occur. This is the rare case when winter treatment would be warranted.

The standard recommendation, given a normal winter, would be to assess the crane-fly population numbers in late March and treat only if needed. Make only one application if applying an insecticide. Alternative biocontrols with nematodes or insect growth regulators have been shown to reduce larval population numbers by up to 50 percent, which could bring the populations below the damage level, but they will not eliminate crane flies completely.

Following a balanced fertilizer program has been shown to improve the turfgrasses' ability to grow away from potential damage caused by feeding crane-fly larvae and eliminates the need to make an insecticide application in many cases.

Gardening runs Friday in Scene and Sunday in Home/Real Estate. It is prepared by Mary Robson, Master Gardener program director; Holly Kennell, Washington State University/King County Cooperative Extension agent; Susan Miller, integrated pest management specialist, and volunteer Master Gardeners.