Cankerworms Are Prevalent in Mount Vernon

It’s a little dull grayish-brown bug that can devastate trees.

The fall cankerworm (Alsophila pometaria) is “very prevalent” in the Mount
Vernon, Huntington and Franconia areas, but not other parts of the Fairfax County, county urban forester Joan Allen recently said. No one knows why they are concentrated in the eastern part of the county at this time.

This winter, county staffers are trying to determine whether fall cankerworm populations are reaching dangerous levels and where. In the past ten years, these insects have defoliated up to 5,000 acres in the Mount Vernon area.  (The spring cankerworm, Paleacrita vernata, is a different species.)

Cankerworm population levels were high enough in early 2012 in the Mount Vernon-Franconia area to warrant spraying 115 acres over 10
days.  The population estimates were based on a survey conducted between December 2011 and January 2012.  The last time county officials found a significantly high enough population to require spraying was 2003.

Severe fall cankerworm infestations can cause widespread defoliation, stress trees and possibly kill trees. Cankerworm caterpillars emerge in the spring and for a month or so and eat the leaves of a wide variety of trees, but they tend to prefer maples, hickories, ashes and oaks, all of which are found throughout Fairfax County.

Determining the Numbers

This winter, staffers from the county’s Urban Forest Management Division are surveying to determine population levels at 300 banding stations in Mount Vernon, Franconia and Huntington. Foresters try to trap adult moths as they emerge from the soil. Surveyors attach black roofing paper to a tree with duct tape and apply an adhesive, called Tanglefoot, with a caramel consistency onto the paper. When the female moths climb up the tree to mate and lay eggs, they can get trapped in the Tanglefoot. Male moths fly into the adhesive too. 

County officials try to get permission from private property owners to put up banding traps and most agree, according to Allen. Traps are designated by a sign identifying the project.

Cankerworm Spraying

For areas determined to have problematic population levels, the county’s Forest Pest Management Branch sprays from spray trucks using an insecticide, a substance called Bacillus thuringensis, in mid- to late April when leaves start emerging.  The spray is naturally found in soil as bacteria and is developed in the laboratory.  It affects caterpillars of moths and butterflies, but not other insects.  County officials try to notify property owners before spraying begins and people can opt out.

Cankerworms are one of several stressors on urban trees, said Allen.

The Fall Cankerworm Cycle

Adult moths appear in the late fall. For an insect, emerging in the late fall and winter is very unusual. Most emerge as adults in the spring and summer. 

The male cankerworm moth is brownish gray and has wings. The female moth is dull gray and has no wings. She emerges from the soil, climbs up tree trunks, awaits males, mates and lays eggs. The moths and their egg masses can be found on the trunks of trees in January and February. The eggs develop over winter and hatch in late April to early May.  

Young cankerworm caterpillars (one-quarter to one inch long) feed on leaves and once they mature, descend to the ground on threads of silk, burrow into the soil, spin a cocoon and pupate.  The pupae stay in the soil until fall and the cycle begins again.

According to the Prince William County government’s website, Prince William is surveying fall cankerworms as well.

For more information, visit http://www.fairfaxcounty.gov/dpwes/environmental/canker4.htm and http://www.pwcgov.org/government/dept/publicworks/Pages/Fall-Cankerworm.aspx

To report cankerworms online in Fairfax County, visit http://www.fairfaxcounty.gov/dpwes/surveys/fallcankerworm.htm.