This year, Fairfax County will spray approximately 2,000 acres to suppress fall cankerworm caterpillars in the Lee and Mount Vernon districts. Helicopters will spray Bacillus thuringiensis (Bt), a naturally-occurring insecticide, in infected areas in the two districts starting in mid-to-late April, according to a recent news release.
Treatment areas for the aerial spray program are chosen through an annual Fairfax County survey of the fall cankerworm population. Because weather conditions are unpredictable, county officials do not know the exact days when spraying will be conducted. Once spraying begins, it may take ten days or more to complete, depending on the weather.
A detailed map showing these treatment areas is available at public library branches, county government centers in the Lee and Mount Vernon Districts and online. Treatment areas are chosen through an annual county survey of the fall cankerworm population. Typically, spraying is done in the early morning and at dusk. However, it may continue all day if weather conditions permit, the news release stated.
Extensive research has shown that Bt is not harmful to people, pets, plants or beneficial insects, such as honeybees. The spray will not harm painted surfaces, and residues can be removed with soap and water, the news release explained.
Fall cankerworm caterpillars can kill trees by defoliating them. The caterpillars feed on various types of trees but especially on native hardwoods, such as oak. Because the population level will be high enough to defoliate trees, the fall cankerworm must be suppressed with an insecticide, according to the recent news release.
Fairfax County residents are encouraged to complete a four question, online fall cankerworm report form at that will assist urban foresters to determine areas where future monitoring may take place.