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Don’t Go Buggy: Learn About Fall Cankerworms in Spring Archived Discussion Room

Fairfax County, Virginia

Don’t Go Buggy: Learn About Fall Cankerworms in Spring

Contrary to its name, the fall cankerworm can be destructive to local trees and plant life during the early spring when caterpillars are active. In normal years the fall cankerworm causes little to no damage, but Fairfax County is experiencing a dramatic increase in population levels which, if left unchecked, may put hundreds of acres of county trees at risk. On Wednesday, April 9 at 2 p.m., join Joan Allen, urban forester in the Urban Forest Management Division, to discuss what fall cankerworms can do to our environment, how Fairfax County is addressing the problem, and what you can do to help.

Joan Allen : Good afternoon. My name is Joan Allen and I am an Urban Forester with Urban Forest Management Division. I am here to answer any questions or concerns with fall cankerworms and the county's aerial suppression program. We estimate that the aerial suppression program will start sometime in the 3rd or 4th week of April (dependent on weather conditions and tree leaf development).

sam : Are these the worms that make webs in trees and devour that tree? These are very destructive. Anything being done about these annual worms, particulary in the Burke Lake Park area and between Old Keene Mill and Lee Chapel Roads along the parkway?

Joan Allen : I believe you are referring to the eastern tent caterpillar. These are very commonly seen in the spring time on cherry, plum, and apple trees. While they may be unsightly they are not harmful to trees and provide a source of food to wildlife.
There is no county suppression program for eastern tent caterpillar. For more information for control contact the Virginia Cooperative Extension Program

Tish Tyson : How does ingesting cankerworms that have been treated with BT effect birds? Also, how does the reduction of the worm populaiton effect migratory bird food supplies? What other insects will ingest BT and be killed by it?

Joan Allen : Bacillus thuringiensis (Bt.) is a insecticide that can be used to suppress populations of caterpillars. It is registered by the EPA for use to control forest pests in residential and forested areas. We believe that this material will only directly harm caterpillars during the time of spraying. The county will only spray areas that have been found to have elevated populations of fall cankerworm and pose a threat to trees.

While Bt only directly harms caterpillars, there may be a shortage of food available to birds in the limited areas being treated. Fall cankerworms are known to exist throughout the county and will be available for wildlife for food.

Anonymous User : We forgot to band our large willow oak this fall. Is there anything we can do now to reduce this year's population in our tree?

Joan Allen : I am very glad to know that you are taking good care of your trees. In a week or so you should expect to see the caterpillar stage of fall cankerworm.

If you are concerned with the potential threat fall cankerworm may pose to your trees we recommend you hire a tree care company to review the trees on your property. For recommendations on hiring an arborist or tree care company:

For tips on homeowner options for controlling fall cankerworm:

Tony : Does BT have any adverse effects on Lepidoptera species and Worm-eating Warbler?

Joan Allen : Bacillus thuringiensis (Bt) is a commercially available insecticide that is used to control populations of caterpillars such as fall cankerworm. If it comes in contact with Bt. species of lepidoptera, a order of insects that includes moths and butterflies, are affected. For that reason, Urban Forest Management limits the application of Bt. to only areas that have elevated populations of fall cankerworm that pose a risk to defoliating trees or eating all the leaves off the trees.
The worm-eating warbler and other birds that feed on caterpillars as a food source may be limited in areas that are being aerially treated. Populations of fall cankerworm are found throughout the county will be available as a source of food to wildlife.

For more information on Bt.: 

Anonymous User : Was there any IPM strategies followed before deciding to use a control? In other words, were there thresholds that were reached to make an application necessary?

Joan Allen : Great question!
We use many Integrated Pest Management (IPM) strategies in the county's fall cankerworm program. Every fall urban foresters set up many monitoring tree bands across the county to gauge the population levels. The data we collect determines whether it is necessary to conduct an aerial suppression program the following spring. If we feel that it is necessary we also conduct an aerial survey to determine the levels of defoliation (or leaves eaten from trees) within the county.

We use many methods to alert the public regarding fall cankerworm and our program including: the county website, publications, public meetings, newspaper ads and articles, podcasts, and several news releases.

Jim Lahey : Why does the county treat only a few areas for cankerworm and not large portions of the county like they did for gypsymoth?

Joan Allen : We only treats areas that we have found through our surveys to have large populations of fall cankerworms. Surveys have always been a part of determining where aerial suppression programs take place.

Anonymous User : Is this insecticide dangerous to any species of birds?

Joan Allen : Bacillus thuringiensis (Bt) is only harmful to caterpillars when ingested. Wildlife that eat caterpillars that have ingested Bt. are not adversely effected.

For more information on Bt.: 

Joan Allen : Thank you everyone for your thoughtful questions. I enjoyed chatting with you today. For more information regarding fall cankerworm and the suppression program visit