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Learn About Fall Cankerworms in Spring Archived Discussion Room

Fairfax County, Virginia

Learn About Fall Cankerworms in Spring

Contrary to its name, the fall cankerworm can be destructive to local trees and plant life during the spring when caterpillars are active. Over the last couple of years Fairfax County has experienced outbreak population levels of this pest. This year, staff have detected population levels stabilizing and suppression efforts this spring will be minimal. On Thursday, April 9 at 11 a.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 Morning! 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 this years small suppression program.

Anonymous User : What is so bad about fall cankerworm?

Joan Allen : Fall cankerworm is a native pest in North America. As a caterpillar, they feed on the leaves of many hardwood trees. Normally, when their populations are low their impacts on trees are minimal. They also provide a source of food to wildlife like beetles and birds. Unfortunately, there are times when cankerworm populations are high and they can damage trees by eating many of their leaves and defoliating them. In this case, predators do not provide sufficient control and control options may need to be considered.

Anonymous User : How can I identify a tree that has been impacted by fall canker worm? Will my tree die?

Joan Allen : Fall cankerworm caterpillars feed on leaves of hardwood trees. Depending on how many cankerworm caterpillars you have in your area, the impacted leaves can appear to have small holes in them to most of the leaf eaten. Pictures of leaves impacted by cankerworm can be found on

Anonymous User : Why is the fall canker worm found in some parts of the county but not others?

Joan Allen : Fall cankerworm can generally be found throughout the county. Historically, high populations have been found in the southeastern region of the county such as in Mount Vernon, Lee, and Mason Districts. It is unknown why fall cankerworm is found more predominantly in this region than others.

Anonymous User : I've heard that we are not supposed to transport wood infested with emerald ash borer. Do we need to be concerned about transporting wood or leaves that have been infested with fall cankerworm?

Joan Allen : Generally, there should not be a concern with transporting fall cankerworm through the transport of firewood. Fall cankerworm caterpillars normally move long distances by wind on a silk thread.
Other pests can be transported via firewood such as gypsy moth, emerald ash borer, and thousand cankers disease. For up to date information on the national firewood quarantine visit:

Anonymous User : How will I know if/when my trees will be treated?

Joan Allen : This year the county is conducting a small ground spray suppression program. We will be treating approximately 66 acres in Mount Vernon and Lee Districts. Last month, we sent letters to effected land owners notifying them of the suppression program. This suppression program is voluntary and residents can opt out. If you would like to know if your property will be sprayed for fall cankerworm you can contact Urban Forest Management Division at 703 324 1770 and

Anonymous User : Is there anything I can do as a private citizen to prevent fall cankerworm from infesting my trees?

Joan Allen : If you are interested in protecting your trees from fall cankerworm damage visit for homeowner alternative control options.

Anonymous User : Is the spray used to treat fall cankerworm harmful to my pets?

Joan Allen : The material being used for this years suppression program is Btk (Bacillus thuringiensis kurstaki). It is registered by the EPA for use to control forest pests in residential and forested areas. Btk will only directly harm caterpillars when applied.

jeff : when will spraying take place?

Joan Allen : We estimate the spraying program to take place sometime in late April to early May. Spraying for fall cankerworm is dependent on leaf development and weather conditions.

Anonymous User : Why are the populations stabilizing? What do you do to determine if the population is changing?

Joan Allen : The Urban Forest Management Division monitors fall cankerworm populations every winter using monitoring bands. By using this method we have some idea of what level of damage to expect from fall cankerworm caterpillars. Between 2012 - 2014 we have noticed an increase in fall cankerworm population. Based on last winter's survey we expect fall cankerworm population to decline.

Anonymous User : Where can I go to get more information on the fall cankerworm?

Joan Allen : For more information on fall cankerworm visit

Anonymous User : What happens after my trees have been treated?

Joan Allen : Once Btk is applied to trees it has a relatively short effective time. Btk breaks down with exposure to UV light and rain.
For more information on Btk visit
Urban Forest will be monitoring for the effectiveness of the spray program and defoliation this May.

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