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Spongy Moth
European Spongy Moths have been a problem in Prince William County since the early 1980s. Accidentally released in Massachusetts in 1869, they have been slowly spreading across the North American continent since then. They are now found across almost the entire state of Virginia.
Spongy moths go through the same life phases as butterflies: eggs, larvae, pupae, and adult. They really cause problems in the larval or “caterpillar” phase. They can completely defoliate large trees as they feed on leaves, growing and building up energy to pupate. Trees can die after just one defoliation if the tree is already stressed due to drought or other insects and diseases, but most healthy trees must be defoliated for 2-3 years in a row before they die.
Spongy moth populations have remained low for the past few years in Prince William County. Their population levels naturally increase and decrease, but we may also be getting help from a fungus, Entomophaga maimaiga, that infects the larval or caterpillar stage, killing them. The fungus was originally released in 1910, and again in 1980 in attempts to help slow the spread of spongy moths. Both attempts were thought to be failures. In 1989 spongy moth populations in Connecticut were drastically reduced, and the fungus was the cause. Since then it has been found in many places in most years, and really likes wet weather. Hopefully, as long as we have rainy Mays, we will also have low spongy moth populations!
What is Prince William County Government doing to help?
The Mosquito & Forest Pest Management Branch monitors for spongy moths every year from August through November to assess the potential populations for the next year. This is done by counting egg masses. In years when the populations are high enough, Prince William County participates in a voluntary aerial spraying program coordinated with the Virginia Department of Agriculture and Consumer Services and the USDA Forest Service. All landowners are notified in advance, and may opt out of spraying if desired.
What chemicals are used on spongy moths?
The most commonly used insecticide is Bacillus thuringiensis kurstaki, or BTk. BTk is a naturally occurring bacterium that is effective at controlling spongy moth caterpillars. It does not harm humans or pets, but can kill other caterpillars present at the time of spraying.
Other insecticides are also available for use in fighting spongy moths.