Gypsy Moth Diagnostic Guide
Gypsy moth, Lymantria dispar, is one of the most significant exotic pests in the history of the US. Introduced to Boston in the 1860’s, it has continued to spread throughout the eastern United States. The gypsy moth larvae defoliate trees leaving them weakened and vulnerable to secondary fungal and insect invaders. Repeated defoliation of hardwoods can kill the tree directly and a single defoliation may cause severe dieback and decline on softwood species especially when coupled with drought or other abiotic stress. Gypsy moth will affect trees in natural settings, forest plantations, and urban environments, often defoliating thousands of trees in a single outbreak.
Trees at Risk
Gypsy moth is known to feed on hundreds of woody plant species. Preferred species are oak, aspen, willow, linden, hawthorn, apple, and alder. Less preferred species include elm, maple, hickory, beech, hemlock, pine, spruce, cedar, and sassafras. Less preferred hosts are usually fed upon when gypsy moth populations are high and preferred species become scarce.
Signs of Damage
- Shot holes in leaves beginning in spring resulting in partial or complete defoliation by midsummer.
- Crowns of trees will be thin initially and will be partial to completely defoliated under heavy pest pressure.
- White, 1 ½ inch long, webby egg masses on trunks and limbs.
- Young caterpillars are black with orange spots on the back.
- Mature caterpillars grow up to 2 inches long and have five pairs of blue spots and six pairs of red spots in rows across its back.
- Pupae are tear-drop shaped and brown.
- Eggs hatch and larvae emerge as leaves begin to unfold in the spring.
- Young larvae feed in April and May and Caterpillars feed in spring and summer.
- Caterpillars pupate early to mid-summer.
- In summer, hair covered egg masses are laid in crevices, under picnic tables, and on vehicles.
- Over wintering takes place in these egg masses.
- One generation per year.
Heavy pressure from gypsy moth has the potential to cause mortalities on shade trees especially in stressful urban sites where gypsy moth feeding is coupled with stress from abiotic factors. Healthy trees can tolerate a single defoliation event; however, multiple defoliation events can cause dieback and decline even on healthy trees. Mortalities on stressed trees can occur after a single defoliation event. Commercial insecticide treatments on individual high value shade trees are the only way to ensure predictable protection during high pest pressure. Threshold levels of gypsy moth are assessed by using egg mass counts in the winter months and can be used by arborists to help make decisions about treatment.
Other Treatment Practices
- Promote health and vigor with proper irrigation, mulching, proper pruning and prescription based fertilization practices.
- While aerial sprays with Bacillus thuringiensis cannot be counted on to provide acceptable levels of control on individual high value urban trees, it is a tool for managing Gypsy moth infestations in locations where large numbers of susceptible tree species are growing in remote locations. Bt is typically applied using aerial spray applications.
- Plant less favorable species like sycamore, ash, tulip poplar, holly, and/or walnuts.
- Lepitect™ applications provided control of gypsy moth larvae in less than five days on smaller trees. Control on larger trees will not occur as quickly (2-3 weeks). Lepitect™ soil applications last for 30-45 days after treatment.
- Conserve has a short residual (4-7 days) but is softer on beneficial insects, whereas Up Star Gold will provide slightly longer residual, but is broad spectrum.
- Use Lepitect infusible on trees that cannot be sprayed or treated with soil applications.
A Diagnostic Guide is designed to help you identify a pest issue and management solutions. Always refer to product label for all rates and approved uses. Some images courtesy forestryimages.org. Use of the images does not imply endorsement of treatments by forestryimages.org