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.
Heavy pressure from gypsy moth has the potential to cause mortality of 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. Mortality 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. Contact your local department of Agriculture or local extension agent to find out the current status of the egg mass counts in your area. Gypsy moth populations will typically rise and fall. The rise continues over a 2-4 year period before reaching a peak which is followed by population decline.
When using Lepitect, use the highest dosage rate and during prolonged spring periods reapply a second application 30-45 days later. Lepitect soil applications become less predictable with trees greater than 25-inchs DBH. Use Mectinite or Lepitect Infusible astree injection treatments for trees that cannot be treated with foliar sprays or other systemic options. Trunk injected systemic options are more predictable on trees greater than 25-inches DBH.
Insecticide sprays can be used to treat for the caterpillar stage beginning at bud break in early spring, however, do not use insecticides that are toxic to bees and other pollinators while trees are in flower. Sprays should target the earliest instar stage possible as sprays become less effective on larger caterpillars.
For severe infestations consider applying at least one foliar spray and a systemic option to carry longer residual length of control and to avoid gaps in coverage.
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. Lepitect does require trees to be actively translocating in the spring for the product to move up into the tree. Lepitect has a 30-45 day residual and should be applied 30 -45 days later on large trees > 15 inches or for severe infestations. 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.
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 partially 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
Photo: Louis-Michel Nageleisen
Departement de la Sant des Forts, Bugwood.org
Photo: Milan Zubrik
Forest Research Institute – Slovakia, Bugwood.org
Photo: John H. Ghent
USDA Forest Service, Bugwood.org
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.
- Eggs hatch and larvae emerge as oak leaves begin to unfold in the spring of the year.
- Young larvae feed in April and May and Caterpillars feed in spring and summer (90GDD-507GDD).
- 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.
A Treatment 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 of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments by forestryimages.org