Gypsy moth management

Gypsy moth is once again making itself known in rural and urban forests as larvae are spotted crawling up tree trunks, feeding on leaves and moving across the ground searching for new trees to infest. Gypsy moth larvae will feed on leaves of oak, aspen, willow, alder, larch, mountain ash and other tree species. Significant defoliation can stress trees even where rainfall is adequate.

Many over-wintering tanned egg masses of gypsy moth were found this past fall and spring in areas around the province and entomologists are expecting lots of defoliation during June. In early June, larvae will range from just hatched, black, about ¼ inch long to about 1 ½ inches long with black hairs along the length, 5 pairs of blue spots and 6 pairs of red spots along their backs.

By mid to late June larvae can grow up to 2 inches long before pupating in July. One larvae can consume 12 to 15 large oak leaves to maturity. Adult moths will emerge later in July and August. After mating, each female moth will deposit only one egg mass of 100 to 1000 eggs in a foamy tan coloured mass which will lie dormant until next spring.

Some trees can re-grow a new canopy after springtime defoliation, however repeated or severe defoliation can kill trees. Young trees can be defoliated overnight by larvae that hideout in ground cover during the day. Where larvae are numerous, large quantities of fecal matter falling from high atop deciduous canopies, bouncing off leaves and hitting ground cover or roof tops can make it sound like it is raining on a clear day. Older larvae will feed day and night, while the younger stages are mostly nocturnal.

City planners and forest managers may implement aerial spray programs this time of year to help reduce the populations of young gypsy moth larvae. A good website, as one example of a gypsy moth management strategy, is the Hamilton Conservation Authority's gypsy moth web-page.

In addition to lifecycle photos of gypsy moth and biological information, the web-page explains where and when aerial sprays are being applied with convenient maps, and the pest control product that is being sprayed. Each region has its own plan.

Typically, the organic insecticide Bt (Foray or Dipel) is applied by helicopter or aircraft for forest cover, air blast ground sprayers for tree nurseries, and is effective at reducing populations of the youngest gypsy moth larvae, while protecting beneficial species and other non-pest insects. For older stages of larvae in tree nurseries, the insecticide Spinosad (Success) may be effective and has some contact killing properties.

For methods of how to control gypsy moth on yard trees or in small deciduous plantations using non-chemical methods, the Hamilton CA website also presents guidelines and pictures on how to reduce gypsy moth infestations and protect trees from extensive defoliation.

Newly hatch larvae

Late instar larvae

Learn more

• Hamilton Conservation Authority's gypsy moth web-page