Lymantria dispar caterpillars

The gypsy moth (Lymantria dispar dispar) is an invasive insect that originates from Europe and was introduced as an experiment intended to develop a silk industry in North America. The experiment failed but the gypsy moth remained and has taken hold in its new home. It feeds on tree foliage as its population cycles from an ever present, but manageable low to alarming highs associated with cyclical outbreaks.
Gypsy moth populations have been building in Ontario since 2019, first in the southwest and now across large parts of southern Ontario. Forested areas from Cobourg to Belleville had severe gypsy moth defoliation in 2020 and are expected to be equally hard hit this summer, unless mother nature decides otherwise.
You may have noticed gypsy moth egg masses on your trees this winter. They resemble a quarter-sized mass of beige felt which covers up to a thousand eggs in which tiny gypsy moth larvae, or caterpillars, have spent the winter. They emerge from the eggs in May and by the time the spring leaves have unfurled, they are making their way to the tips of branches to feast and grow.
During a severe outbreak, your trees may be eaten bare. Fortunately, the caterpillars soon mature (July) and wrap themselves up in a brown tear-drop shaped pupal case to begin their transformation into their adult moth form. And the feeding stops. Your deciduous trees begin to recover and put out new leaves to replace the ones that were eaten. Gypsy moth caterpillars may also feed on coniferous trees, especially pines. These cannot replace their needles once they have been eaten and will likely die if the defoliation is extensive and/or repeated over several years.
female Lymantria dispar with egg masses on tree
During an outbreak, you can’t get rid of the gypsy moths, you have to wait it out. But there are things you can do to protect valuable trees on your property. The City of Toronto website has excellent instructions on how and when to manage gypsy moth:
https://www.toronto.ca/services-payments/water-environment/trees/forest-management/threats-to-trees-insects/european-gypsy-moth/
A gypsy moth outbreak may last 2 to 4 years. As the population density reaches its peak, diseases and parasites begin to overcome the gypsy moths and the population begins to decrease rapidly. Nucleopolyhedrosus virus (NPV) is a naturally occurring virus that is the major cause of gypsy moth population collapse. Entomophaga maimaiga (EM) is a fungal disease that was introduced to help tackle the gypsy moth problem. Its spreads readily during cold wet weather and can also lead to a population collapse. A species of wasp called Ooencyrtus kuvanae is known to parasitize gypsy moth eggs and can reduce gypsy moth populations.
You can find out more about gypsy moth, including mapping of the current Ontario outbreak, at https://www.ontario.ca/page/gypsy-moth or by sending an email to .

Lone Pine Land Trust

Lone Pine Land Trust is a non-profit charity based in Northumberland County.