Anyone who has been sitting along the shore of Mazinaw in June will have been peppered with frass (caterpillar poop) and will have witnessed the extensive defoliation caused by Gypsy moth caterpillars. This infestation is not unique to Mazinaw and has had widespread impact throughout southern Ontario. Although oak leaves are a favourite food for these caterpillars competition for food resulted in them attacking many other deciduous trees and even white pine. Deciduous trees often have a second budding of leaves and will generally survive such an insult if it occurs no more than 2-3 years in a row. Pines however are less resilient and may succumb to these caterpillars if the loss of needles on any given tree is extensive.

The hatch of Gypsy moth eggs in the Mazinaw region occurred between May 22-25 this year and the caterpillars reached maturity and began to pupate in late June and early July. Currently there is an enormous population of (small brown) male Gypsy moths searching for flightless (white) female mates. Egg masses will be laid over the next month and these eggs will overwinter in preparation for a repeat next spring.

Why now?

Gypsy moth outbreaks seem to occur in cycles of 10-15 years. They were transported to Massachusetts from France in the late 1860s with the hope that gypsy moths would interbreed with native silk moths to produce a variety of silkworm that was not susceptible to a troublesome fungus. Numerous moths escaped and small outbreaks occurred in the immediate region with gradual spread to many parts of north America over the next 100 years.

What can be done to control Gypsy moths?

Many measures have been tried to eradicate gypsy moths with little success. Local measures during small infestations can limit the damage and may be worth trying in years between major outbreaks. These include 1) aerial spraying of Bacillus Thuringiensis, 2) capturing and killing caterpillars that hide overnight in a burlap wrap around individual trees, 3) luring males into traps using a female pheromone, 4 )scraping egg masses off trees in the late summer and fall into a can and either immersing them in soapy water or burning them.

When an outbreak is as extensive and advanced as it is this year options are limited. Spraying must be done in the 2-3 weeks after hatching for optimal results. The BT bacteria degrades within 1 week in sunlight so repeat spraying may be required. Different varieties of BT target different insects and although the variety of BT for Gypsy moths has shown no direct adverse effects on birds, fish, animals, or humans there is uncertainty about how it may affect other caterpillar species. It is unlikely that killing caterpillars or trapping males will have a significant impact this year as the number of insects is just too great.

What does the future hold?

Fortunately, in 1989, a fungus that specifically attacks Gypsy moth caterpillars (E. maimaiga) made its appearance in North America.  E. maimaiga is very specific to the gypsy moth and in the years after 1989, it spread rapidly at more than 100 km per year until it infected gypsy moths in all parts of its range. The fungus has been a very effective control agent, depressing gypsy moth populations at all density levels. Part of its effectiveness has to do with the longevity of the resistant resting spores. These may remain alive for up to 10 years.

It is likely that this fungus is already in our region as many dead caterpillars can be seen on and around trees on Mazinaw.  Because of this fungus, the pest will become much less noticeable than in the past. Experts acknowledge that outbreaks will probably still happen, but the large potential for dispersal of E. maimaiga and the huge, long-lived reservoir of resting spores in the soil will mean that such outbreaks are likely to be much smaller in the future. (See Weseloh article for details).

What is the Ontario government doing now?

Little to nothing! After reaching out to the government of Ontario we were provided with a 1-800 number to call that didn’t work, and another email to register our findings on the Invasive Species Registrar. The local Bancroft MNRF District Manager has yet to respond.

The Belleville Intelligencer reported that the MNRF is conducting aerial and land surveys to gauge the extent of the outbreak.

Reference:  Ronald Weseloh. People and the Gypsy moth. American entomologist Fall 2003

By Bob Reid and Vern Haggerty