caterpillar on tree

The very hungry caterpillars

Note: this blog is about the non-native, highly invasive moth species Lymantria dispar dispar, which we have previously referred to as the Gypsy Moth or by the acronym LDD. In this article, we will refer to the moth using its new common name, Spongy Moth.

If you’ve seen an Ontario oak tree recently, you’ve likely been introduced to the invasive Spongy Moth (Lymantria dispar dispar).

Spongy Moth caterpillars were first introduced to North America in the late 1860s and are voracious eaters! Their favourite cuisine is oak leaves, but in particularly bad outbreak years — like this one — they can spread to many other tree species.

How do I protect myself and my campsite?

We know many visitors have questions about this, and may be wondering what to do if there are Spongy Moth caterpillars in the park during their visit.

When it comes to your camping comfort, avoid handling the caterpillars with your bare skin. They don’t bite, but their hairs can be irritating and cause rashes.

You should also bring a tarp or dining shelter for your campsite sitting area. Tree leaves are high in fibre, and these caterpillars are certainly feeling the effects.

Leaf me alone, moth!

Having eaten their fill, the caterpillars will pupate in July

Spongy Moth caterpillars (or larvae) change as they grow. Young caterpillars emerge from egg masses in late May and feed on leaves until early July. Mature caterpillars can be as long as 2.5 inches.

As the caterpillars go through their life cycle, trees can seem to lose their leaves overnight. This often makes campers and visitors concerned about the health of our park trees and ecosystems.

While insecticidal sprays do exist, they are very expensive.

Bacillus thuringiensis (or Btk), the product commonly used to control Spongy Moth outbreaks, can kill other Lepidoptera (butterfly and moth) caterpillars when the spray is applied. [1] [2]

Although the timing and method of application can minimize impacts to some species such as Monarchs, Ontario is home to over 1600 butterfly and moth species, many of which are in their larval form at the same time as Spongy Moth caterpillars.

Butterfly and moth caterpillars are critically important in June – they are what nearly all of our breeding bird species feed their young. A single Black-capped Chickadee pair needs thousands of caterpillars to raise a single clutch of nestlings. Many of the butterfly and moth species themselves are also rare and in need of protection.

Decisions around managing invasive species often require balancing several land values. Other land managers may decide that spraying is necessary to protect their trees.

Ontario Parks has decided not to risk potential impacts to other lepidoptera, so there are no plans to spray for Spongy Moth in any provincial park this year, but spraying may occur outside of park boundaries. Ontario Parks’ ecologists and park managers will continue to monitor the situation closely in the future.

Should we be worried about park trees?

Fortunately, healthy trees are quite resilient to these leaf-munchers. [3]

Although losing their leaves means that trees won’t be able to grow much this year, many will develop a second or even third flush of leaves later in the season.

Our trees also have an army of natural defenders. Birds like Yellow-billed and Black-billed Cuckoos, Blue Jays, Orioles, and Eastern Towhees find Spongy Moth caterpillars delicious, while small mammals like mice and squirrels will eat the pupae.

Beneficial wasps parasitize egg masses, and Black-capped Chickadees feed on the eggs through the winter. [4] Cold winters are another Spongy Moth nemesis; their eggs often don’t survive when temperatures drop below -20oC for prolonged periods. [5]

As an outbreak progresses, more devastating natural controls begin to kick in.

Two diseases, Nuclear polyhedrosis virus (also known as NPV) and a fungus called Entomophaga maimaiga easily spread through Spongy Moth populations during outbreak years. [6] [7] Between predation and disease, populations usually collapse within a couple of years.

The Ministry of Northern Development, Mines, Natural Resources and Forestry monitors Spongy Moth outbreaks in Ontario. You can find their survey results and more information about this species on their website.

How can I help?

This is something our trees have seen before, and they will see it again. But there are many things you can do to help:

Gypsy Moth
Female Spongy Moth

Act on climate change. The effects of invasive species like Spongy Moth can be made worse by climate change.

Spongy moth outbreaks are associated with warm winters, while cool and wet spring conditions promote the spread of naturally occurring fungi in the population that play a key role in population collapse in outbreak years. Climate change disrupts the climate patterns that keep many invasive, or potentially invasive species, in check.

Spongy Moth eggs can be killed in sustained cold temperatures, so continued warm winters may lead to more frequent outbreaks in the future or extra stress on trees.

Don’t move firewood long distances. While Spongy Moths, Emerald Ash Borer beetles, and other pests may already be living at your destination, there are many others we want to keep out. A single piece of infected firewood can destroy millions of trees. Purchase your firewood at the park office when you check into your campsite or buy from a supplier local to the park.

Educate yourself and others about threats to our trees and forests. In particular, our now-weakened oak trees are in danger from Oak Wilt, an invasive fungus just across the Canada-US border.

To help prevent infection, landowners and managers should avoid pruning or cutting their oak trees between April and July. When visiting Ontario Parks alert a park staff member if you see an oak tree with leaves dropping in July or leaves that are browning at the tip.

Gypsy moth

An ounce of prevention is worth a pound of cure

Once an invasive species is established in an ecosystem, control efforts can sometimes do more harm than good. In this case, we should learn the lesson that prevention is better than cure and focus on keeping new invasive species out of Ontario.

We love our trees, but remember: they are just a single part of the biodiversity webs in our parks. Our amazing park staff work with insect and forest specialists to protect all of the ecosystem connections that make life in Ontario possible.

We should remember that when it comes to invasive species, prevention is vital. Show trees you care by keeping new invasive species out of our parks.

[1] Heimpel A.M. and T.A. Angus. 1959. The site of action of crystalliferous bacteria in Lepidoptera larvae. Journal of Insect Pathology 1: 152-170.

[2] Broderick, N.A., C.J. Robinson, M.D. McMahon, J. Holt, J. Handelsman and K.F. Raffa. 2009. Contributions of gut bacteria to Bacillus thuringiensis-induced mortality vary across a range of Lepidoptera. BMC Biology 7(11).

[3] Eisenbies M.H., C. Davidson, J. Johnson, R. Amateis, and K. Gottschalk. 2007. Tree mortality in mixed pine-hardwood stands defoliated by the European Gypsy Moth (Lymantria dispar L.). Forest Science 53(6) 683-691.

[4] McCullough, D.G., K.A. Raffa, and R.C. Williamson. 1999. Natural enemies of Gypsy Moth: the good guys! Extension Bulletin E-2700, Michigan State University Extension.

[5] Andersen, J.A., D.G. McCullough, B.E. Potter, C.N. Koller, L.S. Bauer, D.P. Lusch, and C.W. Ramm. 2001. Effects of winter temperatures on gypsy moth egg masses in the Great Lakes region of the United States. Agricultural and Forest Meteorology 110(2001) 85-100.

[6] Podgwaite, J.D., K.S. Shields, R.T. Zerillo, and R.B. Bruen. 1979. Environmental persistence of the Nucleopolyhedrosis virus of the Gypsy Moth Lymantria dispar. Environmental Entomology 8(3) 528-536.

[7] Andreadis, T.G. and R.M Weseloh. 1990. Discovery of Entomophaga maimaiga in North American gypsy moth, Lymantria dispar. Proceedings of the National Academy of Sciences 87 2461-2465.