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. In this article, we will refer to the moth using its scientific name and the acronym LDD.

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

LDD 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.

Leaf me alone, moth!

Having eaten their fill, the caterpillars will pupate in July

LDD 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, and there are no insecticides currently available for broad-scale use in Ontario that target only LDD Moths. Bacillus thuringiensis (or Btk), the product commonly used to control LDD Moth outbreaks, can kill other Lepidoptera (butterflies and moths) caterpillars when the spray is applied. Proper application of Btk, particularly with respect to timing of LDD life stage, is important to minimize potential impacts on other species.

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 LDD 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 deciduous trees are quite resilient to these leaf-munchers.

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 LDD 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. Cold winters are another LDD Moth nemesis; their eggs can freeze when temperatures drop below -20oC for prolonged periods.

As an outbreak progresses, more devastating natural controls begin to kick in. Two diseases, Nucleopolyhedrosis virus (also known as NPV) and a fungus called Entomophaga maimaiga easily spread through LDD Moth populations during outbreak years. Between predation and disease, populations usually collapse within a couple of years.

The Ministry of Northern Development, Mines, Natural Resources and Forestry monitors LDD 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 LDD Moth

Do what you can to reduce your carbon footprint. The effects of invasive species like LDD Moth can be made worse by climate change. LDD 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. LDD moth eggs don’t survive 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 LDD 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

How do I protect myself and my campsite?

As for 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.

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.