What’s with all these caterpillars?

Prince George, British Columbia, where I live, is in the midst of a forest tent caterpillar outbreak. The number of these little caterpillars has been increasing each year for the past three or so years. And that means that people are noticing them and asking questions. This morning I received a phone call from an affiliated pair of local radio stations and gave them an interview. That has not been my first inquiry on the topic and so, because Twitter is not the best long-term repository for such answers, I’m hoping that this blog post will answer many of the questions that folks might have. I’ll update this post as I receive new questions. So fire away.

What are some of the basic facts about this insect? The forest tent caterpillar is a native species with a range that extends across much of Canada and the USA. They generally present wherever its host trees reside, and they numerous hosts depending upon the geographical locale. Here in western Canada they often feed on aspens and other poplars. They also eat the leaves on some other leafy-tree (angiosperm) species, although there are also some that they avoid. But they do not attack conifers.

Despite their name, forest tent caterpillars do not construct tents. They get their name because they are related to other species – notably eastern tent caterpillars – that do. Forest tent caterpillars do spin silk and often leave large patches of silk in areas where they congregate. Although they do not have tents, they do aggregate in groups and they also move around in little parades following each other from branch to branch and from tree to tree. In some major outbreaks the number of aggregating and parading caterpillars can be so high as to make roadways slippery and dangerous to drive on.

What is the life cycle like? How long will these caterpillars be around? Forest tent caterpillars spend the winter in the egg stage. They hatch out around the same time as bud burst, which means that they have leaves to eat as soon as they leave their eggs. The new caterpillars are quite small, but grow rapidly as they defoliate trees. They move in groups from one tree to the next when food is depleted. The caterpillar stage lasts for a few weeks – usually from about mid-May until the end of June or early-July depending on the local climate. At that point, once they have grown to a good size, they pupate in little cocoons in sheltered locations. The pupal stage lasts a few weeks and then adult moths emerge in July and early-August. The adult moths only live a few days, during which time they mate and the female lays a band of a few hundred eggs around the branch of a host tree. Then the next generation is ready to take on the winter and to emerge the following spring. If you see a lot of adult moths one year (as we did last year here in Prince George), there is a good chance that you’ll have more caterpillars the next year. So keep an eye on the number of adult moths in your area this summer.

Why are there so many of them this year? Forest tent caterpillar populations are cyclical. On occasion – perhaps every ten years or so in any particular location – there can be a population explosion for a few years. The explosions are always self-limiting, as are most biological phenomena. A variety of factors are likely involved in ending an outbreak: disease, predators and parasitoids, starvation, or even untimely inclement weather. Sometimes one of these is all that’s needed to knock the populations down to sub-outbreak levels; often several of these factors work in concert to have that effect.

Are they going to kill my trees? Probably not. Most healthy trees can survive a few years of defoliation. In fact, many trees put out a second set of leaves after losing their first set. Add to that the fact that even in a large infestation, caterpillar populations in any given area may focus on one stand of trees one year, and another the next. So not every tree is necessarily going to be fully defoliated in every year. Defoliation takes away the tree’s food source, because trees, like other plants, make their food by catching sunlight and carbon dioxide with their leaves. So forest tent caterpillars reduce yearly growth in trees. In fact, researchers can study tree rings, which are indicators of growth, to track past outbreaks of defoliators.

A few trees will undoubtedly die if they are already stressed or if an infestation continues on for a number of years before the caterpillar population collapses. But if you see any large tree in an area that harbors forest tent caterpillars, you can bet that it’s already survived a number of previous outbreaks.

What can I do about it? Not much. Once populations get to this level, pesticide spraying is mainly futile, particularly in small areas such as a few trees in your backyard. At most you will spend money and time on a treatment that really won’t have much of an effect. Killing a few caterpillars may make it seem like you’re doing something, but there are plenty more where they came from. At worst, you will kill beneficial organisms (including some that would otherwise be happily killing forest tent caterpillars); you will have deleterious effects on your local ecosystem; and you could be exposing your family and pets to pesticides.

As pointed out by another entomologist on Twitter, there are some cases where larger scale use of Btk, which is not toxic to most creatures other than tent caterpillars, is advisable:

But those are usually special, large-scale situations, often involving aerial applications.

In most cases you can take a non-pesticide approach in your yard. You may want to do this if you have young trees in your yard that may not be as resilient as older, larger trees. You can remove caterpillars by hand or with your garden nozzle. And you can use sticky bands on the trunk so that nomadic parades of caterpillars can’t get to the leaves by climbing up the tree (although they may descend from above on little silk lines). Besides that, though, it’s best to just let nature take its course. The population will collapse soon enough, and in the meantime it is an interesting biological phenomenon to observe.

Have you done any research on forest tent caterpillars? Yes. About a decade ago I was involved in work on tree responses to having their leaves fed on by this insect. In one study we surveyed all of the genes that were turned on and off in leaves while the tree was being fed on. In another study, we found that while the caterpillar was feeding on some leaves, undamaged leaves in other parts of the tree began to release chemical signals into the air. We think that those signals are used to attract in enemies of the caterpillars. In other words, it seems that the tree is calling for help when it detects that it is being fed upon. There is still more work to do on that, however.  For instance, we are not sure which of the chemicals that the attacked tree is releasing – if any – serve to attract enemies of the caterpillar.

Can you make these things into wine? Yes.

Where can I find more information? Along with some of the links above, you can look here, here, and here.