Tag: outbreak

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To spray or not to spray?

Earlier this afternoon I was interviewed by the local television station news program on the occasion of today being Canada’s National Day of the Honey Bee.

As I noted in my previous post, our city is currently being inundated with forest tent caterpillars. They are everywhere, and it’s hard to take a walk in one of our city’s wonderful green spaces without literally bumping into them at every turn as they rappel out of the trees above. Yesterday evening I spent a bit of time in my yard and then spent most of the rest of the evening removing strands of caterpillar silk that had festooned me.

What does this have to do with Honey Bee Day? Well, as the planned news story is bound to point out, the temptation for caterpillar-plagued homeowners is going to be to spray the heck out of the little leaf-munching critters with whatever pesticide they can get their hands on. That urge, I would argue, is a mistake for a number of reasons:

  • At this point in the tent caterpillar infestation, they have done almost as much damage as they can do. I have been observing that they are growing quite well (unfortunately for us!) and are going to be entering their pupal (cocoon) stage shortly. In other words, spraying now won’t do much to reduce any remaining damage that they may still do. The damage is mainly done.
  • In any case, in the face of such a massive infestation, spraying a can of pesticide at a few of the caterpillars is analogous to facing up against the JTF2 with a BB gun. You may inflict some minor damage for a moment, but you’re going to be overrun anyhow.
  • For smaller trees and shrubs (many of which these caterpillars only eat reluctantly at best anyhow), physical removal is as effective as spraying, and definitely much better for the environment.
  • And, spraying WILL impact other arthropods that are beneficial, including enemies of the forest tent caterpillar… and pollinators such as honeybees and various native pollinators (bumblebees and others).

That final point, I believe, is going to be one of the messages of the news item later today. Specifically, don’t lose sight of the forest for the trees.

Or, in other words, don’t lose sight of…

  • the honeybees
  • the native pollinators
  • the spiders that eat garden pests
  • the parasitoid flies that dine on tent caterpillars
  • the ladybugs that eat your aphids
  • the seed-dispersing ants
  • the dragonflies that eat mosquitoes
  • that beautiful swallowtail butterfly that brightens your day
  • …you fill in the blank…

…for the caterpillars.

I realize that even after reading this, some folks are still going to want to buy a can or two of pesticide and use it in their yards. If that is you, then:

  • be sure to carefully follow the directions on the label because they are there for a reason.
  • remember that these are powerful chemicals and that more is not necessarily better.
  • do your best to limit your application to the area in which you deem that it’s needed.
  • protect yourself, your kids, and your pets during and after spraying.

I’ll close with a personal story. The other day I was in a garden store buying a few bedding plants and some soil for our gardens and containers. Near the checkout there was a display of pesticide that is labeled for use against forest tent caterpillar. A customer and a store employee were talking about how best to use the stuff. Being a nosy entomologist I joined the conversation and made my case. Following more discussion between the three of us the customer finally said, “well, I know that this won’t really help with the problem in my yard, but I’m just so grossed out by them that I want to do something.”

I’m not sure if she ended up buying the product or not. But I suspect that a lot of spraying goes on for that very reason – i.e. a general dislike of insects – particularly in vast quantities – combined with a desire to do something… anything.

So, one last plea – please carefully consider your need use a pesticide in this situation. This plague will be over for the year soon enough. If we are lucky, natural enemies and disease will knock the population down this year and we won’t be seeing these creatures in any substantial quantities for quite a few years to come. In the meantime pesticides will not alleviate the problem, but they might end up hurting some friends that you may not even know that you have.

If you would like more information on pollinator conservation, please see this page hosted by the Xerces Society.

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(And a small side note: I’ve been seeing a few “friendly flies” around lately. So hopefully their population levels will pick up and they’ll help to wipe out this infestation. Fingers crossed! Keep in mind that these creatures are called “friendly” for a reason. Specifically, they like to land on various surfaces, including people. But they are harmless to everything except for forest tent caterpillar cocoons. If they are going to be a factor in knocking down the tent caterpillar infestation, there are going to be a lot of them around very soon. Here is a picture of one so that you know what to look for. Click on the photograph to enlarge. Notice the stripes on the thorax, the pattern on the abdomen, and the nice, big reddish/burgundy eyes.)

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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.