Wednesday, August 22, 2012

Pest Control: Tent Caterpillars


As the caterpillar chooses the fairest leaves to lay her eggs on, so the priest lays his curse on the fairest joys. – William Blake

A close-up view inside a tent caterpillar "tent".

Hmmm. I’ll just let you take from that quote what you wish. I just find ‘em. I don’t write ‘em…

Malacosoma americana moth. Photo: Wiki CC
Most of us think moths are pretty little nighttime creatures. Sadly not all of the stages of their lives are quite so attractive.

Tent caterpillars are the larvae of a moth of the genus Malacosoma. The species we have in Nova Scotia are Malacosoma americana, or Eastern Tent Caterpillar.

Tent caterpillars feed on deciduous trees (ones that lose their leaves in the Fall) in most of southern Canada. Luckily severe infestations are cyclical, meaning they happen on a regular, but not yearly basis.

Unluckily it appears that this may be the year for an infestation. Outbreaks happen about every ten years and can last up to two years.

You’ve seen their “nests” if you live in a southern part of Canada. Huge tents of silk in trees with what looks like a dark mass in the centre. That dark mass is actually a swarm of caterpillars. Sometimes hundreds and hundreds of them. 

When they mature, and emerge from their tent, they can skeletonize a tree in no time flat – and neighbouring trees. Every leaf – or nearly so – eaten. The caterpillars return to their woven “lair” every evening.


Gross. A swarming mass of Eastern Tent caterpillars.
Photo: Wiki CC
What are tent caterpillars?
In Canada we have three common types of  tent caterpillars: Eastern, Western (sometimes called Gypsy Moth) and Forest. Each is slightly different looking in hairiness, spots and stripes.

Eastern and Western tent caterpillars form massive web/pouches in trees. Forest caterpillars are slightly different. They form a flat mat that the caterpillars rest on, and leave to do their destructive eating.

Eastern tent caterpillars like apple and cherry trees (including wild chokecherry and chokeberry) but I have seen them in birch and sumac this year as well.

In the spring, as soon as foliage appears, the eggs hatch into young caterpillar larvae that make communal tent webs. The Eastern's "tent" is usually built in the forks of trees while the Western tent encloses the tips of branches. The tent increases in size as the caterpillars mature inside.

On a chokecherry tree.
Tent caterpillars are social. Caterpillars from one egg mass stay together. Caterpillars from two or more egg masses may actually unite to form one large colony. That spells real trouble for the host tree.

Tent caterpillar outbreaks require several specific climactic conditions. Although they seldom kill the infested tree, they cause severe damage, often nearly defoliating the entire tree. If the tree is healthy, it usually will bud again later in the summer. However if a tree is repeatedly attacked it can stress it so much it becomes susceptible to other environmental problems.


How to “manage” outbreaks
Tent caterpillars only produce one generation per year so once you get them they won’t be coming back again. Until the next year…

Since they are a native species, birds and insect predators help control them somewhat. But if an infestation is severe nature may need a little help – especially if they are appearing in trees that you value and/or supply you with food. Remember, Eastern tent like apple and cherry trees…

So besides just sitting back and hoping nature takes care of them, what can you do?

In spring look for the egg cases on trees that are susceptible to tent caterpillars. Remove as many of them as you can.

Up in our apple trees. WAY up...
If the tents hare already developed the branches can be removed and the tent burned. It seems cruel to the caterpillars inside, but so is losing important trees...

Where I grew up some folks didn't remove the branches but stuck burning torches up in the tents. I think I would prefer removing the branch.

The torch method causes the caterpillars to drop on the ground, where they may escape, and has to do some damage to the tree itself – more than cutting off a branch.

Microbial insecticides also work. Bacillus thuringiensis (Bt) is commonly used. It is not toxic to humans but it isn’t choosy as to what kind of caterpillar it kills. So you’ll be killing everything – butterfly caterpillars included.

"Bt" is supposed to kill caterpillar larvae within 5 days. It works by stopping them for eating so they starve. This insecticide won't affect other insects that don’t have a caterpillar larvae stage.

Another option to try is to pre-treat trees that are susceptible by spraying a dormant oil in late winter. This smothers whatever is inside the egg cases. Dormant oils are thick oils used mainly on fruit trees. They are also effective against other over-wintering pests like scale, mites, etc. They can be harmful if not used according to directions.

If you’re not into chemical help you can also encourage predators to visit and stay on your property by building bird or bat houses. If the animals have a food source nearby they’ll settle in and gladly help you along.


Tent caterpillars are a nasty thing to find in your trees. If left unchecked they can cause serious damage to your – and your neighbours' – property. Not only do they introduce the risk of other diseases to your trees (which could spread) but they also leave unattractive bare branches behind.

Oh, and they’re icky. Just look at them.

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