Caterpillars Galore!

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There can be uncountable numbers of tent caterpillars on a single tree. Source:  J.R. Carmichael, Wikimedia Commons

There are hundreds of caterpillars in your yard? Maybe thousands? They’re black with blue marks, a white streak down their back and longitudinal rows of reddish hair? And, above all, every night they gather in a silky shelter commonly called a “tent”? You’re dealing with tent caterpillars, one of the most common—and visible—tree predators.

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The tent protects the caterpillars from predators and rain. Source: Esc861, Wikimedia Commons

There are, in fact, several species of tent caterpillars found in North America and Europe, most in the genus Malacosoma, but there are also other tent caterpillars in other genera. The latter may differ in appearance, but have similar habits. All can do much damage, partially or totally defoliating their host tree.

They’ll attack many species of broadleaf tree, but in home gardens, fruit trees seem to be their favorites. And they don’t just hang out on their host tree, but disperse to other nearby trees and shrubs, returning, however, to their tent each night. Thus, as they wander off looking for a meal each day, you’ll find them in your garden, on your walkways and, just to really freak you out, circling your kids’ sandbox.

Most years, tent caterpillar nests are rather sporadic: you only see a tent here and there and the damage is therefore limited. In other years, however, most host trees in the area will be infested and there may be several tents per tree. In such years (there’s about a 10-year cycle), defoliation can be considerable.

A Tenting Lifestyle

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Tent caterpillar cocoon. Source: D.-Monkman

Tent caterpillars hatch in the spring, when their host tree starts to leaf out. They’re tiny at first and do fairly little visible damage, plus their nest is still small. As a result, they often escape detection for several weeks. However, as they increase in size (there are 5 to 6 larval instars), the damage becomes more and more extreme and the tent grows in size and visibility. Towards the end of their 7- to 8-week period of activity, defoliation can be extreme. Afterwards, the caterpillars leave their nest and their host tree and seek a protected place nearby to pupate. That can be in the ground, in wood piles or on human made structures. They spin a cocoon of white or yellowish silk and pupate inside. The adult butterflies emerge about 2 weeks later and quickly mate. The female then flies to a suitable tree, lays her eggs discreetly on a branch and dies almost immediately. It’s from these eggs that the next generation will emerge the following spring.

To Treat or Not to Treat?

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Tent caterpillars heading back into their nest at night. Source: www.planetnatural.com

In the wild, there is usually no need to intervene. Tent caterpillars are, after all, part of Mother Nature’s plan. A healthy tree can withstand being defoliated occasionally and will in fact cover itself in new leaves just a few weeks later. Also, infestations tend to be occasional: most of the time, they don’t come back to the same tree year after year, which could indeed weaken or kill it. Also, tent caterpillars play an important role in nature as fodder for a wide range of animals, from other insects to squirrels, bats, frogs, skunks and bears. In fact, more than 60 species of birds, including orioles, jays, chickadees and juncos, feed on them in North America alone. So, in a natural site, just let Mother Nature do her thing.

In your garden, though, when they move into an ornamental tree or, even worse, a fruit tree, control may be necessary … and it’s so easy to accomplish!

Their habit of returning each evening to the silky tent they stretch between branches is their weak point! At night, just cut off the branches on which the tent is installed and drop the tent into a plastic bag. Then seal it and put it out with the trash.

Some people recommend burning the nest, but most environmental agencies now discourage this practice for fear that the fire could escape and cause damage.

A Sticky Trunk

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A band of sticky glue will prevent tent caterpillars from roaming. Source: www.tanglefoot.com

If a nest of tent caterpillars is out of reach, apply Tanglefoot, a glue that never dries, all around the trunk in a band about 6 inches (15 cm) wide. This will at least prevent them from wandering all over your yard. If you prefer not to stain your trunk with glue, surround it tightly with a strip of plastic and apply the glue to the plastic.

BTK to the Rescue

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Source: www.naturalinsectcontrol.com

Another possibility is to spray the tree with BTK (Bacillus thuringiensis kurstaki). This organic pesticide is a caterpillar disease widely found in nature and harmless to other insects and animals, including humans. Once the caterpillars ingest the spores, they become ill, stop eating and die about a week later.


Tent caterpillars: after you get over the shock of finding them in one of your trees, you’ll discover they really aren’t the terrible pests they seem!20180620A J.R. Carmichael, WC

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Fall Webworm: Spectacular Damage, But Not That Harmful

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Fall webworm nest in a crabapple tree. Photo: G. Barriault

The fall webworm (Hyphantria cunea) is the caterpillar stage of a rather inconspicuous white moth. Its small size belies the damage it does or, should I say, they do, for they are very gregarious and live in colonies: often huge webbed nests filled with skeletonized leaves and droppings and 2 to 3 hundred creepy crawlers.

Fall webworms can be compared to that other web-producing moth larva, the tent caterpillar, but they occur in late summer or fall, while tent caterpillars (Malacosoma americanum and others) are limited to spring. Also, tent caterpillars leave their nests daily and feed elsewhere, while fall webworms mostly remain within their nest, increasing its size to encompass more and more edible foliage. Only in the final stages do they wander out on their own. And tent caterpillar nests tend to be built at a crotch of the host tree while fall webworms normally start at branch tips then work they way downward.

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Fall webworm. Photo: Melissa McMasters, Flickr

Not many people study the caterpillars themselves, but if you do take a peek inside their nest, they are highly variable in color, with a black to reddish head and a light yellow to green body and tufts of both short and long gray to white hairs emerging from two rows black tubercles along their back. They can actually be quite attractive… at least to my eyes!

Range

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Distribution of fall webworm. Illustration: entnemdept.ufl.edu

Originally a North American pest and found all across that continent to the limits of the deciduous forest (they don’t infest conifers), fall webworms were accidentally imported into the Old World have now spread throughout Europe and are advancing rapidly in Asia. They thrive in both warm and cold climates.

Life Cycle

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Female fall webworm laying her eggs under a leaf. Photo: UF School of Forest Resources & Conservation

In fall, the larvae leave their nests to pupate in cracks in bark and among leaf litter and in the soil at the base of the host tree. Adults emerge in summer and the female lays masses of hundreds of hairy white eggs under a leaf of the new host. They hatch about a week later and soon start to build a new web to protect themselves from predators and the elements. Damage usually becomes evident in mid-August, increasing into September, but in warmer climates, the cycle starts sooner and there can be a second generation.

To Treat or Not to Treat?

In fact, you don’t actually have to treat fall webworms. Since they start their cycle late in the season, when host trees (almost any broadleaf tree seems to do, although they do seem to prefer crabapples and walnuts in home gardens) are winding down for winter and have stored up most of their reserves for the following season, even defoliated branches tend to come back in perfect shape the next spring. And why treat something that’s doing no harm?

Also, fall webworms are an important food source for over 40 species of birds and have dozens of insect predators as well. If you leave them alone, you’re actually benefiting local wildlife.

Possible Treatments

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When a single tree has several nests, it can be completely defoliated… yet will generally still grow back in perfect health the next spring.  Photo: entnemdept.ufl.edu

Still, the nests are unsightly, doubly so when there are several nests in the same tree and you may therefore feel impelled to react. So here are some suggestions about what to do.

The traditional treatment is to cut off and destroy the nests while they are still small and the caterpillars are still young. That works fine … when you can safely reach the nest (they can be out of reach, high up in the host tree). Plus indiscriminate pruning can seriously harm the tree’s symmetry.

If you do prune off infested branches, you can bury the nest or soak it in soapy water to kill the caterpillars. Or put simply it out with the garbage in a plastic bag. Again, traditionally you’d be told to burn the nests, but then you’d be polluting the air.

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You can blast the nests with a strong spray of water. Photo: http://www.northeasttreeinc.com

In my opinion, a better way of eliminating them is to blast their nest to smithereens with a high-pressure water hose. That will get rid of most of them. To kill the remaining caterpillars, now spray nearby leaves with Btk (Bacillus thuringiensis kurstaki, a biological insecticide). When they eat the leaves, the Btk will make them sick and they’ll soon stop feeding and die.

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BTK makes caterpillars sicken and die, but harms no other wildlife. Illus.: laidbackgardener@blog

Although you can also use other commonly available pesticides, Btk has the advantage of killing only caterpillars and not harming beneficial insects or in fact any other life form. Also, caterpillars treated with Btk can still be consumed by birds and beneficial insects without harming them.

When the caterpillars have reached near maturity and begin leaving their nest to wander about on your patio furniture, something they do just before they pupate, you can simply squish them, spray them with a mild insecticide like insecticidal soap or neem (neither are harmful to birds), or drop them in soapy water. Staying indoors for a few days will also work.

Many people also pay quite handsomely to have arborists come in and treat the trees or remove the nests … a bit of overkill, I’ve always felt, considering the limited damage actually caused to the tree. However, when the same tree has several nests, and that can happen, one does feel that something should be done.

Will They Be Back?

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One nest is not so bad, but dozens?  Photo: Michigan Department of Natural Resrources

That’s always a concern, but there seems to be no way of predicting whether fall webworm nests will be abundant in a given year or nearly absent and their numbers do fluctuate considerably. Spraying tree trunks with dormant oil (horticultural oil) in early spring may help kill pupae overwintering on the bark, but is unlikely to stop an infestation if one is about to occur since most pupae are in the soil or leaf litter. Also, female moths fly, so even if they come out of pupation on a tree trunk or at the base of a given tree, they’re likely to travel a certain distance before laying their eggs. So spraying with oil often has little noticeable effect.


Fall webworms: they cause such visible damage that you often feel compelled to react, but tree life goes on in spite of them, so it’s up to you to decide whether to intervene or not.20170915D, Michigan Department of Natural Resrources