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.
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!
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.
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.
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.
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.
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?
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.