The Marvelous in Nature

Today was the first day of fall, as of 11:44 morning. It’s interesting how “they” know with such precision just when summer rolls into fall. It’s always seemed to me that the calendar’s division of the seasons is so arbitrary, segmented into three-month blocks that were chosen to most closely approximate the time of year they represented. But the actual seasons are defined by something much less tangible than dates on a calendar. Technically fall only just arrived today, but for me, it’s been fall for the last three weeks. You can smell it in the air.

There’s always the cues from nature, of course. The changing colours of the trees. The goldenrods and asters blooming. The birds and monarchs heading south. The shortening days and cooler nights. Now, one would figure that the critter ensconced in the wispy cobwebs above would fit into this category, as well, simply by its name – the webs were made by the caterpillars of the Fall Webworm, Hyphantria cunea.

These particular photos were taken when I visited my parents’ a couple weeks ago. It was the second week of September by then, but many of the webs were empty, their occupants departed, and had probably been there for at least a couple weeks already. I had noticed some on the road near my own home at the start of September. It would seem a portent of the impending autumn that the webs should show up at the beginning of the month like that, and I rather suspect that, even though they can be observed year-round in some areas, they really are most frequently seen in the fall, hence the name.

The Fall Webworm is a moth, the adults varying from all white in the north to heavily spotted with black or brown in the south. It’s a type of tiger moth, which includes many hefty-bodied species that don’t resemble tigers at all. Despite the superficial appearance of the nests to those of the Tent Caterpillar, they aren’t closely related. You can tell whether your infestation is of tent caterpillars or fall webworms, for one, by the time of year. Tent caterpillars are springtime species, while the webworms, as their name implies, are a fall species. Also, tent caterpillars usually build their nests in the crotch of a branch, and it rarely gets much larger than a foot across for the largest.

Webworms, on the other hand, start out at the end of a branch. The female moth lays up to 1,500 eggs on the underside of a leaf, covering them with hairs from her abdomen to protect them while they develop. The larvae hatch and begin spinning a silk web containing the surrounding leaves. While tent caterpillars march out several times a day, following silken trails, to munch on leaves, the webworms wrap their food inside their web so they remain more protected. The downside to this approach, however, is that once the leaves inside the web are all eaten, they can’t just change the trail they follow. So they end up expanding their nest to encompass more leaves, and then even more. By the time they’re ready to leave the nest they could have wrapped up the entire branch. A very large colony of webworms is capable of enshrouding the full tree. Sometimes very large trees, like the one in the first photo that was nearly as tall as the nearby telephone pole.

Caterpillars go through 11 instars, or larval stages. There are two different, distinct races, distinguishable by the colour of the head: larvae of the north have a dark head, while those of the south have a reddish-orange head. Northern caterpillars also have white hairs coming out of black and orange bumps, while southern caterpillars have brown hairs coming out of reddish-orange bumps. Earlier instars are generally paler.

Like with the tent caterpillars, the webworms rarely kill their host trees, even though they may completely defoliate them and leave them looking barren and dead. Because their timing coincides when the tree is starting to wrap up its growing season and thinking about dropping its leaves anyway, it doesn’t lose a whole lot (tent caterpillars work the other way; they eat before the tree gets going, so it still has time to recover during the summer). Webworms are generalists, not targeting any one specific type of tree; their larvae have been recorded on 120 species of tree and shrub in North America.

It’s found right across North America, from southern Canada south to northern Mexico. However, it’s also found in Europe, as a non-native invader. It was introduced to Yugoslavia sometime in the 1940s, and has since spread to encompass much of the continent. It was also introduced to Japan in 1945, later spreading to China and Hong Kong. Throughout its entire range it’s been documented on 636 species of trees, and is considered one of the most, if not the most, polyphagous of insects (fancy scientific name for generalistic feeders). Basically, anything deciduous is fair game.

Most of the caterpillars have left their nests now, the webs empty of larvae or leaves, just frass remaining, trapped between the web layers. They’ll trundle off to find a safe place to pupate, in the bark and leaf litter at the base of trees. There, they’ll spend the winter, cozily wrapped up in their silken cocoon, interwoven with bits of detritus from the soil. Next spring, the adults will emerge, once the risk of frost has passed, around May here at my latitude, and they’ll start the cycle again.