The Fall Webworm, the Eastern Tent Caterpillar and the Bagworms

The Fall Webworm,
the Eastern Tent Caterpillar  
and the Bagworms

Webworm infestation

When I was driving along our country road on my way to the supermarket I saw these big webs around tree branches with brown leaves inside. Most of the branches were facing south east, ready for the first morning sunshine. Some of the trees had many of those webs and looked very unhealthy. When I came back, I checked the trees around the house and, unfortunately, I also found one. It is on a tree with the best location - just above the pond, facing southeast, and enjoying sunshine most of the day, but I could not see it from the house. As most of the Eastern Tent Caterpillars had passed on by now, I was concerned. Was this a new infestation of the very destructive tent caterpillars that could damage our trees? What was happening so late in the year?

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Eastern Caterpillar Tent Infestation

After doing some research, I found out that what I was seeing was the Fall Webworm tents. An entirely different animal from the Eastern Tent Caterpillars that we have all come to fear.

Why are the Eastern Tent Caterpillars so despised? Well they start to form their web-enclosed colonies in the Spring in the forks of tree branches- which makes them hard to address. Cutting off the branch below the nest will generally lead to a harmful removal of an energy producing significant branch. So the only solutions were chemical (we don’t want those toxins today), biological (do we want to add biologic agents into the environment as pathogens that we don’t completely understand), or manual destruction. A heavy infestation can defoliate a newly leafing tree and kill it!

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Webworm White Moth

One of our neighbors owns over 200 acres of woods, which he maintains for logging and income. His wife Georgia was tasked each year to go through the woods with a long pole and poke apart all the Eastern Tent Caterpillar nests she could find, allowing birds and other predators to kill off the colonies. Perhaps her late Summer efforts were unnecessary.

What we were seeing now, were Fall Webworm colonies - not Eastern Tent Caterpillars. So what are Fall Webworms? Fall Webworms appear as a white moth with a wingspan of about 1 ½ inches from May to August. Those moths deposit up to 1500 eggs on the lower surface of the leaves of the host tree. When the eggs hatch, larvae begin spinning their webs over the leaves on which they feed. The web enlarges to cover more leaves as the larvae continue to feed.

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Our own webworm infestation

One of the  important differences here is that Webworms remain within the tent to feed on leaves and come late in the year, when most of the energy the leaves are producing is already spent. So leaf destruction is confined to within the tent. Tent Caterpillars come in the Spring and leave their tents to feed all over the tree - possibly killing it. So Webworms are not very destructive.

Webworms prefer maple, birches, walnuts and willow trees. They damage trees by feeding on the leaves of those trees. Nevertheless, the feeding takes place in the Fall, shortly before trees shed their leaves anyway. It seems to be more of a cosmetic problem than a serious health risk for the tree. 

One should however remove the web, if one can reach it, and destroy it by crushing or put it in an alcohol solution. Chemical measures are not needed, since it is late in the season. If the webworm is spotted in the beginning of its web building, chemicals can be applied. Insects are more susceptible when young.

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Eastern Catarpillar Tent Egg

The Eastern Tent Caterpillar has a very different life cycle. It overwinters in an egg mass which has been deposited around a small twig. It is ½ inch long, oval-shaped and brown and contains around 300 eggs. When the leaves appear in early spring, the eggs hatch and the larvae begin spinning their tents in the twig crotches. 300 caterpillars will feed on tender young leaves which is very bad for any tree. By the end of May the caterpillars will leave the tree to form cocoons. By late June, adults will emerge and lay eggs on twigs of a host plant and those eggs will not hatch until the following year.

The damage of caterpillars is done when they feed on new leaves after spinning a tent or web in the fork of a tree. Depending on the size of the tent population an infestation of Eastern tent caterpillars is capable of seriously damaging a tree.

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Bagworm Infestation

There are also the Bagworms. Bagworms are very unique and probably will escape your notice. Their well disguised ‘houses’ are covered with woodland detritus. They may appear like pine cones or other seed pods, but they are an illusion. They can be found on junipers, spruce and pines. Bagworm eggs hatch in May or early June. A worm the size of the lead in a pencil will start eating foliage and will spin a silken bag, camouflaged with tree bits,  around its body. By late summer the bag is several inches long and very difficult to control. A mature bag will repel chemicals and can only be hand picked. This evolution is so elegant because it hides itself with the natural detritus of the environment! So the challenge is on you, as the steward.

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All three pests have different life cycles and should be dealt with. A healthy tree however, can handle one webworm nest. If there are more, one has to do something. The other two infestations need your attention, better sooner than later.