If you have been travelling around in the Land Between lately, you have probably noticed large web-covered branches on several trees. If you have thought to yourself, “What’s the deal with those caterpillars?”, you are not alone. In late summer, broadleaf trees by the road are covered in what looks like white cotton candy. These intricate tents are homes for communities of Fall Webworms (Hyphantria cunea) – caterpillars that work together to build webby homes. There is no need for concern, as these insects do not cause any serious damage.
The Fall Webworm is often confused with the Eastern Tent Caterpillar (Malacosoma americanum), which emerges earlier in the summer. They are both defoliating insects that live in forests and create those big white tent webs you see in the trees. The Forest Tent Caterpillar (Malacosoma disstria) is a related species that creates a silken mat on tree trunks rather than a large tent or web. Contrary to popular belief, these insects are actually native to Ontario, and play important roles in our ecosystems.
The easiest way to tell Fall Webworm apart from Eastern Tent Caterpillars is by looking at where they build their webs. Eastern Tent Caterpillars build their webs in the crooks of branches, while Fall Webworms form their webs on the ends of branches. As the name suggests, Fall Webworms also emerge later in the season than Tent Caterpillars which form their webs in late summer.
The Fall Webworm overwinters in cocoons in pupae form, and the small white adult moths emerge in the spring. Female moths lay their eggs on the underside of leaves, which then hatch into hungry, light-coloured caterpillars. These caterpillars are the ones who build those large webs we are so familiar with. The webbing gives them warmth and shelter as they feed on leaves. They extend the web outwards as they eat off more and more of the tree.
Tent Caterpillar larvae hatch in May, then spend the next few weeks growing and feasting on leaves from broadleaf trees. When they are not eating, they are protected in their tent homes, and then follow their own silk trails back to feeding sites each day. After about six weeks of eating leaves, the fuzzy caterpillars build themselves a cocoon and metamorphosize into brown-coloured moths. The female moth will lay a cluster of eggs directly onto a tree branch or trunk before she dies.
While homeowners or business owners do not like the look of defoliated trees or the mess that the caterpillars create with their droppings (aka. frass), neither Fall Webworm nor Tent Caterpillars cause serious long-term harm. Most trees are able to recover and re-grow the leaves that have been lost. What’s more, the Fall Webworm does not eat new buds, and instead feeds on the leaves that will soon be dropped in the fall anyway!
Caterpillars go through a natural boom-and-bust cycle, and are controlled by many natural predators and diseases. For instance, Black-Billed Cuckoos, squirrels, and black bears all love to eat caterpillars. Bats and frogs also feed on the adult moths. Forest Tent Caterpillars can cause a bit more damage than the Fall Webworms, with outbreaks occurring every 10-12 years. The outbreaks can spread as far as millions of hectares. Even still, only trees that have been previously injured are at risk of dying from losing a few more leaves. The Ministry of Natural Resources and Forestry tracks the insects’ disturbance patterns, and may intervene when populations become very concentrated.
In addition to these native caterpillars, you may have seen or heard about the outbreak of another, more problematic species, the Gypsy Moth (Lymantria dispar dispar). The Gypsy Moth is a defoliating insect native to Europe, that began seriously spreading into Ontario forests in the 1980s. This invasive species prefers Oak trees, but will eat most tree species, and is able to defoliate entire trees and shrubs over large areas. They overwinter in the egg stage on tree bark, then hatch in the spring. Gypsy Moths do NOT build tent webs.
This summer (2020), Eastern Ontario has been experiencing the worst outbreak of Gypsy Moths since the 1980s. It has become concerning because their caterpillars are now infecting coniferous trees like White Pines and Cedars – when conifers are infected, they are unable to regenerate leaves in the same way that broadleaf trees can. An article in the Belleville Intelligencer suggests that property owners can help control the spread by scraping off egg nests or placing burlap around infected trees to trap caterpillars (see original source for further details).
So what should you do about the tents you see in all those trees? In short, nothing. Fall Webworms are generally not a cause for concern. In fact, you can marvel at the structural integrity of their silk, and appreciate the teamwork required to build these homes!
A Summary of the Four Caterpillar Species:
|Eastern Tent Caterpillar||
|Forest Tent Caterpillar||
Written by Emma Halupka, September 2020
- A Fall Webworm tent web. (Photo: Emma Halupka)
- Adult Fall Webworm moth (Photo credit: Katja Schulz. Attribution required)
- Eastern Tent Caterpillars in their tent web. (Photo: Jacob W. Frank. Public domain)
- Gypsy moth. (Photo: Ontario Government. Source)
- Baldwin, Derek. 2020. Eastern Ontario in grip of worst Gypsy moth outbreak in 30 years. The Belleville Intelligencer. (Link).
- Crawford, Blair. 2020. ‘We’re getting hammered’: Gypsy moth outbreak devastating Eastern Ontario forests. Ottawa Citizen. (Link).
- Government of Canada. 2013. Tent Caterpillars. (Link).
- Government of Ontario. 2020. Gypsy Moths (Link).
- Natural Resources Canada. 2015. Fall Webworm. (Link).
- Ontario Parks. 2018. Blog: What’s With all the Caterpillars? (Link).
- Scholwalter, TD, and D.R. Ring. 2017. Biology and Management of the Fall Webworm, Hyphantria cunea (Lepidoptera: Erebidae). Journal of Integrated Pest Management, Volume 8, Issue 1. (Link).
- Schowalter, TD. 2017. Biology and Management of the Forest Tent Caterpillar (Lepidoptera: Lasiocampidae). Journal of Integrated Pest Management, Volume 8, Issue 1. (Link).