A Native Pest: Fall Webworm

September-October 2010

FALL WEBWORM IS PROBABLY one of the most noticeable pests we encounter in the landscape. Everyone has noticed the thick, webbed larval tents on the ends of branches. Although their host list is a mile long, we see fall webworm most commonly on birch, black walnut, ash, cherry and apple. Larvae construct these webbed nests so they can hide in the relative safety of the thick web during the day. If you’ve ever tried to open up a webbed nest, you’ll experience just how tough they are.

At night, late instar larvae may leave the web to feed on leaves once all leaves inside the web have been consumed. Early instar larvae chew on upper and lower surfaces of leaves, leaving a lacy leaf skeleton behind. Older larvae consume the entire leaf leaving only the petiole. Webbed nests often persist well into fall and winter and reduce the aesthetic value of the tree. Although the larvae consume leaves, it is so late in the season that even moderate defoliation has a limited impact on tree health. However, consecutive years of moderate-high defoliation can impact tree health over time. This is a rare occurrence thanks to the natural abundance of natural parasitoids – parasitic flies and wasps lay their eggs inside the body of fall webworm larvae.

Biology Basics
Fall webworm (Hyphantria cunea) is a moth belonging to the Arctiidae family, the tiger moths. Among other traits, the larvae of tiger moths are recognized by stiff hairs that emerge well out from the body. Woolly bear and yellow bear caterpillars also fall into this family. Larval colouration varies greatly in this species. The head capsule is usually black but there is a red-headed colour race that we encounter commonly in southern Ontario. Dark-headed larvae have a cream-coloured body with rows of black or orange bumps with groups of stiff, white hairs extending out of them. Some of the hairs are quite long. The red-headed larvae have a tan-coloured body with rows of orange bumps with groups of stiff, white hairs extending out of them.
The adult, a pure white moth, begins to emerge in mid-June. Egg masses are deposited on lower leaf surfaces and covered with a fine mat of hairs. Larvae hatch about a week later and spin a web over the branch so they can feed on the leaves within. The web enlarges over time and the larvae feed for about six weeks until they pupate in the soil. A second generation of adults may emerge in late summer whose larvae will feed into late September. This 2nd generation of larvae overwinter as pupae in the soil.

Tree Health & Happy Clients
So to recap: fall webworm is largely an aesthetic pest that rarely impacts tree health. It has a significant following of natural enemies that routinely impact the successful completion of its life cycle. Despite this, clients are routinely asking you what can be sprayed to save their trees, right? And the client is King, right? Right.

Experience tells us that contact insecticides are not going to be able to penetrate that thick web enough to impact larval mortality. We may be able to reduce larval populations with Bacillus thuringiensis (B.t.), but only when the larvae are feeding outside the web which is a) when they first hatch or b) when late instar larvae are looking for new feeding sites or feeding outside the web at night. In both cases, it’s going to take some significant monitoring to be able to time a successful B.t. application. Which brings us back to pruning out larval nests in July, i.e. mechanical control. If we can remove the first generation of larvae, we’re going to have a significant negative impact on subsequent generations. Just like all IPM programs, pruning will be even more successful if we can complete it on neighbouring properties as well.

We’ve had several outbreak years of fall webworm where larval populations have exploded and nest sites were taking over almost entire trees. 2009 was a good example of that. Typical of nature’s rise and decline rhythms, natural predators and parasites caught up with the pest and fall webworm numbers were reduced substantially this year and likely will be for the next year or two.

When we see high levels of webs and defoliation, we tend to think it’s the behaviour of an exotic pest. On the contrary, fall webworm is native from Canada to Mexico and therefore has some well established biocontrol organisms here. Unfortunately, fall webworm has been introduced to other continents, including Europe, Japan, Korea and China. As you can imagine, fall webworm is a dreaded pest of trees in these areas where it behaves aggressively and thrives in a longer growing season without the presence of natural predators and parasites. It’s not often that we hear about North American insects becoming invasive alien pests in other parts of the world.

Contact Information

This column is written by Jen Llewellyn, Ontario Ministry of Agriculture and Food and Rural Affairs (OMAFRA) Nursery Crops Specialist

Email any questions you have directly to Jen and we'll publish her response.

P: 519-824-4120 ext. 52671 • F: 519-767-0755

OMAF website: http://www.omafra.gov.on.ca/scripts/english/crops/agriphone/index.asp

Nursery Landscape Agriphone: 1-888-290-4441

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