Seeing Webs Again? It May Not Be a Second Round of Tent Caterpillars

We have been receiving many calls about what appears to be a second phase of tent caterpillars around Campbell River and the Comox Valley.

However, western tent caterpillars have only one generation each year and are usually active during spring and early summer. The fresh webbing appearing in trees during late August may be caused by a different insect, such as the fall webworm.

The two insects can look similar from a distance, but understanding the difference can help you decide whether anything needs to be done.

Tent caterpillars or fall webworms?

Western tent caterpillars generally appear from May through June. Their tents are usually built where branches meet, and the caterpillars leave the tent to feed on nearby leaves.

Fall webworms become noticeable later, commonly from mid-July through mid-September. Their webbing begins near the outer ends of branches and expands around the leaves as the caterpillars feed. The caterpillars remain inside the web, which can eventually cover a large section of the branch.

This late summer activity can look like tent caterpillars have returned, but it is likely a different stage of seasonal insect activity. The Province of British Columbia identifies May through June as the typical period for western tent caterpillars, while fall webworm tents are generally present from mid July through mid September. Province of British Columbia

The good news for your trees

Although the webbing can look alarming, fall webworms rarely cause serious damage to healthy, established trees.

Because their feeding occurs near the end of the growing season, the tree has already completed much of its annual growth and stored energy for the year. Even noticeable leaf loss is usually more of an appearance concern than a threat to the tree’s survival.

Natural Resources Canada notes that even severe fall webworm infestations generally have little effect on otherwise healthy trees. However, repeated defoliation or feeding on an already stressed tree may contribute to further decline. Natural Resources Canada

These caterpillars are part of the food web

Caterpillars are an important source of food for birds, predatory insects and other wildlife. They are also affected by naturally occurring parasitoids, diseases and environmental conditions that help keep their populations in balance.

Removing every web or spraying as soon as caterpillars appear can interfere with these natural relationships. In many situations, allowing the colony to complete its short life cycle is the most appropriate response.

A healthy landscape is not always perfectly tidy. Feeding, predation, seasonal damage and recovery are all part of a functioning ecosystem.

When should you consider intervening?

Limited intervention may be appropriate when:

• A newly planted or very young tree is being heavily defoliated.

• A fruit tree is likely to lose a significant portion of its crop.

• The tree is already stressed by drought, root damage, construction or disease.

• Severe defoliation has occurred repeatedly.

• The webbing is creating an access or safety concern.

• You are unsure which insect is present.

Small webs may sometimes be removed by hand or pruned out when doing so will not require the removal of a significant branch. Avoid burning the webs while they are on the tree, as this can cause serious damage to the bark and surrounding foliage.

Large branches should not be removed simply because they contain webbing. The pruning wound may have a much greater and longer-lasting effect on the tree than the temporary loss of leaves.

Support the tree instead of treating the insect

In many cases, the best response is to support the tree’s overall health.

Water slowly and deeply during dry weather, protect the root zone from soil compaction and avoid unnecessary digging or disturbance around the tree. Mulch can also help retain soil moisture, moderate soil temperature and support root health when it is applied correctly.

Before using any pesticide, the insect should be properly identified, and the condition of the tree should be considered. Even products intended for caterpillars can affect other moth and butterfly larvae exposed to the treatment.

A little patience may be all that is needed

If you are seeing new webs in your trees this August, it does not necessarily mean the spring tent caterpillars have returned. It may be fall webworm or another web-producing caterpillar taking its turn in the seasonal cycle.

For a mature, healthy tree, observation and patience are often the best treatments. The webbing will eventually disappear, the leaves will fall naturally, and the tree will likely return with fresh growth next spring.

If the tree is young, already declining or experiencing extensive feeding, proper identification and an assessment of its overall health can help determine whether intervention is worthwhile.