Question:

What are these caterpillar-filled white web masses on branches of my apple and walnut trees? And what should I do to control them?

- Gardener in Sierra County

Answer:

From your photos, it looks like you have a broadleaf tree pest that is common at this time of year: fall webworms (Hyphantria cunea). These fall webworms make white nets shaped like bags covering branch tips and if you get close enough you can see those caterpillars hanging out inside. 

The larvae of fall webworms look like fuzzy one-inch caterpillars. They spin the silken webs and live communally, munching on leaf tissues within the web before dropping down to the ground to pupate in soil litter and emerge as moths in a few weeks. The adult moths mate and usually lay eggs on the underside of leaves which then hatch in the spring. Adult fall webworms are deceptively beautiful with snow white wings adorned with black spots. There are two races of the larvae: red-headed and black-headed.

Do not confuse webworms with bagworms, like I did. Bagworms create tight sleeping bag-like cocoons made out of little leaf trimmings for camouflage that dangle from tree limbs and can cause considerably more damage than fall webworms.

Here is the good news: because these fall webworms are defoliating your apple tree shortly before normal autumn leaf drop, the damage is not long-lasting. If you had taken these photos in the springtime, the web structures would mostly likely be caused by a different insect defoliator, the tent caterpillar (Malacosoma spp). The tent caterpillar can be more damaging than the fall webworm because defoliation in the spring limits photosynthesis and causes a reduction in energy reserves that may not be replenished that season.

Healthy trees are generally able to handle infestation with fall webworms (and tent caterpillars) without too much trouble. However, trees that are already experiencing some other stressor, like water stress, may not be as quick to bounce back.

Fall webworms are considered to be more of a nuisance than a damaging pest. Even though they are unlikely to kill a tree, these web tents built by fall webworms are not particularly pretty. Control is easier if you catch them earlier in the summer and remove the tents. If you can safely reach the infected branch you can either use a stick or rake to break the silky net and knock the caterpillars into a box for disposal or, if the branch is small enough, you can cut the entire limb out. Take care that your pruning does not cause greater damage than is caused by the caterpillars!

Share photos of your own fall webworms on our Facebook page (DesertBloomsNM) or Twitter feed (@NMDesertBlooms)!

Keep the questions coming!

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!