The Fall Webworm (Hypantria cunea), a native pest found throughout North America, is now upon us. The fall webworm forms a large web sometimes one metre long over the ends of branches of birch, ash, Manitoba maple, flowering crab, and many other deciduous trees. The webs are can easily be seen during August and September and this year have begun already in late July. The caterpillars that feed in the webs are pale yellow and hairy. Fall webworms pupate overwinter in cocoons under soil debris or tree bark. Adults, which are a pure white moth, emerge in late May to early July and lay eggs in masses covered with hairs on the undersides of leaves (or in my case even inside closed umbrellas). The eggs hatch in a few days. Larvae feed in large groups for 4 to 6 weeks inside the web, which extends as the foliage inside is eaten. The last-stage larvae wander all over the tree, then pupate in soil debris.
I first noticed some nests on my crab apples about two weeks ago and today noticed more on the hawthorns and lilacs at the back of my property as well as in one of my birch trees. Host plants include over 85 varieties of deciduous forest, shade, and fruit trees.
My solution is to cut out the nests and submerge them in soapy water. You can also cut open the nests and let birds and predatory insects (beneficial wasps and hornets) feed on the caterpillars. I cut out nests only in the ornamental gardens, choosing to break open nests I can reach in the woodland areas. While the nests are unattractive, any damage is largely aesthetic. Fall Webworms do not do a lot of long term damage to the tree since the nests appear and grow late into summer and the trees have already had sufficient time to use their leaves for photosynthesis.
You can also use BTK (Bacillus thuringiensis var. kurstaki) but you need to do so smartly. Since the spray will not likely enter the web you need to either spray the leaves around smaller webs (so the caterpillar eat the BTK as they expand the nest) or spray the entire plant as older larvae start wandering outside their web to feed. I don’t like to wait this long so I find removing the nests easier. In larger trees you could also break open the nest and spray inside with BTK to help.
Don’t confuse the Fall Webworm with the Eastern Tent Caterpillar which arrives early in spring.