what to do about fall webworm? (usually, nothing)
IT’S A BANNER FALL WEBWORM year in my corner of the Northeast, more than I’ve seen in 30ish years here. A longtime local nurseryman friend with a U-pick apple orchard and fields of landscape trees and shrubs he grows to sell agrees: epic. What’s an organic gardener to do about fall webworm?
Mostly nothing, except perhaps on the most vulnerable landscape plants—a young ornamental tree, perhaps, that might have trouble bouncing back if it suffered total defoliation.
The signature bags, or webbing, of caterpillars of Hyphantria cunea—a native moth species—started showing up extra-early, too, at the start of July in 2018 instead of more like August. And now? Everywhere—even on some large-scale herbaceous plants, like my Petasites hybridus (leaf, top of page), something I have not seen before, either. Usually it’s just the woody plants here (like the twig dogwood Cornus sericea ‘Silver and Gold,’ just above).
These “fall” guys build their webs on the end of branches—unlike the Eastern tent caterpillar in springtime (another native insect you may have seen, in Malus and Prunus species in particular), which take hold in branch crotches (photo below, in a crabapple).
A gardener’s first impulse: Make it stop! Spray something! Kill them! But in the natural forest environment things basically take care of themselves on the webworm score. So except when fall webworms attack an ornamental—especially one that is young or small, as mentioned—with a particular vengeance, most other plants bounce back, and no intervention is generally required. Here’s why:
4 reasons i rarely intervene against fall webworm
1. This is a native insect (as are Eastern tent caterpillars I might see in spring on fruit trees, but unlike, say, imported pests like gypsy moths).
2. Fall webworms eat this season’s leaves, but not the buds that will produce next year’s foliage. Meaning: the leaves they’re eating are winding down their photosynthetic assignment for the year, anyhow—the plant doesn’t expect much more of them from here out.
3. Unlike some Lepidopteran species with very specific host plants, this one (a white moth, sometimes with dark dots) is known to do its thing on more than 90 (some sources say 80) types of woody plants. That means they’re generalists—and generalists don’t decimate one particular species of plant the way a specialist pest (think: emerald ash borer, hemlock woolly adelgid…) might in a large, targeted outbreak. A list of some of the many things they eat is here. (Above, adult moth by Tamp AGS on Wikimedia.)
4. The fall webworm has many natural enemies—parasitic insects including wasps and predatory insects. Entomologists say that fact them attack fall webworms (instead of eliminating them yourself) may actually have environmental side benefits:
“In fact, fall webworm is thought to be an important late-season food source for many of these natural enemies,” says this Michigan State fact sheet (pdf), “enabling their populations to persist until the following year, when they can again prey on gypsy moths and other tree-feeding pests.” Bonus!
if intervention is required..
IF INTERVENTION is required against fall webworm on a vulnerable landscape specimen, always be safe and sane:
When webs first appear, before extensive feeding occurs, mechanical (non-chemical) control can be achieved by pruning off affected bits, or using a bamboo cane or even a stiff jet of water from the hose to dislodge the webs. Then drown the insects in a bucket of water, or squash them.
As it will with any foliar-feeding caterpillar, spraying a solution of Bacillus thuringiensis (Bt) according to product directions will kill the larvae, but not harm other insects.
Me? I’ll be over here watching in total fascination, basically.