Welcome back to Ask a Naturalist: your own personal “Google” for information on all things natural on PEI! It seems like everyone is asking about these web-like masses appearing on trees and shrubs across the Island right now (Photo 1). These are the protective silk coverings of Fall Webworm caterpillars (Hyphantria cunea), a harmless native species.
I understand why property owners could be concerned about the presence of these insects. A scary-looking web appears on your favourite tree or shrub; it’s filled with hungry caterpillars that set about eating all the leaves inside – that can’t be good, right?
It may look bad, but on a healthy tree it isn’t anything more than an aesthetic problem. Those leaves have fulfilled their purpose already, having turned sunlight into food for the plant all summer. They are nearing the end of their lifespan, and even complete defoliation of a healthy tree this time of year is unlikely to kill it. (If your tree is stressed for other reasons, that’s a different story). While Fall Webworms are around annually, it’s rare to have them on the same tree year after year, let alone on the same branch.
Fall Webworms may look ugly, but they rarely do any damage and can actually be beneficial. As a native insect, Fall Webworms are great food sources for our insect-eating birds including Chickadees, Flickers Jays, Robins, Sparrows, Vireos, Warblers, and Wrens, among others. They also contribute to nutrient cycling by breaking down leaves into finer forms of organic matter used by all sorts of organisms.
In my opinion, the biggest threat from Fall Webworms comes from well-meaning people trying to control them. Burning the webs on the tree has obvious risks to not only to the tree, but also to the surrounding vegetation (and possibly to the person doing the burning!). Chopping off the affected branch and burning it is a common solution, but every time you cut a tree you are opening a potential pathway for pests and disease. Spraying pesticides is not only unnecessary, it’s also a waste of time and money: those protective silk webs are hard to penetrate, and the product you use will just run down the outside of the web. If you feel you MUST remove the webs and caterpillars, you can do it manually with a rake, pole, or strong stream of water. A more wildlife-friendly option is to poke holes in the webs to make it easier for birds and other predators to get at the tasty caterpillars inside (Photo 2).
Within their protecting covering, Fall Webworms mature through several stages over a period of about six weeks. Once fully-grown, the caterpillars will leave the webbing and become pupae in cocoons under leaf litter or decaying vegetation, where they’ll overwinter. In late spring or early summer, they’ll emerge as adult Fall Webworm Moths which are usually all white and less than four centimetres (about 1.5 inches) long. During the summer, moths will mate, lay eggs that hatch into caterpillars, and the cycle starts over again.
As a final note, Fall Webworms are not the same as Tent Caterpillars (Malacomosa spp.) which appear in spring, or Gypsy Moths (Lymantria dispar dispar) which don’t form webs and have only been found in isolated pockets on the Island. On PEI, it’s traditional to call Webworms and Tent Caterpillars “Army Worms”, but those are a different species yet again (Spodoptera spp.) and don’t form webs.
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