Has anyone else recently found themselves going outside alone only to come back in with multiple “visitors” attached to their hair, clothing, or anything else that comes in contact with the multitudes of cankerworms dangling from silken threads in our trees? Cankerworms are everywhere right now! While their normal lifecycle makes them a regular occurrence each year, there are some years where everything lines up for an abundance of these annoying defoliaters. Guess what this year looks to be? As if oak pollen isn’t hard enough to get through, now we have to worry if it’s a strand of oak pollen tickling our neck, or a cankerworm hitching a ride! Today we’ll talk about what plants are most susceptible, and what treatment works best to control cankerworms.

Cankerworms in a group on pavement.
Worm climbing up a window screen.

Descriptions and Control of Cankerworms

Cankerworms are also called inchworms but they are really caterpillars. They are called inchworms because of the way they pull up the rear of their body and use it to gain momentum to thrust the front of their body forward. It’s rather entertaining to watch them move, but that is where their cuteness stops. These caterpillars can vary in color but are predominately marked with green, pale-yellow or brown stripes along the lengths of their 1 inch bodies. They also just happen to look like the tassles of oak pollen, ew! (See pic below and tell me at first glance that oak pollen doesn’t look like a cankerworm!) Larvae hatch from eggs just about the time that buds are breaking and leaves are developing on the trees in spring (and fall) and feed for 3 – 4 weeks. Cankerworms can quickly work to defoliate broad leaf trees. 

It’s important to note that while defoliation can occur on all broad-leafed trees, it is the newly planted, newly transplanted, young or weakened trees that are more susceptible to any severe damage by cankerworms. Established, older trees may still experience some defoliation but will generally put out a fresh flush of foliage and not sustain long-term damage. All this being said, I did recently find some cankerworms happily munching away at my prized blue hibiscus petals! Not cool, inchworms, not cool!

Cankerworms on a railing next to oak pollen.
Caterpillar in hibiscus petals.

Bt (Bacillus thuringiensis) is an organic spray that can help control cankerworms, although is best to know that this spray (like most insecticides) tends to work best when cankerworms are still young (when they are around a half inch long). The good news is that you don’t have to apply subsequent spray throughout the season because they do not produce multiple generations, although you may have to battle cankerworms again in the fall. Spinosad and Sevin are other products that work to eradicate caterpillars. Remember that butterflies and other beneficials are laying their eggs on many host plants and any insecticide, organic or not, can harm these beneficials as well. If you can avoid spraying, choose so.

What it comes down to is that you should be most on guard if you have newly planted, broad-leafed trees that appear to be getting munched on. Look for BB sized holes in leaves and a chewed on appearance to the foliage. Cankerworms will work on eating all of the leaf except for the center rib vein for the most part. If you are finding this on your young trees, you might want to spray to protect them from becoming weakened or enduring long-term damage. If you have this appearance on your foliage of otherwise healthy, established trees, skip the spray; this too shall pass.

Cankerworms are a temporary, heebie-jeebie invoking, nuisance for the most part. You’ll spend some time this April sweeping cankerworms off the exterior of your home, plucking them from your hair, feeling the squish between your toes when you walk barefoot, and twisting and writhing like a crazed maniac in the streets when you get tangled in their silk. And then they’ll be gone. We’ve gotten through a lot in the past year; we can get through this too.

~The Happy Gardener, Lisa Mulroy