Barberry geometer caterpillars feasting on barberry leaves. Photo: ask.extension.org

Question: Help! My barberries are being eaten by little caterpillars. I’m sure there must be a thousand of them and they’re chewing all the leaves. What should I do? 

LilyLady12

Answer: They’re under attack from the barberry geometer caterpillar or barberry looper (Coryphista meadii), found throughout the United States and southern Canada. It’s a small black caterpillar with a white lateral stripe dotted with orange and an orange head. It’s one of the “inchworms” and moves with a characteristic looping movement. It has a voracious appetite for barberry (Berberis spp.) and mahonia (Mahonia spp.) leaves and dozens are found on each shrub (though definitely not thousands), often defoliating the plant entirely. 

The adult is a rather nondescript brown moth that is active from April to October. There can be several generations per year, although usually only one in the northern part of its range.

Typically, barberry geometer caterpillar attacks suddenly, strips the shrub of most of its leaves within days, then disappears never to be seen again. The plant then puts out new leaves and recovers fully. Given the rapidity with which it strikes, its quick disappearance and its sporadic nature, you may not need to treat it. 

If you want to, try knocking the little guys into a bucket of soapy water or spraying them with BT (Bacillus thuringiensis kurstakii), an organic insecticide sometimes sold as “caterpillar killer.” (That has a nice ring to it, don’t you think?) This product is only effective against caterpillars, so won’t harm other insects, including bees.