Throughout the growing season I receive calls from gardeners wanting to know what is the insect feeding on their plants.
Many of the calls in recent weeks have been from gardeners concerned about a green caterpillar eating their delphiniums. These caterpillars are unofficially named delphinium defoliators by local gardeners because they consume a substantial number of delphinium leaves early in the growing season. The caterpillar, which is the larval stage of a nondescript, night-flying moth in the genus Polychrysia, feeds as it grows then eventually wraps itself in the leaves to pupate.
Judging from the number of calls, it would seem that their population is high this year, so you might want to take a look at delphiniums growing in your garden. While you’re looking at the delphiniums for signs of feeding, also check your globeflowers (Trollius) and larkspur because the delphinium defoliator has been known to feed on these closely related plants.
If you notice feeding damage, or frass (the nice name for insect poop), on your delphiniums and you also see the green caterpillars, it is time to act swiftly to lessen the damage to your prized perennials. Once the caterpillars begin folding the leaves around themselves they are much harder to control and most of the damage is done. Although I’m not aware of delphinium defoliators killing a healthy perennial, they will undoubtedly cause unsightly foliage and reduced growth this season.
At the most basic level, you can simply pluck the offending caterpillars off your plants as you hunt for them. If you have a large area of delphiniums and aren’t able to hand pick the caterpillars, you can try using a pesticide with Bacillus thuringiensis (Bt), Kurstaki strain, as the active ingredient. Bt, a bacterium found in soils, produces proteins that are toxic to certain immature insects. Kurstaki strains are exclusively toxic to larval stages of butterflies and moths, so there is less of a concern about damaging other insects that may be on or near your delphiniums.
Bt becomes activated in the gut of the caterpillar, so the caterpillars must be actively feeding during the time of application. This means that by the time the caterpillars have ensconced themselves in the folded leaves it is too late for an application of Bt, or any contact insecticide for that matter. Bt is available locally at many nurseries and garden centers.
As an added measure, make sure to practice good sanitation in your garden. Cleaning up delphinium stalks and leaf litter in the fall will remove overwintering sites for this insect and help prevent their establishment in future years. Even gardeners who clean up leaf litter every fall have called with defoliator sightings this summer, so it won’t eliminate the problem completely, but it may help reduce the frequency and severity of infestations in your garden in coming years.
Darcy Etcheverry is a program assistant with the University of Alaska Fairbanks Cooperative Extension Service agriculture and pest management program in Fairbanks. She can be reached at 474-2422 or firstname.lastname@example.org.