An interesting insect was found defoliating currants in St. Paul (Ramsey Co.) recently. The currant spanworm, Itame ribearia, is generally smooth-skinned and bright yellow with white patches, and many black spots. It is a relatively small caterpillar, growing to about 1 1/4 inch long. Like other inchworms, the currant spanworm has just two pairs of prolegs on its abdomen and makes a characteristic looping motion as it walks.
These caterpillars overwinter as eggs which hatch in spring. They feed on currants and gooseberry (Ribes spp.) from late May through June. Currant spanworms first chew the tips of the leaves down to the midrib before generally defoliating shrubs. They pupate soon after they are done feeding and emerge as moths in summer. They lay eggs on their host plants during summer, especially on the lower branches. There is just one generation a year.
A currant spanworm is generally not a common insect, but it can become abundant in localized areas. Healthy, vigorously growing shrubs should tolerate its feeding damage, although moderate to severe defoliation can reduce the attractiveness of the plant. It is too late to treat currant spanworms now, but if you had a problem this year, scout susceptible plants next spring, starting mid to late May. If you discover an infestation then, you can treat them before defoliation becomes significant. There are many insecticides that are effective, including low impact products, such as insecticidal soap, spinosad, and Bacillus thuringiensis.
Originally published in Yard and Garden Line News, July 2008