Figure 1. Defoliated sunflower leaf with webbing and frass and thistle caterpillar. Courtesy: Philip Rozeboom
Thistle Caterpillars in Sunflower
During sunflower scouting last week, there were several sunflower plants that had defoliated leaves with webbing present (Figure 1). The insect causing this defoliation was the thistle caterpillar, which is the immature stage of the Painted Lady butterfly (Figure 2). Thistle caterpillars feed within the leaves that they roll and hold together using webbing (Figure 3). The thistle caterpillar is rarely an issue in sunflower; however, it is possible that these caterpillars may cause enough defoliation to warrant management.
Figure 2. Painted lady butterfly. Credit: A. Varenhorst
Thistle caterpillars are aptly named due to the spines that are present on their body, which give them a prickly appearance. The caterpillars can vary in color from brown to black, but will have yellow lines that run down each of their sides. These caterpillars produce a webbing that is used to pull leaves together near the site where they are feeding. These “rolled” leaves will often have large holes present with concentrated areas of frass or waste.
Figure 3. Thistle caterpillar feeding within a rolled leaf. Credit: A. Varenhorst
Scouting & Management Recommendations
To scout for thistle caterpillars, 10 random sunflower plants should be scouted and the process repeated for five areas within a field. For each of the selected plants, evaluate the amount of defoliation that is present and record the growth stage of the sunflower. The threshold for sunflower defoliation is 25% and the observed thistle caterpillars are less than 1 ¼ inches in length. This insect is sometimes managed using spot spraying for areas where infestations are more severe. For a list of foliar insecticides that are labeled for thistle caterpillar management, please refer to the current edition of the South Dakota Pest Management Guide: Sunflower.
Figure 4. Defoliation chart. Developer: Marlin E. Rice