Last week we received calls about insects feeding on potato plants. The culprit was the Colorado potato beetle, which is a major pest of potatoes. Although the name suggests that this beetle would only feed on potatoes, it can also be a pest of other plants in the nightshade family, including tomatoes and eggplants. The Colorado potato beetle can also feed on weeds in the absence of garden hosts and it was originally reported on buffalo-bur.
Colorado potato beetles overwinter as adults and emerge in the spring to feed on newly sprouted plants. In South Dakota, there can be 1-2 generations of Colorado potato beetles per year. The larvae and the adults will both feed on host leaves. However, the later stage larvae are responsible for approximately 75% of plant defoliation.
Early instar larvae of the Colorado potato beetle are red. As the larvae mature, they will turn pink. All of the larval stages will have a black head and two rows of black spots on each side of the body (Figure 1-A).
Adult Colorado potato beetles are oval in shape and approximately 3/8 of an inch long. They have an orange head and prothorax (i.e., segment behind the head) as well as an orange underside. The prothorax will have black markings present. The elytra (hardened forewings) are cream colored with five black stripes on each side (Figure 1-B).
Potatoes can tolerate 30% defoliation during the vegetative stage without any issue. However, when tubers begin to bulk, the plants can only tolerate 10% defoliation. Tuber bulking occurs shortly after the plants flower. If the thresholds have been reached and Colorado potato beetle larvae or adults are observed on plants, management is likely necessary.
At this time in the season, beetles and larvae can be removed by hand and destroyed if there is a limited number of plants to scout. This process won’t be realistic for larger gardens. Insecticides containing the active ingredients azadirachtin or spinosad can be used. Colorado potato beetles are resistant to products containing the active ingredients carbaryl, cypermethrin, deltamethrin, lambda-cyhalothrin, imidacloprid, permethrin and pyrethrins. If using an insecticide, closely monitor populations after application. If the application doesn’t kill the Colorado potato beetles, reapply