One of the familiar pests of the garden is the Colorado potato beetle, also known as the "potatobug." It's interesting to note that potatobugs were very familiar to Iowans through the first half of the 20th century but that they seemed to disappear for a while only to re-emerge as a major pest problem in the 1990s and remain widespread and destructive to this day. Colorado potato beetles feed on the foliage of potato, tomato, eggplant, pepper and other related plants. They are most damaging to potatoes where the defoliation reduces yields and may even kill plants.
Colorado potato beetles are approximately 3/8 inch long with an oval, convex body. There are 10 alternating yellow and black stripes on the wing covers. The adults spend the winter hiding in wooded areas and other protected locations and then begin to lay eggs on host plants in early spring. Eggs hatch into dark red, humpbacked larvae with a dark head and two rows of black spots along the sides of the abdomen. Larvae often feed in groups, causing isolated severe defoliation. When the larvae have grown to one-half inch in length they burrow into the soil to transform to new adults that appear in mid-summer and repeat the cycle.
Colorado potato beetle is a difficult pest to control. Hand picking has been used since before the development of modern pesticides. Hand-pick beetles, eggs and small larvae from infested plants as soon as possible (practical for a few insects on a few plants, but impractical for larger gardens and fields). Especially remove overwintering beetles that appear on young plants in the spring.
Another way to avoid severe loss is to adjust planting time. Defoliation of potatoes is most damaging in the period just before and during flowering. Planting either earlier-than or later-than normal allows the potatoes to be in bloom before or after, respectively, the peak of beetle activity. This strategy is most effective when combined with an early-maturing variety.
When insecticide treatment is warranted consider timing, coverage and insecticide choice. Timing is critical. Small larvae are much easier to control and spraying when the larvae are small is much more effective (and required with certain insecticides) than delaying and spraying after the larvae are grown. Early treatment is also necessary to prevent crop loss. Complete and thorough coverage of infested plants is necessary for good control. With that in mind, control is generally more effective with liquid sprays than with dust applications.
Because of decades of repeated insecticide use, the Colorado potato beetle may be resistant to many available insecticides, including Sevin and malathion. Consider other controls available at your local garden store, including pyrethroids and biorational ingredients such as spinosad; Bt tenebrionis, Neem (azadirachtin) and the pathogenic fungus, Beauveria bassiana. Note that the biorationals are only effective against very young larvae; they will not kill large larvae or adults.