The adult Colorado potato beetle is about 1/2 inch long and yellow with brown stripes. The larva is orange to red, with black spots, and can be 1/2 inch long when full grown. Both the larva and adult eat the leaf margins and entire leaves of potato, pepper, tomato, and eggplant.
Colorado potato beetles overwinter as adults in the soil. They become active in the spring as early potato plants emerge. They feed on the young potato plants, and they lay bright orange eggs in clusters on the stems and foliage. Larvae also feed on foliage. Typically, two to three generations develop each summer in Illinois.
Thresholds used in commercial potato production give some idea of what constitutes an infestation that needs to be controlled in gardens as well.
- On young potato plants in the spring, control is probably warranted if feeding by adult beetles or larvae results in more than 20 to 30 percent defoliation (20 to 30 percent of the plant's leaf area is lost); this usually happens when more than 2 to 3 adult beetles are feeding on each plant.
In the summer, when potato plants are blooming, some yield reduction is likely if defoliation exceeds 5 to 10 percent; this usually occurs when infestations exceed 5 larvae or 3-5 adults per plant.
- Natural enemies rarely keep Colorado potato beetle in check, so where infestations occur, hand-picking and application of insecticides are the most common approaches to controlling this insect in home gardens.
In addition to conventional insecticides, microbial insecticides that contain Bacillus thuringiensis tenebrionis can be used to kill Colorado potato beetle larvae.
- These microbial insecticides kill the target pest without killing other insects. Populations of Colorado potato beetle are resistant to one or more insecticides in some areas. If a registered insecticide fails to give control, switch to another insecticide in a different class. Do NOT retreat using a higher rate of the same insecticide.