The primary damage of Colorado potato beetle is leaf feeding by larvae and adults, although young fruits can also be eaten if the host is eggplant or tomato. Leaf feeding has the greatest effect on potato growth if it occurs within two weeks of peak flowering; leaf feeding during the last few weeks before harvest or very early in the growth of the crop has little effect on yield. Host plants include potato, tomato, eggplant, pepper, tobacco, and other plants in the nightshade family.
The adult Colorado potato beetle is 3/8 inch (10 mm) long, with its rounded outer wings forming a hard "shell" marked with black and yellowish-white stripes. The head is tannish-orange with black markings and the beetle has six tannish-orange legs. The eggs are yellowish-orange, found in clumps of 20 to 40 on the underside of leaves. The larva (immature form, or "soft-shell" beetle) has a soft, reddish-tan body with two rows of black spots along each side, six black legs, a black head, and is 1/8 to 1/2 inch (3 to 13 mm) long. The pupa is a yellowish color and is found in a motionless condition in a round cell in the soil.
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The Colorado potato beetle lives in Ohio year-round. Adult beetles overwinter in soil or under litter in fields, gardens, or fencerows, and they begin to emerge from these locations in May. All of the beetles in an area may emerge over a short period of time, but if weather conditions are dry or cool, beetles may emerge over a prolonged period. After they emerge from the soil, beetles walk in search of suitable host plants where they feed and lay eggs on leaves. If they do not find suitable food while walking, they will fly in search of food. Eggs hatch into larvae in 4 to 15 days, depending on temperature.
Larvae begin to feed on leaves during June, often in exposed locations on the upper part of the plant. Early-season infestations are sometimes not evenly distributed within a garden but are clumped on one side of the planting or on a small number of plants. After 2 to 4 weeks of feeding, larvae are fully grown and they drop from plants and dig into the soil where they go through their pupal (resting) stage. After 5 to 10 days in the soil, the pupae turn into adult beetles and crawl above ground. The new adults feed on leaves, then mate and lay eggs. The second brood of larvae is usually active in July and August. There are usually two broods of larvae per year in Ohio, although there can be one or three broods in some years.
Because of resistance problems, chemicals alone are no longer adequate for control of Colorado potato beetle. Infestations should be prevented by using appropriate cultural practices. If insecticide sprays are needed, careful thought should be given to the selection of the chemical and the spray timing.
Potatoes planted in early-May are likely to be infested by potato beetles at the worst possible time, which is during flowering. Gardeners have an opportunity to avoid beetle problems by planting all of their potatoes either a month earlier or a month later than early-May.
If potatoes are planted very early (in early-April) so that plants bloom before June, beetle damage will occur too late to affect yield. Use an early maturing variety and aim for an early-July harvest. Although the early-planted potatoes can tolerate much leaf feeding, the gardener must learn to tolerate the presence of infested plants. If the gardener can't stand the sight of infested plants, it would be better to try the late-planting strategy.
If potatoes are planted late (in mid-June), most beetles will have left the area before plants emerge. Again, use an early-maturing variety so that the crop will mature before October.
Promote rapid emergence and growth of potatoes by selecting a well-adapted cultivar (variety) for the site. Larger plants are better able to tolerate defoliation than smaller plants.
Use early-maturing cultivars and harvest as soon as the crop is mature. This reduces the food source for late-season beetles and can weaken them for their overwintering phase.
Hand-picking can effectively control the Colorado potato beetle in small plantings. Inspect plants once or twice per week, remove all larvae and adult beetles, drop them into a container of soapy water, then dispose. Dislodge beetles by lightly beating plants with a broom; hold a snow shovel under plants to catch the dislodged beetles. Eggs can be crushed by hand. A vacuum cleaner can be used to remove beetles from young plants.
Lightweight row covers can be placed over plants as a barrier to prevent adult beetles from colonizing the plants.
Several natural enemies of the Colorado potato beetle can help reduce the pest population, but none has yet been found that alone can control this pest. Potato beetle eggs and larvae are preyed on by two kind of stink bugs: the spined soldier bug and the two-spotted stink bug. Ladybird beetles ("ladybugs") feed on eggs of potato beetles, and a parasitic fly and a fungus can attack potato beetles. Ladybird beetles also feed on pollen so they can be attracted to gardens with many pollen-producing plants. Natural enemies may be protected by use of microbial insecticides that are toxic to the Colorado potato beetle but not toxic to the beneficial insects mentioned above.
A relatively new group of insecticides is made from naturally-occurring bacteria that cause certain kinds of insects to become sick and die. One type of bacteria called B.t. or Bacillus thuringiensis, varieties san diego and tenebrionis, has been developed for control of Colorado potato beetle larvae. The bacteria must be eaten by larvae. These products are slower acting than conventional insecticides; larvae may not die until 4 to 5 days after treatment, although they are too sick to feed during that time. B.t. must be applied as a spray when eggs are just starting to hatch. B.t. is more effective at warmer than at cooler temperatures because insects feed faster as temperature increases. An advantage of the B.t. products is that they are not toxic to natural enemies of the potato beetle.
Representative brand names and producers: Colorado Potato Beetle Attack (Ringer), B.t. Leaf Beetle Attack (Safer), Colorado Potato Beetle Beater (Bomide), Colorado Potato Beetle Pre-Adult Control (Dragon), M-One and M-Trak (Mycogen).
Small larvae are most easily killed by insecticides, large larvae are more difficult to kill, and adults are hardest to kill. Insecticides are thus likely to be more effective if applied when most of the beetle population is in the early larval stage.
The Colorado potato beetle can develop resistance to insecticides. In areas of intensive potato production where insecticides have been heavily used, beetles are resistant to nearly all insecticides. Because resistance to one chemical is often followed by resistance to related chemicals, it is best to alternate chemical classes when you make applications; you would use one class during May and June for first-brood beetles, then switch to a different class during July and August for second-brood beetles. Five major classes of insecticides are listed below. Be sure to follow directions on the label of any product.
NOTE:Disclaimer - This publication contains pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author, The Ohio State University and Ohio State University Extension assume no liability resulting from the use of these recommendations.
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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