Colorado Potato Beetle

Colorado potato beetle is a major pest of potatoes that is native to America and Mexico. It has been in Iowa for over 150 years and has a long history of devastating outbreaks. This article provides some basic information on Colorado potato beetle identification, life cycle, and damage to potatoes. With the help of Dr. Ian MacRae at the University of Minnesota, I put together an IPM plan for this devastating pest.

Identification

Adult: Adult Colorado potato beetles are oval in shape and 3/8 inch long. They have a yellow-orange prothorax (the area behind the head) and yellowish white wing covers with 10 narrow black stripes (Photo 1). 

Egg: Eggs are oval and bright yellow to orange in color (Photo 2), turning dark red just before hatching. They can be found in clusters on the underside of leaves.

Larva: Newly-emerged Colorado potato beetle larvae have brick red bodies with black heads. Older larvae are pink to salmon colored with black heads. All larvae have two rows of dark spots on each side of their bodies (Photo 3).

Pupa: Fully-grown, fourth-instar larvae drop to the ground to form a pupa in the soil (Photo 4).

Biology

Adult Colorado potato beetles overwinter in potato fields, field margins, and windbreaks approximately five to ten inches below ground. They become active in the spring at about the same time potatoes emerge (typically in May). Adults feed for a short time in the spring and then begin to mate and lay clusters of 10-30 eggs on the undersides of leaves. Each female can lay up to 350 eggs over 3-5 weeks.

Eggs begin to hatch within two weeks, depending on temperatures. Larvae remain aggregated­ near the egg mass when young but begin to move throughout the plant as they eat the leaves. Larvae can complete development in as little as 10 days if average temperatures are in the mid 80’s. It will take over a month if temperatures average near 60°F. Larvae mature through four instars before they drop from the plant, burrow into the soil, and pupate. There can be two generations in Iowa. Because eggs are laid over time, all life stages of Colorado potato beetle may be present at the same time in a potato field by July.

Plant Injury

Both larvae and adults feed on the foliage of potatoes (Photo 5) and, if left untreated, can completely defoliate plants (Photo 6). In addition to potato, they may also feed on eggplant, tomato, pepper, and other plants in the nightshade family (Solanaceae). Older larvae (i.e., 4th instars) are responsible for as much as 75% of feeding injury.

Scouting

Scout fields regularly since injury can happen quickly. Check 50 plants every three to four days. Record the percent defoliation across each entire plant, and note the life stages of insects you find and the size of any larvae. This information will help determine whether enough plant injury has occurred that warrants treatment, and the most effective management option depends on the life stage(s) present.

Potatoes can usually tolerate substantial defoliation, up to 30%, when they are in the vegetative stage. Once tubers begin to bulk, they are much more sensitive and can only tolerate about 10% defoliation. Tuber bulking begins soon after flowering, making this time critical for Colorado potato beetle management. Consider treatment if defoliation exceeds these thresholds.

Management

In general, Colorado potato beetles are very difficult to suppress because of their biology. Successful suppression will take an integrated approach with a focus on being proactive. We often recommend using IPM (integrated pest management) to help with field crop pests, and Colorado potato beetle is no exception. It should include some or all of the following tactics:

Tracking development: Like all insects, Colorado potato beetle development is somewhat predictable based on air temperatures and growing degree days (GDD; Table 1). The lower developmental threshold, a point at which no development will occur, for Colorado potato beetle is 52ºF. The very first adult you see in the spring sets the “biofix” where accumulating GDD begins. Target scouting and suppression efforts on the first generation, approximately 120-200 GDD after the biofix.

Cultural: Much of what farmers/gardeners do to grow plants favors insect development. If we can mix up our growing conditions to interfere with the habitat pests need to survive, that will greatly discourage them from devastating our crops. For example:

• Sanitation, or removing potential food and habitat, is an effective starvation technique. If potatoes or solanaceous plants (e.g., eggplant, peppers, tomatoes) are not available when Colorado potato beetle adults first emerge in the spring, they will seek out alternate hosts, such as nightshade and ground cherry. Remove old plants and weeds in and around potato plots before, during, and after the season to eliminate food sources.
• For small plantings, hand removal can be effective. Drop adults and larvae into a pail filled with soapy water. Also, remove or crush the eggs on the underside of leaves. Adults can fly in, so be sure to check your potatoes regularly. Hand removal may be less practical in larger plots.
• Crop rotation, or only growing potatoes only every other year, may help reduce beetle populations if no potatoes are being grown within a radius of ¼ to ½ mile away and temperatures are not excessively warm. Moving the area that is planted with potato, is largely ineffective because adults can fly long distances when temperatures exceed 70°F.

Date of planting and tillage are not very effective management strategies for Colorado potato beetle. Exclusion cages work well for small potato plantings but are likely not feasible for large fields and can be quite expensive.

Biological: There are few natural enemies of Colorado potato beetle, like predatory stink bugs and beetles. There is also a naturally-occurring fungus, Beauveria bassiana, that will kill larvae and adults. Unfortunately, biological control has minimal impact on CPB populations compared to other crop pests.

Genetic: Planting an early-maturing variety will allow you to escape much of the damage caused by adults emerging in midsummer. Check seed catalogs for varieties that mature in less than 80 days. Yields on early-maturing varieties are not as large, and often these varieties do not store as well as the popular Russet Burbank potato.

Chemical: Depending on the size of the potato plots and the historical outbreaks on the farm, insecticides may be required to protect the crop. This method can be used with other IPM tactics and in moderation (e.g., a last resort when other tactics are not working). Plan to scout and use thresholds to determine if applications are needed (see previous sections). There is a wide spectrum of chemical control products that target CPB, ranging from organic to restricted-use pesticides. Colorado potato beetle has evolved resistance to many insecticide modes of action, so be sure you are aware of local product recommendations.

• Microbial insecticides are generally approved for organic production and considered low risk to humans. This includes products that contain Bacillus thuringiensis var. tenebrionis (Btt) or spinosyn (derived from Saccharopolyspora spinosa), which are both derived from bacteria that have insecticidal activity against beetles. Btt is effective for young Colorado potato beetle larvae (1st and 2nd instars), but does a poor job on larger larvae and adults. If choosing Btt, spray every few days when you see egg masses hatching to ensure that small larvae are targeted. Spinosyns have a greater level of efficacy than Btt and can be used on larvae and adults. Spinosyns have a long residual (10-14 days).
• Plant-derived insecticides are not always approved for organic production, but most are. This includes products that contain neem oils (azadirachtin) or pyrethrins. For both of these, small larvae should be targeted for best results. Plan to spray every few days when you see egg masses hatching. Expect a residual of less than a day on these products. Some Colorado potato beetles have developed resistance to IRAC Group 3 pesticides, which includes pyrethrins.
• Synthetic insecticides have many more options but pose a higher risk to humans. Additionally, many of these products are no longer effective on Colorado potato beetle due to excessive use. To date, Colorado potato beetle has developed resistance to 57 active ingredients according to the Arthropod Pesticide Resistance Database.

If an insecticide application is required, it is important to read the label and follow all directions, particularly how soon you can harvest potatoes after applications (PHI). Keep the following considerations in mind:

• It is easier to kill young larvae compared to older larvae and adults. 
• Products are more effective when making direct contact with the body. Getting small droplets to contact Colorado potato beetle can be challenging since they feed and aggregate on the undersides of leaves. 
• Targeting suppression efforts on the first generation can greatly reduce the impact of the second generation when plants are more sensitive to defoliation.
• Most chemicals are broad-spectrum, meaning they will kill most insects that make contact with the application (e.g., pollinators, predators, etc.). This is true of organic and synthetic insecticides.