A3678

Colorado K.A. DELAHAUT Life cycle The Adult Colorado potato ( decemlineata) is a Colorado potato beetles over- are easy to spot. They have bright common and destructive of winter as adults in Wisconsin. In the yellow bodies with black stripes potatoes and in home fall they burrow 2–8 inches deep in running the length of their backs and gardens and fresh-market gardens. If the , choosing protected areas black spots behind their heads. The 3 left unchecked, Colorado potato near trees or in grassy edges sur- beetles are about ⁄8-inch long with beetle larvae and adults can com- rounding gardens. Adults emerge in hard, rounded shells. pletely defoliate plants, causing the spring, at about the time the first serious yield losses or plant death. shoots of early season potatoes or Once a garden has been infested, volunteer plants appear. potatoes and will suffer progressively worse damage in sub- sequent . The beetles prefer to feed on potato, but they will also feed on eggplant and such as night- shade, groundcherry, jimsonweed, horsenettle, and mullen. Weeds provide a widespread source of food, allowing beetles to colonize new areas. Symptoms and effects

Both adults and large larvae are voracious feeders. They can 1 chew holes larger than ⁄8 inch across into the of susceptible plants. They often consume entire leaves Larvae and adult Colorado potato beetles beginning with young, succulent devouring leaves from a potato plant. leaves. Larvae typically feed in groups and may completely defoliate plants. The larger larvae (later stages) do the most feeding damage. Heavy defoliation will severely reduce plant yields, particularly if it occurs when potatoes are flowering.

Look for clusters of bright yellow-orange on the undersides of leaves beginning in early May. COLORADO POTATO BEETLE

Control Females lay up to 500 bright related to temperature: cool weather yellow-orange eggs in clusters of Begin checking for Colorado slows growth, warm weather acceler- 15–25 on the lower leaf surfaces potato beetles in early May after ates it. So unless the weather of a before dying. Larvae hatch from the potato plants have emerged. Continue given is that of the “normal” eggs 4–9 days later and begin feeding checking both potatoes and eggplants year, recommended treatment dates immediately. The larvae will molt through the end of July. Examine the may not coincide with the most vul- four times before pupating. Each lower leaf surfaces for clusters of nerable life stage of an pest. stage between molts is called an bright yellow-orange eggs. In areas Using a system based on the daily . First instar larvae are blackish- that were infested the previous year, high and low temperatures instead of brown and tiny, about the size of a look for adult beetles on plants near calendar dates will help you better comma. They congregate on field edges in early May. Because of anticipate pest outbreaks. This system expanding foliage on the ends of the cool weather at this time of year, converts daily average temperatures stems. Because of their small size, feeding by adults is minimal. into degree days. A degree day is a feeding damage is inconsequential. Potato plants can tolerate varying unit of measure for each degree above Second instar larvae begin to assume levels of defoliation before they will a base temperature (52°F). To use this the more typical brown-red larval col- suffer yield loss. The level of tolerance approach, begin keeping track of the oring, and although feeding damage depends on the plant’s growth stage. temperature when you find the first is more evident, damage from second Flowering plants can tolerate only mass. Then add the daily high will not be severe enough to 5–10% defoliation; after flowering, and low temperatures, and divide by require treatment. Third and fourth plants can tolerate up to 30% defolia- 2 to calculate the average tempera- instar larvae are reddish-brown with tion before yields are affected. Control ture. Subtract 52°F from the average black heads and legs and black spots measures for potato are not necessary to get the number of degree days. along the sides of their bodies. Fourth while defoliation remains below these Keep a running total of the number of instar larvae are about the size of a levels. Eggplants, however, can be degree days to chart insect develop- 3 pea ( ⁄8 inch). The last two instars defoliated at any stage of develop- ment. Table 1 identifies the number of consume increasingly more foliage ment, totally eliminating any yield. degree days needed for each stage of and will cause serious crop damage. Most chemical pest management Colorado potato beetle development. After passing through four instars practices are timed to calendar dates. over 2–3 weeks, larvae return to the But insect development is directly soil to pupate. Within 10–14 days, the second generation of adult beetles emerges. Second generation adults nor- Table 1. Rate of beetle development using degree days mally appear in mid-July and may Accumulated cause severe defoliation. When tem- Life stage Degree days degree days Treatment peratures are above normal in June and July, second generation adults Egg 120 120 Not susceptible— do not treat may produce a second generation of larvae. Under normal conditions, First instar 65 185 Most effective time these adults produce only a partial to apply Btt second generation and then seek Second instar 55 240 Most effective time to apply overwintering sites. There are usually conventional 1–2 generations per year in most of Third instar 60 300 Wisconsin. Fourth instar 100 400 Pupae 275 675 Not susceptible— do not treat Natural control Physical barriers, such as row Another product available for A number of predaceous bugs, covers and plastic-lined trenches, can control of the young larvae is the bio- two of parasitic tachinid , be used to prevent adult beetles from logical and some may reduce popula- finding plants and laying eggs. Row var. tenebreonis (Btt). This product is a tions of Colorado potato beetle. covers should be in place before bacterium that is harmless to people, However, none of these natural con- plants emerge and left there until , and plants, but causes a trols are particularly effective. Cultural and mechanical control mid-June. Plastic-lined trenches lethal in Colorado potato should be situated between fields and beetle larvae. It is only effective A variety of cultural methods are overwintering sites before plants against first and second instar larvae available to reduce the number of emerge. Dig trenches 18 inches deep and must be applied as an early spray Colorado potato beetles. In home and 18 inches wide, line them with against small larvae. Bacterial insecti- gardens, the large larvae and adults plastic, and cover the plastic with a cides only persist 1–2 days and thin coating of dust. should be applied weekly for 2–3 can be hand picked or removed with a Chemical control net. Be sure to wash your hands applications if used alone. As with before touching your or mouth resistance is a very other , there is a danger after hand picking Colorado potato serious threat to continued effective that overuse of this product will lead beetles because the contain a control of Colorado potato beetles. to the development of resistant chemical which can burn and blister Wisconsin beetle populations are still beetles. Bacillus thuringiensis is avail- sensitive skin. Also, planting crops susceptible to a wide range of insecti- able in several formulations. other than potato and eggplant for a cides, but repeated use of any one However, only Bt var. tenebreonis is year will reduce the population. material can rapidly lead to loss of effective against Colorado potato Hand picking is not feasible in effectiveness. In fact, races of the beetles. The more commonly available many fresh-market gardens, so other beetle exist that are resistant to every Bt var. kurstaki (Btk), which is effective strategies are needed. Crop rotations class of insecticide currently regis- against , will have no effect that avoid solanaceous plants such as tered for control. In Wisconsin, many on beetle larvae. eggplant and potatoes will substan- Colorado potato beetles are resistant Scientists have genetically engi- tially reduce the beetle population. to the insecticide (Sevin), ren- neered potatoes that have Btt present This practice must be combined with dering it essentially useless in the in the plant. Since the is the removal of alternate host weeds to home garden. always present, no sprays are needed. be effective. An alternative approach The best time to control Colorado Currently, only the Russet Burbank is to plant strips of potatoes on field potato beetles using carbaryl (Sevin) variety Newleaf® is available as a edges next to overwintering sites. The or insecticides is when transformed resistant plant. Other beetles are attracted to these “trap” larvae are at the second and third varieties are expected to be released crops, where they can be removed instar stages. Treatments at the fourth soon. It is important to understand using propane flamers, vacuum instar stage will be made too late to that using resistant plants exclusively suction, or other physical controls. prevent yield loss. Eggs and pupae is not the answer to controlling this are not susceptible to chemical control. pest. If large amounts of resistant plants are planted in the same loca- tion year after year, Colorado potato beetles will become resistant to Btt. References to products in this publication are for your convenience and are not an endorsement of one product over other similar products. You are responsible for using chemicals according to the manu- facturer’s current label directions. Follow directions exactly to protect the environ- ment and people from chemical exposure. Copyright © 1997 University of Wisconsin-System Board of Regents doing business as the division of Cooperative Extension of the University of Wisconsin-Extension. Send inquiries about copyright permission to: Manager, Cooperative Extension Publishing, 432 N. Lake St., Rm. 103, Madison, WI 53706. Author: K.A. Delahaut is horticulture outreach specialist, College of Agricultural and Life Sciences, University of Wisconsin- Madison and University of Wisconsin-Extension, Cooperative Extension. Produced by Cooperative Extension Publishing, University of Wisconsin-Extension. University of Wisconsin-Extension, Cooperative Extension, in cooperation with the U.S. Department of Agriculture and Wisconsin counties, publishes this information to further the purpose of the May 8 and June 30, 1914 Acts of Congress; and provides equal opportunities and affirmative action in employment and programming. If you need this material in an alternative format, contact the Office of Equal Opportunity and Diversity Programs or call Cooperative Extension Publishing at 608-262-8076. This publication is available from your Wisconsin county Extension office or from Cooperative Extension Publishing. Call toll-free, 877-WIS-PUBS (947-7827), or online at cecommerce.uwex.edu. A3678 Colorado Potato Beetle I-03-97