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The Colorado potato beetle, originally native to Mexico, moved into the Great Plains, east of the Rocky Mountains, following the spread of its Mexican host plant, buffalo burr (Solanum rostratum). About 40 years after settlers first moved potatoes into Iowa, the potato beetle adapted to this new host, spreading as a serious pest of potatoes through the USA and eastern Canada since 1860, as well as into much of Europe and Asia (interestingly, in Mexico the beetle still ignores potatoes). The Colorado potato beetle feeds on the leaves of several solanaceous plants and is known as a serious pest of potato and eggplant, as well as an occasional pest of tomatoes. Many beetle populations have developed resistance to the pesticides that have been widely used against them. Description: The larva of the Colorado potato beetle is a red to orange soft grub about 1/2 inch (12 mm) long when mature. It has a black head, black legs and, when large, two rows of black spots on each side of the body. The larvae are often found either on the underside of leaves or at the top of the potato plant. The adult is a distinctive yellow and black striped beetle. Ten black stripes run along the length of the wing covers. The beetles are convex, about 1/8 inch (8 mm) long. The eggs, orange-yellow in color, can be found in clusters of about 20 on the undersides of leaves.
Life Cycle: The beetle overwinters as an adult in the soil. It emerges early in the spring and typically walks to find new host plants. After feeding and mating, females begin laying up to 500 eggs over a 4-5 week period. Eggs hatch in 4-9 days and larvae begin to feed immediately upon emergence. The larvae go through 4 molts over a period of 2-3 weeks before entering the soil to pupate in a spherical cocoon. The new adults then emerge in 5-10 days and the cycle is repeated through a second generation. Damage: Adults and larvae both feed on host plants. Adult damage is generally considered less severe, except in cases where they "stem" young host plants early in the season. These severed plants often die. Moderate plant defoliation, less than about 15%, has no impact on yield, particularly if it is early in the season. However, potato beetles are frequently sufficiently abundant to totally defoliate plants, often killing them prematurely and severely reducing yield. Control: There are several effective means of dealing with this pest, including cultural, biological, physical and chemical techniques, several of which work best in combination. Cultural: Cultural techniques include the use of early plantings of fast-maturing potato varieties that can be harvested before beetle damage becomes significant. Crop rotation often helps delay the arrival of beetles if new plants are kept 200 yards or so from previous hosts. Rotations can work well with early plantings. An alternative is to delay planting so that potatoes emerge after populations of overwintered adults have declined. This approach has been proven effective when using a heat-tolerant variety such as "Caribe." Straw mulch can also delay beetle development, as well as enhance biological control. Biological: Biological controls include several native insects, both predators and parasitoids, that can be important in controlling beetle populations, particularly later in the season. The difficulty with biological control is that in most cases there are so many beetles early in the season that plants are either defoliated or treated with insecticides before the biological control agents have a chance to work. In conjunction with cultural techniques mentioned above, the blue-green ground beetle Lebia grandis often plays a significant role in controlling the beetle, as do two species of predatory stink bugs, a parasitic fly, and at least two ladybug species. A native fungus, Beauveria bassiana, often kills high numbers of beetles late in the season, leaving a characteristic white cadaver of beetle adults and larvae. Restraint in pesticide use will maximize the effectiveness of these natural biological control agents. There is also a commercially available biological control product, Bacillus thuringiensis, var. tenebrionis, that is effective in killing the Colorado potato beetle. Sold under many trade names, including Novodor, Foil, and Trident, this bacterium is most effective when used against young larvae and should be used starting when eggs begin hatching. Physical: Physical controls such as flaming and ditching are old approaches that have found renewed interest among commercial potato growers who have difficulty controlling the Colorado potato beetle by chemical means. Homeowners can also use ditches to keep beetles from their crops. This involves digging a ditch with steep sides lined with sheet plastic around the perimeter of a field. Beetles walking to crops from overwintering sites fall into the ditch and are unable to climb or fly out. With small numbers of plants, hand-picking of adults and egg masses can be very effective. Chemical: Chemical controls have become difficult because the Colorado potato beetle is now resistant to most common garden insecticides such as Sevin and diazinon. Rotenone remains effective and is commonly used against this pest, although it also kills the beetle's natural enemies. A botanical insecticide based upon the extracts from the Neem tree has also been shown to be effective against the beetle. This pesticide, which has low human toxicity, has recently been registered for food crops. Genetic Engineering: Genetically engineered potatoes with resistance to the Colorado potato beetle and several other key pests are now available to both homeowners and commercial growers. Although some consumers are concerned about potential effects on human health, the scientific community is more concerned about environmental effects, including accelerated development of pesticide resistance. It is not likely that homeowner use of genetically modified potatoes will have adverse environmental impacts.
By Dr. Richard A. Casagrande, URI Department of Plant Sciences, 1999
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