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Knowledge to Grow by Karen Weiland - Potato Beetle

As potato crop pests go, the Colorado potato beetle is a nasty one. The adult is an oval shaped yellow bug about ¼ inch long with 10 black stripes on its back.

It overwinters some 6-10 inches below the surface of the soil. Then when the soil becomes warm enough to plant potatoes, they emerge looking for a new home. When they find a suitable host plant they start to feed right away.

They do not limit themselves to potatoes – in fact anything in the nightshade family will do. Even though potatoes are their favorites they will settle for tomatoes, peppers and eggplant. After the adults mate, the female beetle lays several hundred orange-red eggs, usually on the undersides of the lower leaves of the potato plant. Depending on the temperature the eggs can hatch anywhere between 4-10 days.

After hatching the larvae will begin to feed on the potato leaves. The larvae are about ½ inch long, reddish in color with a black head and legs and two rows of black spots along each side. They can defoliate a plant within one to two days. After two to three weeks of feeding, the larvae will drop from the plant, burrow into the soil and pupate. The adults emerge two to three weeks later and then the cycle starts all over again. There are usually two generations per year. Good grief, those potatoes at the grocery store are looking pretty good. Just joking! There are methods of fighting these bugs.

One is the use of insecticides. However, most synthetic insecticides available to home gardeners fail to provide adequate control due to issues of insecticide resistance. A relatively new insecticide called esfenvalerate can be tried, but anytime an insecticide fails to kill potato beetles, switch to a product with a different active ingredient. Pyrethrin can be effective but is short-lived. This works best on smaller larvae.

A more organic way to get rid of these pests is to hand pick them off of the plants and then destroy them before they begin to lay their eggs.

Bacillus thuringiensis (Bt), which is considered non-synthetic and therefore allowed for certified organic production, is an effective product available to home gardeners. Bt must be eaten by a susceptible insect in order to work, therefore the spray must be applied to the plant where and when the insect is feeding. Thorough coverage is important. Apply every few days when the egg masses have begun to hatch. Only apply Bt when needed to lessen the chance that insect populations will develop a resistance to it. When eaten, the insect stops feeding and dies within a few days.

Adults normally emerge in mid-summer, so planting an early maturing variety of potatoes will help as will keeping your garden free of weeds they may like to feed on if potatoes are not available.

As always, Happy Gardening!

The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange County.