Anyone who grows potatoes is familiar with the Colorado potato beetle. This insect pest is one of the best known beetles, famous for it’s ability to devour vegetables in the nightshade family: potato, tomato, eggplant and peppers. The adult beetles as well as their larvae can strip the plants of leaves and ruin an entire crop, if left to their own devices.
The rather attractive dome-shaped adults are nearly a half-inch long, yellow, with five black stripes on each wing cover. In late spring, after hibernating in the soil, usually along field edges, they emerge, with one thing on their mind: finding a food source. They walk toward this goal for a few days until they have the strength to fly about looking for potato plants or their relatives. Once they find them, the female beetles start snacking on foliage and laying clusters of 15 to 25 bright yellow-orange eggs on the undersides of the leaves. From these hatch the soft-bodied, hump-backed larvae that look a little like slugs.
They larvae start out small but grow fast, molting four times into larger stages, or instars. The last instars grow to be quite plump, and they’re the ones that eat the most foliage. Then, they crawl into the soil and pupate. From this resting stage, a second generation of adults emerges by the end of the season, ready to eat again before passing the winter underground.
It’s amazing that not long ago, this serious pest of vegetables pest was a harmless, well-behaved insect. It fed only on the buffalo bur, a tough weed that grows along the eastern foothills of the Rocky Mountains. Then, about 150 years ago, the beetles discovered a new food growing in the white man's gardens. It adopted the cultivated potato as its favorite food, spread rapidly, and we’ve been fighting Colorado potato beetles ever since.
It happened like this: with the opening up of the West following the Mormon migration to Utah in1847 and the California gold rush of 1849, pioneers arrived by the thousands and many of them planted potatoes. By 1855 potato growing reached westward to the native home of the beetle, and the insect started to spread eastward along the routes traveled by the pioneers.
By hitchhiking and flying with the prevailing winds, the beetle migrated about 85 miles a year. It reached Nebraska in 1859, Illinois in 1864, Ohio in 1869, and the Atlantic coast in 1874. This caused great alarm overseas, and almost every European country banned the importation of American potatoes. Europe’s potato growing regions remained free of potato beetles until after World War One, when they appeared near Bordeaux, France, where there had been concentrations of American troops and supplies. Now the beetle is widespread in Europe, too.
Prior to the coming of the Colorado potato beetle as a pest, the name "potato bug" was used to describe a different beetle that is a relatively minor pest on potatoes. Now called the “old-fashioned potato bug”, this long slender beetle has two black stripes on each wing cover. The adult feeds on potatoes but lays its eggs in the ground and the predatory young feed on grasshopper eggs and the like.
There are many naturally occurring controls of Colorado potato beetle. Fungi infect them, beneficial insects attack them, and toads and birds eat them. Rarely, however, do these forces combine to offer sufficient control to protect crops. Home gardeners resort to hand picking the beetles, followed by squishing, stomping or drowning them. Over the years, farmers have unleashed an arsenal of insecticides against the potato beetle, only to find it developed resistance to many of them.
In recent years, new pesticide materials have been developed that have lower human toxicity and less environmental impact than those of the past. The most widely used of these is a strain of the B.t. bacterium that specializes in infecting potato beetles. It’s sold under several bio-insecticide brand names. However, development of resistance remains a concern if a single class of insecticide is relied upon too heavily for control.
Cultural practices play an important role in the management of Colorado potato beetle, as they help minimize the need for insecticides, protect natural predators, and prevent the development of pesticide resistance. Such practices include crop rotation, trapping, mulching, row covers, and flaming.
By changing the location of host plants, crop rotation is one way to suppress potato beetle populations because it makes it harder for them to get to their food after they emerge in the spring. The problem is, the pest is rather mobile so long distances are needed between last year’s fields of potatoes and this year’s in order to have much of an effect. A few hundred feet doesn’t do much; a few thousand feet may help; but a mile or two is better. Physical barriers like rivers or roads in the terrain between rotated fields can enhance the effectiveness of crop rotation.
Mulching the potato crop with straw after an early-season hilling may also reduce the Colorado potato beetle’s ability to locate potato fields, and the mulch creates an environment that favors beetle predators. Growers using this system note that the straw should be spread so it’s touching the potato plants, but only a light layer of straw mulch is needed. Another way to protect plants is to place lightweight floating row covers over them and then seal the edges, well before the beetles arrive.
Plastic-lined trenches have been used to trap beetles as they head toward a field of potatoes in the spring. A plow or heavy disk can be used to form a shallow trench around a field or along the edge of a field by a hedgerow or previous potato field where new beetles are likely to come from. Then black plastic is placed in the trenches, sort of like an ‘upside down’ raised bed. Since the beetles can’t fly when they first emerge, they walk toward their food and fall into these shallow trenches. The dust keeps them from getting enough of a grip to get out, and as the black plastic gets hot the beetles get roasted.
Flaming of the early-season adults is another practice that’s employed to roast beetles. As they start to colonize field a tractor-mounted or hand-held flamer can be used to create a very quick pass of intense heat directed at the plants while they are still small. This needs to be done when weather conditions are right for the beetles to be active on top of the foliage. The potato plant can recover from this brief exposure to intense heat, but the beetles are injured, and fall to the ground, unable to recover.
For more information on managing Colorado potato beetle, go to an Extension pest management guide, such as http://www.nysaes.cornell.edu/recommends/ and click on potatoes.
Also check out http://attra.ncat.org/attra-pub/coloradopotato.html
for information on alternative and organic controls.