Corn Rootworms

Host Plants

Corn rootworms are the scourge of corn, particularly in the Midwest of the United States.

Regions impacted

Found throughout North America, especially in the midwestern corn-growing areas such as Iowa. Also present in other temperate climates, including in northern Europe.

Description

Corn rootworms are a devastating pest of cultivated corn in North America. While adults can feed on a variety of plants, their larvae survive exclusively on the roots of corn. There are three primary corn rootworm species in North America:

• Northern rootworm – most prevalent throughout the corn belt. The adult of the northern corn rootworm is tan to pale green and about 1/4 inch (6 mm) long. Newly emerged beetles are usually cream or light brown but gradually turn green with age. Male and female beetles have similar coloration, however, female beetles are typically larger than their male counterparts.
• Western rootworm – also impacts the corn belt. Western corn rootworm adults are yellow to green in color with a black stripe along the sides of their wing covers and are about 5/16 inch (7.5 mm) long. Male wing covers are usually entirely black and darker than those of females, which can appear with striped patterns.
• Southern corn rootworm – found throughout the U.S. but rarely causes serious damage. The southern corn rootworm adult is about 3/8 inch (9 mm) long, yellow to green, with 12 conspicuous black spots on its back. This species is also known as the twelve-spotted cucumber beetle.

It is difficult to distinguish between larvae of the western and northern species. Rootworm larvae are white and slender, about 1/2 inch (13 mm) long when fully grown, and sporting brown heads.

Life Cycle

Western and northern corn rootworms have only one generation per year. Eggs of both species are deposited in the soil by female beetles from mid-summer until fall. The eggs overwinter and begin hatching from late May to early June in most areas of the Midwest. The newly hatched larvae seek out and feed on corn roots. The rootworms pass through three larval instars, then in late June/early July pupate in soil near the corn roots. After 5 – 10 days, adults emerge, with males appearing a week or so before the females. Mating takes place a short time later. Within two weeks of emergence, females begin laying eggs, usually toward the end of July, with peak egg laying occurring in early to mid-August.

Damage and Detection

• Both corn rootworm larvae and adults can damage corn plants.
• The beetles feed on the foliage, pollen, silks, and developing kernels of corn. The latter attacks can interfere with pollination and germination, negatively impacting kernel production.
• Larvae are very disruptive to the root system, feeding primarily on root hairs and outer root tissue. As larvae mature, they burrow more deeply into the roots to feed. Damage can include extensive root injury and the development of secondary infections.
• Roots damaged and weakened by rootworms may topple or grow in a hunched “gooseneck” shape. These plants can pollinate poorly and are more difficult to harvest.
• Western corn rootworm beetles can also wreak damage by feeding on the green tissue of the leaves. Unless feeding in abundance, these beetles are not usually dangerous to the crop but can indicate a large adult population which may impact the next season’s roots.

Prevention and Control

To mitigate the impact that larvae can have on the roots of corn, the following are recommended:

• Space permitting, rotate the field annually. Rotating a distance of a few feet will be sufficient as the newly hatched larvae cannot move very far. Rotating to legumes can starve the bulk of the larvae within the year. Rotation to grasses such as wheat or rye is also effective. Even a 2-year rotation can be effective as rootworm populations take time to move and establish.
• Encourage natural predators such as birds and competing ground beetles by creating some permanent beds or mulched pathways where the soil will remain undisturbed;
• Maintain a tidy garden to give corn rootworm beetles fewer places to hide during the winter;
• If applying a soil insecticide, follow the directions closely. Insecticides do protect the roots but are not as effective as rotation – you can get rootworm population growth in unprotected areas even when soil insecticides are applied.