Across the U.S. Corn Belt, corn rootworm (Diabrotica spp.) represents the number one group of insects that can threaten farmers’ corn yields. Research estimates charted over the years place annual economic losses from these pests at close to $1 billion annually.
Corn rootworms comprise a three-species insect complex: Western corn rootworm, Northern corn rootworm and Southern corn rootworm. All species possess lifecycle similarities, with the four stages of egg, larvae, pupa and adult beetle. They each lay eggs that hatch as larvae (worms) to feed on the roots of corn plants. Conversely, the three species differ in biological characteristics and management methods.
Identification and Lifecycle
The Western corn rootworm adult beetles appear yellow to green in color with a black stripe on their wings. Males may have more black on their wings, and females have larger abdomens.
The adult Northern corn rootworm appears initially as a tan-colored beetle, which turns green as it matures. Both males and females are similar in color, but females are typically larger.
Both Western corn rootworm and Northern corn rootworm beetles produce a single generation of larvae each growing season. Female beetles of both species lay eggs in the soil from midsummer until fall. These eggs overwinter and begin to hatch as larvae in late May or earlier across much of the Corn Belt. By late June or early July corn rootworm larvae have finished the pupae stage of development and appear in corn fields as adult beetles. Western and Northern corn rootworm beetles are small in size – 5/16 to 1/4 inches long. While the larvae feed on roots early in the corn growing season, the adults feed on pollen, silks and leaves.
Southern corn rootworm infestations are most prevalent and damaging to corn in the South, where they will also feed on peanuts, alfalfa, cucurbits and other broadleaf hosts. Sometimes called the spotted cucumber beetle, it is yellow to green in color and easily identified as an adult by 11 black spots on its bright green wings. It’s typically 1/4 inch in size.
Because of the insect’s preferred warmer climates, Southern corn rootworm overwinters in the soil as an adult beetle. The beetles mate in early spring with females, which deposit 500 or more eggs in the soil. Similar to the other corn rootworm beetle species, Southern corn rootworm larvae chew on corn roots, pupate and emerge as adults. Two to three generations are produced each growing season, and they remain active well into November.
Each of the corn rootworm species feeds on corn roots and can reduce yields. Particularly with Western and Northern corn rootworms, entire root nodes can be severely damaged. Moderate root damage may result in lodging and cause problems at harvest. When roots are pruned by corn rootworms, water and nutrients are hindered from moving up the corn plant to support developing ears. High adult beetle populations may feed on silks and pollen, which can affect pollination and kernel set. Similar damage may occur with a Southern corn rootworm infestation, but it’s rarely enough to cause significant economic damage to corn.
Scouting for corn rootworm adults is important to protect corn pollination and determine the damage potential for next year’s crop from high adult populations laying eggs during the current growing season. The University of Nebraska-Lincoln provides a very easily understood explanation of two ways to scout for adult corn rootworm.
Bt corn resistance
Transgenic (GMO) corn hybrids contain Bt (Bacillus thuringiensis) proteins, which typically control corn rootworms at a reliable rate when used according to labels, as approved and mandated by the U.S. Environmental Protection Agency (EPA). Today, there are hundreds of hybrids with approved Bt corn traits commercially available for corn rootworm control.
While Bt corn generally provides good control of labeled insects, Bt corn resistance to corn rootworm has been confirmed in at least four states: Nebraska, Illinois, Iowa and Minnesota. Research entomologists also suspect Bt corn resistance in other states, including Colorado, Kansas, Missouri, South Dakota and Wisconsin. It’s important for farmers to consult with their seed dealers to be well-informed about which insect or insects are controlled by a particular Bt corn trait as well as the respective planting requirements to manage resistance. All the Bt corn traits currently on the market do not have corn rootworm resistance.
Corn rootworm insecticides may be soil-applied in liquid or granular form at planting to control larvae or later in the season as a foliar treatment to keep adult beetles from laying eggs that will hatch into larvae the following growing season and attack corn roots. Insectide resistance to organophosphate, carbamate and pyrethroid chemistries has been confirmed in several Corn Belt states, particularly in continuous corn. Because rootworms can rapidly adapt to repeated use of one insecticide, it’s in a farmer’s best interest to rotate insecticide modes of action.
Corn Rootworm Management Options
Hybrids and seed traits
Farmers should consult with their dealer and review university Extension field trials to evaluate Bt and non-Bt hybrid options to control corn rootworms. Corn growers should make sure they are following all stewardship guidelines and regulatory measures identified by the industry to manage resistance.
Under low to moderate corn rootworm pressure, insecticide seed treatments can offer protection to corn plants. They can control corn rootworm pests in addition to a large spectrum of other corn pests including wireworms, billbugs, white grubs, corn leaf aphids and black cutworms. Many insecticidal seed treatments are adsorbed by the roots and so are ideally placed to control early season pests.
Follow integrated pest management (IPM) guidelines to effectively use insecticides and manage resistance in combination with other control practices. If scouting turns up adult beetles that are above the economic threshold, you’ll want to consider a foliar application of a product such as Baythroid® XL insecticide before the adult female beetles have the opportunity to lay eggs. Baythroid XL insecticide, a pyrethroid, performs on a broad spectrum of corn insects, with fast knockdown and long residual control.
Other cultural practices
Although crop rotation remains an alternative to manage corn rootworms, some adult beetles have been known to migrate to soybean fields and lay their eggs. In fact, Northern corn rootworms have been shown to adjust to a corn/soybean rotation by adapting to a two-year lifecycle, call “extended diapause,” according to University of Nebraska-Lincoln. The phenomenon has been confirmed in North Dakota, South Dakota, Nebraska and Iowa. Extended diapause can take two winter chilling/hibernation periods for eggs to hatch. Experts advise monitoring first-year corn for early detection.
Always read and follow label instructions. Not all products are registered for use in every state. Baythroid XL is a Restricted Use Pesticide.