Insect resistance management (IRM) is critical for corn rootworms because they are highly adaptable pests. It is important to follow an integrated pest management (IPM) plan to mitigate resistance development and ensure profitable corn production.
Scouting for corn rootworm is the best way to determine if management action is needed. Once scouting reveals that corn rootworm is present and causing injury to a cornfield, choosing the best management strategy depends on field history, resistance issues (insecticide, Bt, or crop rotation), financial constraints, and agronomic practices in the field.
Although beneficial organisms are generally not effective at keeping corn rootworm populations below economic levels, it is important to understand that biological control is happening in the field on some level. Larvae and pupae, which are in the soil, are preyed upon by ground beetles, rove beetles, ants, spiders, and centipedes. Adults may be preyed upon by spiders. Entomopathogenic (pathogens that kill insects) fungi, nematodes, and bacteria exist and may attack corn rootworms at various life stages. Some flies and wasps may act as parasitoids of adults.
Crop rotation is still the single most effective way to manage corn rootworm populations in much of the corn-growing region. Western and northern corn rootworm adults have strong fidelity to corn, meaning they almost exclusively feed on and lay their eggs in cornfields. Rootworm larvae are not highly mobile and are generally unable to complete development on other plants. Crop rotation disrupts the availability of corn, the primary host, and the corn rootworm life cycle. Crop rotation is generally effective unless:
- The rotated crop (following corn) is one that adults are attracted to. Rootworm beetles are attracted to plants in the Cucurbitaceae family, such as cucumbers, melons, and pumpkins. Since rootworms typically lay eggs where they are feeding, corn planted after a cucurbit crop can result in high rootworm populations to rotated corn.
- There are northern or western corn rootworm variants in the field. Rootworms are highly adaptable insects and have overcome crop rotation in some areas. Much of the Corn Belt has a strict corn-soybean rotation scheme, which western and northern corn rootworms have exploited. See the Life Cycle page for more information on how these two species have overcome crop rotation.
Crop rotation is excellent at breaking up the corn rootworm life cycle and reducing or eliminating populations, but it also has resistance management benefits. When corn rootworm recolonizes the field later, the population is less resistant to the management tactic (Bt or insecticide) that was used in the field.
Planting corn early can result in plants that have larger root systems when eggs hatch and larvae begin to feed. This can reduce the stress placed on the plant from larval feeding and prevent or reduce lodging. Additionally, early-planted fields (or earlier maturing corn hybrids) may silk (R1) before peak adult emergence in the summer. This is important because adult corn rootworms feed on the silks, which means they will move to fields that are silking in search of pollen. Since females typically lay eggs where they are feeding, less oviposition may occur in early-planted fields (or fields with shorter maturity groups) which may reduce corn rootworm damage to corn the following year.
A good weed management program provides numerous benefits to the crop, including reduced competition for light, nutrients, and water, which leads to better growing conditions for the corn. Additionally, managing weeds eliminates other pollen sources for adult corn rootworms later in the growing season and may force adults to find other areas with food resources for laying eggs. Any volunteer corn should be managed to reduce the chance of non-variant egg-laying in soybean fields that will be rotated to corn the following year.