Corn rootworm has not been seen as much of a problem since the introduction of the Bt toxin Cry38b1 which conferred resistance to corn rootworm. In recent years entomologists in various states have found resistance to Cry38b1 in four states. Resistance has been confirmed by researchers in Illinois, Iowa, Nebraska and Minnesota. Scientists in Colorado, Kansas, Missouri, New York, South Dakota and Wisconsin also are reporting significant damage in fields planted to corn hybrids containing the single Bt toxin. While the corn rootworm resistance problem in the midwest has drawn the most attention, it is not isolated to that region. Entomologists in Pennsylvania and New York are seeing an increase in problems fields as well. While Cry38b1 has been suspect in most of the CRW resistance cases, the practice of continuous corn and the repeated use of a single toxin are putting increased stress on all of the Bt toxins currently available.
Corn rootworms are one of the most damaging pests in the corn belt. According to the USDA, they are responsible for up to $2 billion in lost revenues each year. It is anybody's guess as to how extensive CRW damage will be in 2014. With the development of resistance to Bt toxins it could be beneficial to scout for them, especially in multiple year corn. The main purpose is to be vigilant and possibly detect any potential problems early.
Corn rootworms are important insect pest of corn in the midwest. Two species of rootworms that may cause severe damage to corn as both larvae and adults are the western and northern corn rootworms. The adult of the northern corn rootworm is a tan to pale green beetle about 1/4 inch (6 mm) long. Newly emerged beetles are usually cream or light brown in color, but gradually turn green with age. Both sexes of 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. However, male wing covers are often nearly entirely black or at least darker in pigmentation than that of the female, which usually appears as more regular stripes.
Identifying damage in corn roots after pollination can help producers evaluate current management practices. Start with a plant from a refuge strip or untreated area. Dig 6-8 inches around the plant preserving the root ball. Examine the soil and roots for larvae. Damaged roots may appear rotted and will have pinhole markings where corn rootworms have entered the plant's roots. Examine silks for adult beetles. Identifying adult beetles is a clue that any root damage may have been the result of corn rootworms since damage appears primarily on the roots. Overall plant and stalk health for nutrient deficiencies that may be the result of root damage. Next dig and examine plants from three other treated areas for the same traits as the plant from the untreated area. Treated plants and plants with less corn rootworm damage will be sturdier, with larger root masses and will be more difficult to dig up. Wash the roots and let them soak in buckets of water for 30 minutes. This will allow you to examine clean roots and identify larvae that float to the top of the water.
Options for management include rotating crops, avoiding continuous corn rotations as well as not planting Bt- traited corn with the same single mode of action for more than two years in a row.