Corn producers have a lot to consider when it comes to rootworm management, said Randy Higgins, field crop entomology specialist with Kansas State University Research and Extension.

A BT rootworm corn hybrid and refuge, crop rotation and planting-time insecticides are a few of the options available for producers today. Ultimately it´s a decision of whether or not the crop will receive significant rootworm damage, what is economically feasible and what management strategies will create the highest profit margin for the producer.

BT rootworm corn hybrids are genetically modified to provide protection against rootworm larvae by producing a rootworm-active toxin throughout the rootworm larval feeding interval, Higgins said. Seed companies marketing rootworm-resistant corn also provide corn growers the option of ordering hybrids that contain additional traits
which can help the crop resist damage from other insects and (or) that provide immunity from some post-emergence herbicides.

BT corn must always be used in conjunction with a non-BT corn hybrid planted nearby as a refuge. The refuge provides a place where non-BT resistant insects can develop. These susceptible insects then can move into the BT corn to mate with any BT resistant insects that might be present in order to reduce the rate of insect resistance
development to the BT toxin.

One consideration associated with growing BT corn is the planting time requirement to follow the legal refuge establishment rules set by the Environmental Protection Agency.

"No more than 80 percent of a producer´s acres that are planted to corn can be planted to BT corn hybrids in one year and at least 20 percent must be planted to non-BT hybrids," Higgins said. "Refuge corn should be planted at the same time as BT rootworm corn within or immediately adjacent to the BT corn field and managed similarly."

Non-BT corn planted on rotated ground that was in a crop other than corn last year is not considered an acceptable rootworm refuge if the BT rootworm corn is planted where corn was grown the previous year. Draw a map and compare field histories, distances, and hybrid traits with the literature provided by the seed company that summarizes the planting and management requirements that must be followed.

Refuges can also be protected against rootworm damage, Higgins said. Some options for refuge management include planting-time insecticides and commercially applied systemic seed treatments. Research has not shown, however, consistently high performance of seed treatments against rootworms, particularly when moderate to heavy populations develop.

Insecticide sprays that target adult rootworm beetles are prohibited  in BT rootworm refuges, unless the BT rootworm corn is also treated, which can be expensive.

If a producer is not interested in planting BT corn to control rootworms while continuing to plant corn after corn, he or she may want to use traditional planting-time insecticides. Producers should remember that insecticide performance may decline, though, if excessively dry conditions persist or if very wet soils develop after application, the entomologist said.

A possible problem with planting-time insecticides is that corn producers are moving to earlier planting dates, which can make it more challenging to protect from rootworms, Higgins said.

"Many planting-time insecticides provide protection for four to six weeks, or somewhat longer, and rootworm egg hatching usually occurs in mid-May," Higgins said. "If corn is planted in late March to early April, then eight to 10 weeks may pass before the eggs hatch and protection may not be available at the level the producer expects."

This year´s unseasonably warm temperatures may cause the rootworm hatching season to start earlier, though, he said.

Producers looking for a more traditional and less expensive rootworm management tool may want to use crop rotation. Since rootworms only feed on corn plants, crop rotation will help reduce rootworm populations.

When rootworm eggs laid last year hatch, the larvae will not be able to find adequate nutrition in other crops such as soybeans or grain sorghum when crop rotation is implemented. Subsequently, when this year´s corn crop is planted in last year´s soybean field, rootworm populations should be reduced due to starvation, Higgins said.

If rootworm damage to first-year corn is suspected, producers should contact their nearest county or district extension office or call a K-State field crop entomologist by calling 785-532-5891.

For more information about corn insect management, interested persons can refer to the K-State Research and Extension "Corn Insect Management" recommendations available on-line at www.oznet.ksu.edu/library/. 

K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus in Manhattan.