Knowing that Ohio growers will in all likelihood be planting more Bt transgenic corn hybrids next spring, we continue to remind growers about Insect Resistant Management (IRM) requirements.

The purpose of employing these practices is to help prevent insects from developing resistant to the Bt gene. These are government regulations that must be followed. Growers who fail to follow IRM requirements risk losing access to Bt corn technology.

There are two different Bt types that could be used, Bt for corn borer and Bt for corn rootworms, with three different families of transgenic hybrids available with the different Bt genes alone or in different stacked configurations (YieldGard, Herculex, and Agrisure). There are some similarities and some differences in the requirements for the two Bt types.

First, and foremost, is the need to plant a 20% refuge area to non-Bt corn. A refuge is simply a block or strip of corn planted with a hybrid that does not have the Bt gene for controlling the target insects. The primary purpose is to maintain a pest population that is not exposed to the Bt toxin, allowing susceptible insects to remain alive in the population to
mate with any resistant insects that survive in the transgenic area. This allows any offspring to remain susceptible to the Bt. In terms of the distance of the refuge from the Bt corn, there is a major difference between transgenic hybrids for corn borer and rootworm. For corn borer, the non-Bt refuge can be within, adjacent, or near the Bt field, but has to be
within 1/2 mile, preferably within 1/4 mile. For corn rootworm, the non-Bt refuge has to be within the same field as the Bt corn, or adjacent to it. When adjacent, it can be separated at most by a ditch or a road, but not by another field. When planting the refuge, there are various plantings options, including a separate field refuge (mainly for corn borer), an adjacent field, a separate but adjacent block next to the Bt, a block refuge within the Bt field, the refuge planted along the perimeter, or a split-planter refuge. If planting the refuge using a split planter, the strip width must be at least 4 rows for corn borer (6 preferred) and corn rootworm.

There are other guidelines related to IRM, including management of the Bt field and non-Bt refuge, and use of other insecticides. Both the transgenic and non-transgenic areas should be managed in a similar manner. Growers should plant both hybrids close to or at same time, and select Bt and non-Bt hybrids that have similar growth and development
characteristics. If planting a refuge for rootworm, the cropping history must be the same (i.e., if the Bt is planted following corn then the refuge must be planted following corn). Reducing inputs or placing the refuge on marginal land can reduce the effectiveness of the refuge. In terms of insecticides, there are procedures for controlling the target insect (corn
borer or corn rootworm) and secondary insects in both the Bt corn and in the refuge. The major ones are, for European corn borer, the non-Bt corn refuge may be treated with conventional insecticides only if the target pest reaches economic threshold. However, a foliar Bt-based insecticide cannot be used within the refuge. For rootworms, a soil-, seed-, or foliar-applied insecticide for control of rootworm larvae and other soil pests is allowed in refuge. If an aerial insecticide is applied to the refuge for control of rootworm adults, the same treatment must be applied at same time to Bt corn. Regarding the use of rotations with Bt-rootworm hybrids, if the refuge is planted on rotated grown, the Bt field must also
be on rotated ground. However, if the refuge is corn following corn, than the Bt field can be planted on either a continuous or rotated field.

If growers are planting a stacked hybrid containing both types of Bt, they have two approaches on how to manage their refuge. The first choice is planting separate refuges for each target pest. However, we recommend the second choice, the common refuge approach where corn without any Bt technology is planted. The common refuge, also 20%, must be within or adjacent to the transgenic field, as is the case with Bt-rootworm
transgenic corn. A common refuge can be treated with soil insecticides or seed treatments for rootworms, and also non-Bt foliar insecticides for control of late season pests. If rootworm adults are present at that time, than the Bt stacked hybrid must also be treated in a similar manner. Growers should seed their seed dealer for any additional IRM requirements related to the use of stacked traits.

It is of extreme importance that growers follow these guidelines. Not only are we concerned about preventing resistance to the Bt technology, but also to make sure growers do not loose the ability to use this technology because of making wrong decisions.

There is another suggestion to mention to growers, especially those planning on using transgenic rootworm corn in first year corn. If possible, plan on leaving a strip in your corn without any protection from rootworm to determine if the first year rootworm variant is REALLY a problem in your field next year. Leaving an untreated strip (no soil insecticide nor seed treatment) will be the only way to determine if the variant was actually present AND an economic problem. Corn in these untreated strips should have their roots dug and rated next July to determine the level of rootworm injury.

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