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by Ric Bessin, Extension Entomologist

University of Kentucky College of Agriculture

The EPA(Environmental Protection Agency) recently formalized and issued the required resistance management policies to be used by commercial corn producers when using Bt-corn seed that controls corn borers. Bt corn that controls rootworms has different requirements. The elements of the requirements are:

  • One single protective and practical corn refuge requirement for the primary corn-growing region (20% of the corn acreage) and one corn refuge requirement for the primary cotton-growing region (50%).

  • The refuge be within 1/4 mile of the Bt corn if it will be treated for corn borers and 1/2 mile if it is untreated.

Why Use a Refuge?

Using refuges with Bt corn is important and it benefits all growers directly. This article addresses the some of the key reasons why growers need to plant a corn borer refuge when using Bt corn and discusses how the refuges should be deployed and managed.

Spread Your Risk

European and southwestern corn borer levels are highly variable from year to year. While a producer may have substantial loses to corn borers one year, that does not necessarily mean that the problem will be as bad or worse the following year. Several factors influence the likelihood of corn borer problems. These include corn borer levels in the fall, parasitism rates, overwintering survival, spring weather conditions during moth flight, and corn planting dates.

Because the Bt-corn seed is an added expense, growers want to use it where it will be economically advantageous. In years when corn borers are not a problem, there is no advantage in using Bt-corn and the added cost of the seed is not recovered. Growers who have had a history of corn borer problems can spread their risk by only planting a portion of their crop to Bt corn. This reduces seed costs across the entire farm while protecting a substantial portion of their crop from corn borers. In addition, this acreage of non-Bt corn helps to reduce the potential of ECB developing resistance to Bt-corn.

Evaluate the Need For and Return of Bt Corn

Planting the entire farm with Bt corn does not allow the grower to compare its performance to standard hybrids. The infestation level in the standard hybrids in the refuge area provide an estimate of the corn borer levels on the farm. The corn borer economic scale (see ENT-49) can be used to determine the economic loss due corn borers. This value can be compared to the premium cost of the Bt seed. The results of this comparison can be used to make future planting decisions.

Effective Resistance Management

Planting a corn borer refuge on each farm using Bt-corn is the only strategy to prevent corn borer resistance that growers have. In the unfortunate event that corn borers did develop resistance to Bt corn, growers in that area would undoubtedly lose a tremendously valuable corn borer management tool.

While the possibility of corn borers developing resistance to Bt corn is only a theory, insect pests have a long history of developing resistance to any pest management tactic that is used for a long period of time over a wide area. The examples pests being able to overcome pest management strategies are too numerous to list! Consider the western corn rootworm beetle in Illinois and Indiana. For more than 20 years it was effectively controlled through the use of a corn-soybean rotation. The eggs that were laid one summer in a corn field would hatch the following year in what has become a soybean field. This pest had been a problem only with continuous corn. But it adapted. Now a portion of the female beetles lay their eggs in soybean fields and rootworms are now a serious problem in first-year corn in this area. Don’t underestimate the ability of insect pests to adapt!

Using Refuges

In order for refuges to effectively delay corn borer resistance, they must be designed to take advantage of three key elements. The first element of a refuge is that if a resistant corn borer is able to develop on Bt-corn, we want to make sure that it is most likely to mate with a ‘normal’ corn borer so that these offspring will still be susceptible to the Bt toxin. To do this, there must be a reservoir of susceptible moths around each Bt corn field. At least 20% of the corn acreage on each farm must be non-Bt corn in order to provide a sufficient reservoir.

The next element of an effective resistance management plan is to arrange the refuge such that if a corn borer larva feeds on a Bt plant it always gets a toxic dose. If they move from Bt to non-Bt plant, they may survive by only consuming a partial dose of Bt. Studies have shown that a large percentage of corn borer may move from one plant to another within the same row and that some move between rows of corn. To ensure that a corn borer always gets a toxic dose of Bt when feeding on a Bt plant, they should not be able to move between Bt and non-Bt plants.

The last element is timing. It is important to have the Bt and non-Bt corn in about the same stage of development during corn borer egg laying. In order to do this, the Bt and non-Bt corn need to be planted at about the same time and have similar maturity. It is not necessary to have a refuge each day when Bt corn is planted on consecutive planting dates. However, there should be at least one refuge for Bt corn planted over a 4 to 7 day period.

Planting Arrangements

Mixing of Bt and Non-Bt seed in Hopper Boxes: This is strongly discouraged. This results in a mixture of Bt and non-Bt corn in the same row and may favor, rather than delay the development of resistance. Additionally, it is becoming more common for Bt hybrids to also have tolerance to broad spectrum herbicides. The advantage of herbicide tolerance is lost when the seed is mixed with a non-tolerant hybrid. Another reason why this arrangement is discouraged is that it does not allow the grower to manage corn borers in the non-Bt corn without wastefully treating the Bt corn.

Planting Bt and Non-Bt Corn in Strips: Growers fill some of the hopper boxes on the planter with Bt-seed and some with non-Bt seed. This is discouraged for the same reasons mentioned above.

Planting the Bt and Non-Bt Corn in Large Blocks: This is the recommended method of planting the refuge. One portion of a large field is designated for Bt-corn and the other for the non-Bt refuge. In this situation, corn borer movement between Bt and non-Bt plants is minimized, insecticide sprays or early harvest can be used to minimize corn borer losses in the refuge, and the grower can manage weed control according to the needs of the particular hybrids. If Bt and non-Bt corn cannot be planted in the same field, then adjacent fields or fields within one-third mile of the Bt corn can be used as refuges.

Managing Corn Borers in the Non-Bt Refuge

While the reason for planting a refuge is to maintain a population of Bt- susceptible corn borers, growers should still manage those refuges to avoid serious losses. When using a 20 percent refuge with Bt corn plantings, growers may consider spraying for corn borers if scouting indicates it is economical.

Planting Dates and the Potential for Corn Borer Losses. Extremely early or late planting will increase the potential for borer damage. The first generation can be very damaging to early plantings, while late planted fields are prone to attack by the second. Generally, Bt corn has a greater yield advantage with later planting dates because of increased insect activity. It is the late planted refuges that are most likely to be damaged by corn borers.

Early Harvest. This management option can reduce losses due to broken or lodged plants or dropped ears. Second generation larvae feed primarily in the plant's ear zone and below. This damage is the primary cause of harvest loss. Early harvest, especially of heavily infested fields, can be an effective strategy with corn borer refuges.

Determining the Need to Spray Corn Borers in the Refuge. Careful scouting of fields is the most effective means of detecting economic infestations of the European corn borer and other pests. With the first generation in late May and early June, examine the whorl leaves of 20 consecutive plants in at least 5 areas of the field. Look at the leaves for the "window-pane" type of feeding damage caused by the larvae. Pull the whorl from one damaged plant at each stop and unroll it carefully to look for live larvae and their size. The percentage of infested plants in the field and size of the larvae present can help you determine the need for an insecticide application. Sprays should be considered if 50% of the plants show "shot hole" or "window pane feeding" damage and live larvae are present in the whorl. Once larvae have bored into the stalks treatment is not effective.

The same general procedure is followed with the second generation; however, the leaf axils, leaf sheaths, and ear shank areas should be examined for live larvae. Again, the percentage of infested plants and the size of larvae are the keys to determining the need for treatment. Treatment is suggested if egg masses average one per plant and egg hatch has begun or if 50% of plants inspected have live larvae feeding on the leaves or tassels in leaf axil or behind sheaths. If your examination indicates that half of the larvae have entered the stalk, treatment is not recommended.


  • Each farm using Bt corn must have a non-Bt corn borer refuge.
  • In Kentucky, the refuge should be at least 20% of the corn acreage.
  • Each Bt-corn field must have a refuge within one-quarter to one-half mile.
  • Bt-corn and refuges should be planted on about the same planting date and be of similar maturity.
  • The Bt corn and refuges need to be arranged as large blocks or separate fields.

Issued: 4/99 Revised: 1/04

CAUTION! Pesticide recommendations in this publication are registered for use in Kentucky, USA ONLY! The use of some products may not be legal in your state or country. Please check with your local county agent or regulatory official before using any pesticide mentioned in this publication.


This page is maintained by Pat Dillon, Department of Entomology, University of Kentucky. Please send questions or suggestions to: