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Bt refuges costing North Carolina growers

Along with technology comes the unavoidable precautionary requirements, and in the case of Bollgard II cotton varieties it comes in the form of non-Bt (bacillus thurigiensis) refuge strips.

While growers in other parts of the country contend this doesn't cost extra production dollars, researchers in North Carolina say it cost growers there an average of $37 per acre.

In two tests in North Carolina, entomologists did everything they could to control bollworms, but they still got an average of six percent damage on non-Bt cotton, plus the insecticides cost an average of $56 to control bollworms on non-Bt cotton.

By comparison the Bollgard II technology fee costs less than $20 and the Bt-containing Bollgard II plots showed an average of only 2.4 percent damage over 14 different varieties. Even without the yield boost from Bollgard II, the cost differential is $37 per acre, not counting the Roundup Ready technology fee.

Controlling insects in these refuge areas can be a challenge in addition to costing extra dollars. One concern is insect resistance to Bollgard II's genetic control, and the more realistic concern is with resistance to pyrethroids commonly used to control insects in non-Bt cotton.

Across all tests in North Carolina, entomologists have found no resistance problems associated with use of Bollgard II varieties. North Carolina State University Entomologist J.R. Bradley says his research team has not seen an increase in pyrethroid resistance in refuge strips planted in conjunction with Bollgard II cotton.

Some bollworms escape Bollgard II cotton, and in general, whatever that number is Widestrike varieties are about double that number — with both technologies the escapes are low, Bradley says. “We have conducted a number of tests this year, and we have not seen any change in susceptibility of bollworms to pyrethroids,'' he adds.

“Tobacco budworm is a different story. You cannot effectively control tobacco budworms with pyrethroids. When you come back to the field later, you will find large budworms down in the bolls, and this damage will cause yield losses,” Bradley says.

Currently, growers have three options for meeting the refuge requirements for planting Bollgard II cotton varieties:

  • Ensure that at least five acres of non-Bollgard cotton (refuge cotton) is planted for every 95 acres of Bollgard cotton. This refuge may not be treated with any insecticide labeled for the control of tobacco budworm, cotton bollworm, or pink bollworm. The size of the refuge must be at least 150 feet wide. The refuge must be managed (fertility, weed control and management of other pests) similarly to Bollgard cotton. The refuge must be planted within one half linear mile from the edge of the Bollgard cotton field.

  • Ensure that at least 25 acres of non-Bollgard cotton are planted as a refuge for every 100 acres of Bollgard cotton. All cotton may be treated with insecticides (excluding foliar Bt products) labeled for control of the tobacco budworm, cotton bollworm, or pink bollworm. Ensure that a refuge is maintained within 1 linear mile (preferably within one half mile) from the edge of the Bollgard cotton.

  • Plant at least 5 acres of non-Bollgard cotton (refuge cotton) for every 95 acres of Bollgard cotton. The refuge cotton must be embedded as a contiguous block within the Bollgard cotton field. For very large fields, multiple blocks across the field may be used. For small or irregularly shaped fields, neighboring fields farmed by the same grower can be grouped into blocks to represent a larger field unit, provided the block exists within one mile squared of the Bollgard cotton and the block is at least 150 feet wide. Within the larger field unit, one of the smaller fields planted to non-Bollgard cotton may be utilized as the embedded refuge. This refuge may be treated with any insecticide (excluding foliar Bt products labeled for the control of tobacco budworm, cotton bollworm, or pink bollworm whenever the entire field is treated. The refuge may not be treated independently of the Bollgard field.

“We have data accumulated over several years that show we have enough acreage of other crops that provide non-Bt selected insects and wild hosts to serve as a refuge,” Bradley says.

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