Bt cotton needs help in heavy pest invasions

Cotton growers who want effective bollworm and tobacco budworm control can usually depend on Bt varieties. But for massive infestations of beet armyworms or other pests, there's only so far even the most proven Bts can go.

Scott Armstrong, a Texas Tech University entomologist, is sold on the value of genetically modified cottons containing Monsanto's Bollgard gene. "You can anticipate at least an 80-85% control pattern for bollworms or tobacco budworms," says Armstrong, who had Bt test plots on research farms near Halfway and Denver City, TX, last summer. "I believe it's an easier, cost-effective method of controlling these insects."

However, after witnessing massive beet armyworm swarms in fields last summer - populations of 100,000-200,000/acre in many fields and an incredible 600,000/acre in at least one - Armstrong and others recommended quick-striking pesticide treatments to handle the invasions.

The Bt toxin is high in component parts of Bt cotton plants with the exception of older leaves and the pollen in flowers. "As the plant matures, less Bt exposure is in the plant," says Armstrong.

"This can cause a control problem when bollworms infest cotton late in the season, such as when they move out of corn and into cotton in large numbers in mid-August," says Jim Leser, Texas A&M University entomologist in Lubbock.

U.S. acreage of Bt cotton jumped from about 2.2 million in 1998 to about 3.7 million in 1999. Of the 15-million-plus cotton acres planted for 2000, about 5.7 million contained the Bt gene.

Many enemy insects drop dead soon after taking a few bites of Bollgard cotton. Beneficial insects aren't harmed, nor are aphids, thrips, plant bugs and boll weevils.

Bt cotton wasn't designed to handle heavy beet armyworm infestations, either. The varieties provide 25-40% control in infestations below the sliding threshold of 10,000-20,000 beet armyworms per acre, depending on their feeding habits at the time of monitoring.

"Once you get past that point, growers almost certainly have to spray," says Armstrong.

The Texas Ag Extension Service provides the following control ratings for Bt cotton: bollworms pre-bloom, 90%; bollworms bloom, 70%; tobacco budworms, 95%; pink bollworms, 99%; cabbage loopers, 95%; beet armyworms, 25%; fall armyworms, 20% or less; cotton leaf perforators, 85%; and European corn borers, 85% or more.

"As cotton approaches cutout, bollworm control drops to 50% or less," says Leser.

The cost of Bt varieties is about $21/acre over that of normal varieties. In stacked-gene Bt and Roundup Ready varieties, it's $45 more.

"It's not as bad as most think," says Armstrong. "For bollworm control on non-Bt varieties, a grower will spend $12-15/acre on a pyrethroid insecticide alone. There is little worry about scouting a field for bollworm or budworm. And with non-Bt varieties, it's critical to make timely applications. Bt varieties just make it easier."

Bollgard II, the second-generation stacked Bollgard product from Monsanto, should provide greater insect control, says Armstrong. Monsanto field tests have shown that it provides improved control of fall armyworm, beet armyworm, cotton bollworm and soybean loopers.

"Bollgard II will be in a broader spectrum of varieties that should provide more coverage for us and help prevent resistance buildup," says Armstrong.

Texas Ag Extension Service agronomists and entomologists have looked at various aspects of Bt cotton. And they have several recommendations on when and where Bollgard varieties are needed. They say Bt should be considered:

- In areas where you're making two or more foliar insecticide applications and spending $20/acre or more for tobacco budworm, bollworm or pink bollworm control.

- In areas with chronic bollworm and/or pink bollworm infestations that in the past have required foliar sprays of conventional insecticides.

- In river-bottom areas where tobacco budworm and/or bollworm pressure has been historically heavy.

- Near waterways and homes, where restricted-use insecticides can't be sprayed.

- Where you want to use less conventional broad- spectrum insecticides to promote beneficial insects for biological control of all insect pests.

- In boll weevil eradication programs where outbreaks of lepidopteran pests may be expected.

- In areas where the boll weevil has been eradicated and conventional insecticide sprays are no longer required for that insect.

"Since the insecticides available for pyrethroid-resistant budworm control are expensive, Bt cotton fits well," says Texas A&M entomologist Jim Leser.

"Bt also fits where two or more applications are needed on an annual basis for bollworm control, and where boll weevil eradication programs are in their early stages and considerable spraying is taking place. Sometimes it's difficult to justify Bt cotton on dryland production fields (in semiarid regions) and in areas where irrigated cotton is grown near corn," Leser says.

For further information on Bt cotton in your growing area, contact your local or regional cotton agronomist or entomologist.