August insecticide sprays may be important to yields

With cotton prices still below the loan rate and dry conditions threatening yields, some growers may be thinking about calling off the airplanes and parking the ground rigs when August rolls around.

That could be a mistake, according to the results of an on-farm demonstration conducted by specialists with the University of Tennessee Agricultural Extension Service near Somerville, Tenn.

In two out of the three years of the demonstration, the specialists increased yields substantially when they sprayed field plots for a complex of plant bugs, stinkbugs and escaped bollworms and bollworm eggs with a combination of pyrethroids and imidacloprid in mid-August.

With the advent of Bt cotton — west Tennessee growers planted about 85 percent of their acres in Bollgard in 2001 — and boll weevil eradication, what were called secondary pests in the past have been moving to the forefront in many Mid-South cotton fields.

“Before the arrival of Bollgard and the eradication program, we often were unable to produce a top crop — even with a three to four-day spray schedule,” says Craig Massey, an Extension IPM specialist. “With Bollgard and eradication, we've been able to produce a top crop. But you have to protect that crop against secondary pests.”

When Massey began the demonstration on Mark and Joe McNabb's farm in Fayette County in 1999, he was primarily targeting bollworm escapes with over-sprays of Bollgard cotton.

“But we've turned our focus to plant bugs and stinkbugs,” he said. “We think that's where we're getting a yield increase from these August sprays.”

Until recent years, entomologists considered stinkbugs a minor pest in Mid-South cotton. With the reduced spraying for Bollgard cotton and the eradication program, however, scouts have more noting much larger numbers of stinkbugs in cotton, corn and soybeans.

“The adoption of genetically enhanced crops has no doubted permitted many producers t remain in business despite the recent economic downturn,” Massey said in a paper presented at the Beltwide Cotton Conferences. “However, farmers must continually modify management practices to maintain satisfactory margins as new technology becomes available.”

In the first year of the demonstration in 1999, Massey and the McNabbs divided a 26-acre field into four treatment areas. One was not sprayed; one was sprayed when thresholds triggered on June 15; one on June 15 and July 17; and one on June 15, July 17 and Aug. 16.

In 2000 and 2001, Massey expanded the demonstration to six treatments to evaluate the effectiveness of single insecticide applications targeting early (June), middle (July) and late (August) season pest infestations.

For 1999, the McNabbs harvested their highest yield in the June-July-August treatment (313 pounds above the check). “But the greatest yield increase was considered to be due to the August application since the June and June-July treatments produced yield increases of 73 and 94 pounds above the check,” he noted.

“Results from the 1999 study indicated that insect pest activities were relatively modest during the early and middle portion of the season, but the potential for insect pest injury late in the season was substantial,” said Massey.

“The broad spectrum insecticide application made during the month of August eliminated the growing plant bug, stinkbug infestations and any potential bollworm escapes that can occur in late season Bollgard fields. This in conjunction with the crop rapidly reaching physiological maturity limited boll injury by insects allowing the crop to achieve maximum yield potential.”

In 2000, the single spray on Aug 17 increased yield 578 pounds over the untreated check, according to Massey. “This biggest yield difference was considered to be due to the control of late season stinkbugs and possibly some escaped bollworm larvae. Additionally, beneficial insect populations were permitted to thrive during the early and middle portion of the season.”

The second largest yield increase of 386 pounds above the untreated check came in the strip plots that were sprayed in June, July and August.

“Though the treatment structure differed between the 1999 and 2000 trials, the treatments which contained a broad spectrum insecticide application in August tended to provide the greatest yields,” said Massey. “Again, beneficial insect populations were thought to provide a portion of the yield response observed in the August only application.”

In 2001, the June-only treatment produced a 123-pound increase above the untreated check while the other treatments differed only slightly between each other. Scouting indicated relatively low levels of bollworms, which the Bollgard cotton prevented from developing.

For the three years of the demonstration, the August-only treatment produced the highest average yield increase above the check of 352 pounds per acre followed by the June-July-August treatment at 291 pounds above the check.

“These results suggest that well-timed broad spectrum insecticide applications in Bollgard cotton permit the producer to aggressively manage insect pest populations to achieve maximum yield potential,” says Massey.

“Since Bollgard protects a greater proportion of early season fruit, the crop has the capacity to reach physiological maturity more rapidly, which can reduce late season insect pest management complications.”

e-mail: [email protected]

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