Spray early, but not too early in no-till

Cotton farmers new to no-till or reduced-till should be preparing now to burn down winter weeds in the fields they intend to plant in late April or early May.

While that might seem to be rushing things, an early burndown may be the most effective cultural practice for reducing potential insect problems in no-till, said Roger Leonard, research entomologist with the LSU AgCenter.

Leonard, one of the first entomologists to conduct research on insects in no-till cotton, said that making sure seedbeds are completely free of living vegetation three weeks before planting can minimize insect infestations in seedling cotton.

“Burndown herbicides need to be applied four to six weeks before planting, depending on specific products, to successfully terminate winter and spring vegetation by this time,” said Leonard, a speaker at the recent National Conservation Tillage Cotton and Rice Conference in Tunica, Miss.

Leonard cautioned growers not to apply herbicides too early and warned that they may have to apply multiple products to achieve satisfactory weed control, including a residual herbicide to prevent emergence of spring weeds.

“Additional weeds may become established if herbicides are applied too far in advance of planting or if the herbicide provides no residual control,” he noted. “But herbicides applied too late will not completely kill the vegetation or be ineffective against some weed species.”

If some winter vegetation remains on the seedbed despite the grower's best efforts, he should apply an insecticide at planting to eliminate insects such as cutworms, said Leonard.

Leonard, who works at the LSU AgCenter Macon Ridge Research Station near Winnsboro, La., began researching insects in no-till in the late 1980s.

He noted that more and more producers are switching to some form of conservation tillage to try to decrease production costs and improve profits. But the trend to less tillage has also brought new challenges.

“These changes significantly influence arthropod pest diversity and density as well as the overall pest status,” he said. “Conservation tillage, crop rotation, boll weevil eradication, transgenic crops and new programs such as the Conservation Reserve are interacting to create new problems in insect pest management.”

For openers, the increase in surface residue that is a natural result of reduced tillage provides a favorable microenvironment for soil-dwelling insects.

“The general timing of herbicide applications in cotton has shifted closer to the time of planting compared to the timing of tillage practices,” said Leonard. “This delay in terminating some weed species provides a refuge for insect pests until cotton becomes available.”

The advent of Roundup Ready cotton also has led to increases in the adoption of conservation tillage practices by providing an effective and economical weed management tool, said Leonard.

The application of glyphosate herbicides (Roundup, Touchdown, etc.) as a burndown treatment provides satisfactory control of most native species found in cotton fields — if properly timed,” he notes. “However, applied too early or against weeds larger than their susceptible stage, the glyphosates can be ineffective.”

Cutleaf eveningprimrose, smartweed and horseweed can remain in fields and on field borders when herbicide are mis-timed or mis-applied. “These plant species are non-crop hosts for insect pests, including cutworm, tarnished plant bug, false chinch bug and cotton aphids,” he said.

While providing benefits such as increased surface residue and lowering nematode populations, crop rotations can also leave growers open to more insect problems.

“The total impact of crop rotation on insect pests has not been well-studied in the southern United States,” Leonard notes. “One change in insect ecology relates to heavy residue from the previous corn harvest and its impact on soil-dwelling insect pests.”

Heavy residue from the previous corn crop can cover the soil surface and mediate soil temperature and moisture levels. “This, in turn, increases the probability of insect pests successfully overwintering in those fields,” he said.

Bollworm, the same insect species as the corn earworm, and stinkbug are common insect pests that can overwinter in cornfields. Corn can also serve an as alternate host for insects such as cutworm, bollworm, stinkbug and tarnished plant bug.

With more growers reported considering more corn, more of those pests could be migrating from corn into adjacent cotton fields in 2002.

Leonard suggests that farmers begin planning now to make sure their burndown applications are properly timed to take out winter weeds in their fields and along field borders this spring.

“Most insect pests are highly mobile and crop consultants should scout field borders and adjacent fields to observe potential sources of these pests,” he notes. “Field corn, ditch banks, fallow fields and WRP/CRP areas are potential refuges for insect pests.”

It's true, Leonard said, that predators and parasites are increased in conservation tillage systems and through plant diversity in crop rotations.

“However, these biological control agents are not always capable of providing successful control of cotton insects unless other integrated pest management tools are used,” he said. “Crop consultants should consider the influences of such areas on pest migration and frequently monitor cotton fields with a high probability of pest infestations.”

e-mail: [email protected].

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