Early Season Boll Weevil Suppression and Control for the New Mexico High Plains

Guide A-236

Jane Breen Pierce, Assistant Professor, Agricultural Science Center at Artesia
Wallace Cox, Lea County Extension Program Director and Extension Agriculture Agent

College of Agriculture and Home Economics New Mexico State University

This publication is scheduled to be updated and reissued 3/05.

Boll weevil has been successfully eradicated from most of the southeastern United States. Virtually all of the Cotton Belt is in or planning eradication programs. The areas of Texas immediately adjacent to southeastern New Mexico began boll weevil eradication programs in the fall of 1999. In New Mexico, Luna County and the Mesilla Valley have established eradication programs. Some Lea County farmers joined the west Texas eradication program on a voluntary basis.

The following management practices are offered as options to help reduce boll weevil numbers in the New Mexico High Plains. Most of these tactics focus on cultural controls and are useful regardless of eradication program status. The advantage of cultural controls is that they can suppress boll weevil populations throughout the season and they cost little or nothing. Also, since there are no boll weevilresistant plants or effective biological controls for this pest, cultural controls are the only other option to insecticides.

Control weeds. Field tests in Eddy and Lea Counties over the last three years have indicated that the best overwintering habitat for boll weevils in the New Mexico High Plains is dense vegetation. Typically, these habitats are weedy borders and ditch banks, urban weedy lots and gardens, and shinnery oak. Weevil survival is higher in these habitats because in late spring, the relative humidity is higher and the temperature lower than in other habitats. Boll weevils overwintering in dense vegetation are more likely to emerge after mid-June, when cotton squares are available. Although weed control during the winter is preferable, weed control in the spring still can be effective in reducing boll weevil survival.

Avoid early planting. First and most important, do not plant early. If possible, do not plant before April 25, and certainly not before April 20, even if soil temperatures are high enough to plant. Fields planted earliest act as trap crops, attracting boll weevils from a 1 mile radius. Those fields will be the hardest hit by boll weevils.

Delay planting if possible. Boll weevils must find and feed on cotton to reproduce and will not live long (up to 10 days) without it. Peak emergence from overwintering habitat can be as late as June 8. If boll weevils survive 10 days beyond that point, they will infest fields that have hostable squares (the size of a pencil eraser) on June l8. A delay of just a few days in planting can make the difference between having a boll weevil infestation and having a weevil-free field until mid-August, when the crop is essentially finished.

Data collected for 10 years at the Agricultural Science Center at Artesia indicate that until May 5, each day of delay in planting results in a 1-pound reduction in per-acre yield. However, reduced boll weevil damage, fewer insecticide applications, and, later, reduced eradication costs will more than offset this cost.

In Lea County, costs for delayed planting should be similar to the figures above. Roosevelt and Curry Counties probably will not be able to delay planting.

Plant fields near good overwintering habitat last. The best overwintering habitats cause boll weevils to emerge late, when cotton is most likely to be available. Boll weevils do not move far from overwintering habitat in spring, so fields closest to good overwintering habitats are the hardest hit. Planting these fields last allows time for many of these boll weevils to starve, reducing the size of the critically important first generation.

If possible, orient rows east to west. Tests at the Agricultural Science Center at Artesia have indicated that rows oriented east to west have much higher rates of boll weevil mortality than rows oriented north to south.

Use seeding rates that will produce 3 to 4 plants per foot. Planting at very high rates results in delayed maturity and increased competition among plants. Without boll weevil pressure, a stand with 6 plants per foot will produce lower yields than one with 4 plants per foot. With boll weevil pressure, that loss will be magnified. The delay in maturity means that a higher number of the most valuable squares will be vulnerable to the high population of late-season boll weevils. Thinner stands also result in higher mortality of boll weevil due to desiccation, particularly in the first generation.

Plant on beds. High temperatures and low relative humidity can cause extremely high boll weevil mortality, particularly early in the season when there is little shade. Planting on beds makes it more likely that the shed squares will end up in the center of the furrow, where they will be exposed to lethal conditions.

Apply insecticide when squares reach the size of a matchhead. The following recommendation is made with the assumption that farmers want to suppress boll weevil populations. This recommendation is aggressive and uses a very low treatment threshold. Treat a field if the total of all trap captures is at least 2 boll weevils in the two-week period before matchhead squares appear, using a trapping rate of at least 1 trap per 10 acres.

Make an insecticide application one to two days after you find the first matchhead-sized square. The timing of the insecticide application is critical. Apply too early, and you waste money because most of the weevils would have died anyway. Too late, and the weevils will have laid eggs in your field, establishing a population that will be impossible to control without multiple applications. Remember that one pair of boll weevils could, if left unchecked, produce a billion weevils by late September.

The early application is very effective-even though it only targets a small population of boll weevils. In areas where boll weevils are not well established, it is even possible to prevent their establishment by delaying planting and applying insecticide when matchhead squares appear.

There has been some confusion among cotton growers who think that if they delay planting and make an insecticide application, the field will not be infested and should not have boll weevils for the entire season. If there are few boll weevils in an area, and planting is delayed, and the application mops up the last few boll weevils in the field when hostable squares first appear, it is possible to eradicate them from a field. This has been and is being done around the country to make entire areas of the Cotton Belt weevil-free. If no more overwintered boll weevils enter that field, it will remain free of weevils-until about August 10. But at that time, weevils will move in from other fields and infest any fields that have available squares.

Consider boll weevil control when making applications for other pests. If you need to spray cotton for other insects, consider using an insecticide that also controls boll weevil. If you have boll weevils in the two-week period before squares reach matchhead size, assume that boll weevils will be established in those fields for the rest of the season. This is a more reliable indicator than in-season trap counts, because the pheromone traps have to compete with pheromones produced by boll weevils in the field.

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Printed March 2000
Electronic Distribution April 2000