Early Season Pest Management of Boll Weevil in New Mexico
Guide A-233
Jane Breen Pierce, Assitant Professor Agricultural Science Center at Artesia
Carol Sutherland, Extension Entomologist
New Mexico State University
This publication is scheduled to be updated and reissued 1/04.In areas without boll weevil eradication programs, New Mexico cotton producers should consider voluntary, early season management of boll weevils for several reasons.
Early season is when boll weevils are easiest to control.When boll weevils emerge from overwintering sites, they are significantly stressed by heat, low humidity, and not enough suitable food. They must eat cotton pollen to reactivate their reproductive systems.
Early in the season, cotton plants are small and easy to treat with readily available ground equipment. Because of their size, the plants require only a small volume of pesticide, even for thorough coverage.
If insecticide treatment is necessary, early season is the only time in the year where one insecticide application alone will give good control. After boll weevils are established, three applications may be necessary to have a similar impact. Early season applications also are less likely to have a large or lasting impact on beneficial insects, compared to applications after bloom. While beneficial arthropods are of limited importance in controlling boll weevil, they are extremely important in keeping many other potential cotton pests at subeconomic levels, especially foliage- and boll-feeding caterpillars, some bugs, aphids, whiteflies, and mites.
Simple cultural controls such as delayed planting can produce a dramatic reduction in boll weevil numbers. The stressed survivors of overwintering can starve to death searching for squares or blooms. In areas with relatively low populations of overwintering weevils, delayed planting can eliminate any need for an insecticide application for overwintered boll weevils.
Southern New Mexico rarely has high winter mortality of boll weevils. As of early February 1998, for example, tests at the Agricultural Science Center at Artesia still showed 60 percent survival of boll weevils placed in overwintering habitats. High winter kill depends on temperatures well below freezing for extended periods. But frequently in New Mexico, freezing temperatures occur only overnight or for a few hours prior to sunrise. Insects acclimated to the area are prepared to overwinter and survive these short-duration freezes and oscillating temperatures.
High winter mortality can occur in areas where high moisture is associated with cold temperatures. In the Pecos and Mesilla valleys there is little precipitation, particularly during the winter when irrigation also is curtailed. Lea and Roosevelt counties may have higher winter kill from low temperatures but also may have more favorable overwintering habitats than south-central New Mexico.
Tests conducted in 1996-1998 indicate that shinnery oak and Conservation Reserve Program (CRP) grasslands are good overwintering habitats for boll weevils in the New Mexico High Plains cotton-growing areas. In the Mesilla and Pecos valleys, urban and suburban landscapes and weedy borders are the most important havens for overwintering boll weevils. In both urban and rural areas, good weed control has a significant impact on boll weevil overwintering success.
RECOMMENDATIONS
1. Control weeds.
In the absence of shinnery oak and CRP grasses, weedy borders are the primary source of overwintered boll weevils. In 1997-1998 tests in the Pecos Valley, weedy borders produced more than three times more overwintering boll weevils than other habitats. Use of appropriate herbicides, cultivation, timely burning, or periodic mowing, especially around cotton fields, can be helpful not only for reducing overwintering habitat but also for limiting weed colonization and reproduction in some species.
2. Delay planting.
Boll weevils that establish a population in a field in the spring stay in that field and generally do not infest new fields until August. If you can prevent or delay a population getting established in the spring, the field may not develop significant numbers of boll weevils until mid-August or later. A planting delay of just 1-2 weeks has a significant impact on boll weevil control. Delay also may eliminate the need for an insecticide application for overwintered boll weevils, especially where overwintering success is already low.
Emergence and death of boll weevils from overwintering habitat before cotton squares are available is called "suicidal emergence." Boll weevils can only reproduce on cotton, and only when it is producing squares big enough to support the developing larvae. Squares the size of an eraser head (1/4") are large enough to feed a developing boll weevil. Overwintered boll weevils that feed on the terminal growth alone cannot survive more than 7-10 days. Development of reproductive systems in overwintered adults begins only when squares or blooms are available for pollen-feeding by adult boll weevils.
You can greatly reduce the possibility of establishing a boll weevil population in your field if cotton doesn't begin to produce hostable squares until after peak numbers of boll weevils already have emerged and died.
Peak emergence in New Mexico may vary from year to year and depends on the weevils' overwintering habitat. In 1997 in Lea County, peak emergence was in early June in the most favorable overwintering habitats-urban areas, shinnery oak, and CRP grasses. In 1998, peak emergence was earlier in Eddy than in Lea County. In Eddy County, peak emergence for all habitats was the second week of May. Peak emergence in Lea County was 1 to 3 weeks later than in Eddy County depending on habitat, with boll weevils in urban areas emerging last.
The potential impact of delayed planting is perhaps best illustrated by an example. Boll weevil captures in the fall of 1997 in Eddy County indicated that there should have been economically damaging numbers in 1998. However, farmers in the hot spot near Carlsbad planted late, with the majority planting after April 25. Many planted after May 1. In mid-May, emergence of overwintering boll weevils peaked. Most fields, however, did not produce 1/4", hostable squares until June 17-26. Since the peak emergence of boll weevils was over by mid-May, there were relatively few boll weevils surviving to infest fields in late June when they could successfully reproduce.
By mid-June, the highest number collected from one trap was only 3 boll weevils, compared to 33 boll weevils captured in a single trap in mid-May. This reduction allowed some farmers to avoid treatment but also gave them an opportunity to be more aggressive in suppressing the remaining population. The threshold for treatment was reduced from 1.5 boll weevils per trap to 2 boll weevils per field, regardless of the number of traps-a very aggressive, eradication-level threshold. This prevented not only economic damage in these fields but also further expansion in the Pecos Valley.
3. Delay planting cotton fields near good overwintering habitat.
Fields near good overwintering habitat are most susceptible to damage. These fields are near towns or weedy areas, particularly if there is a nearby river or stream. A delay in planting those fields will reduce the number of boll weevils that become established in them. Boll weevils do not move far from overwintering habitat. Few move more than 2 miles, and most move into cotton fields adjacent to overwintering habitat. Also, in fields near good overwintering habitat, boll weevils emerge over a longer period. Even a few days' or a weeks' delay in planting those fields can make a difference in damage compared to less susceptible fields.
4. If possible, plant cotton rows in an east-west direction.
Orienting the rows this way allows an even distribution of light into the canopy. Damage was 28 percent higher in rows oriented north-south in tests conducted in the Rolling Plains area of Texas. Yields were higher in rows oriented east-west due, in part, to reduced boll weevil damage. Ongoing tests in Artesia indicate that the canopy in east-west rows is significantly hotter and drier than the canopy in north-south rows. This microclimate also reduces survival of bollworm eggs.
5. Consider applying an insecticide when squares are the size of a match head (3/16").
An average of 2 boll weevils per trap indicates a population high enough to produce significant damage the same year. We are recommending a slightly more aggressive threshold of 1.5 boll weevils per trap for areas where they are well established and no eradication program has been initiated. Where boll weevils are not well established, treatment is recommended when at least 2 boll weevils are captured per field in 3 weeks prior to match head square (3/16") regardless of the number of traps. Ideally, the application should be made 2-3 days after the first match head square is found.
When scouting for squares, look for the most developed plants. Open the bracts to determine the size of the square. The size of the largest square will define when the application should be applied. Boll weevils cannot complete development in a square until it is hostable size, or the size of an eraser head (1/4"). However, final decisions about applications should be made at match head square to allow time to arrange for the application and to allow for the likelihood that some larger squares were not sampled.
6. For growers planting varieties other than Acala 1517, consider an early maturing variety.
Among the Bollgard cotton varieties, Paymaster 1215 and Paymaster 1330 are early maturing varieties that have yields comparable to the longer season varieties. Early maturing varieties escape high damage by late-season boll weevils because the boIls mature and become unsuitable for food and egg laying before boll weevil populations reach damaging levels. This also helps keep boll weevil numbers down for the following year.
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Printed January 1999Electronic Distribution March 1999