Sporadic pests target cotton fields

Changes in cotton production systems not only have made plant bugs and stink bugs more significant pests, but have opened the door for damage from sporadic pests, such as slugs, grasshoppers and three-cornered alfalfa hoppers, according to entomologists at Cotton Incorporated's Crop Management Seminar, in Tunica, Miss.

According to Ron Smith, professor emeritus, entomology, Auburn University, three factors have led to the emergence of these pests — the elimination of the boll weevil, which has ended the application of broad spectrum insecticide sprays; the planting of genetically altered Bt varieties; and the increase in conservation tillage acreage.

Sporadic pests in the Mid-South begin with a slimy, but damaging pest — the slug, according to Extension entomologist Angus Catchot, Mississippi State University.

Slugs and their defoliation of young cotton dominated the early-season pest complex in 2004, according to Catchot. Slug infestations were confined to no-till fields, with several fields requiring complete or partial replants.

“No control is achieved using any of the traditional insecticides labeled for use in cotton. While slug baits have been used in the Midwest with some success, they have had no use in the Mid-South since they are costly and slug infestations have been unpredictable.”

Three-cornered alfalfa hoppers were extremely numerous in soybeans across the Mid-South in 2004. “Consequently, it was expected that this sporadic pest would show up in a few cotton fields,” Catchot said. “These pests are of little economic importance and feeding is generally restricted to leaf petiole girdling.”

Treatment is rarely justified unless plants are stressed and adjacent to a legume host of the three-cornered alfalfa hopper that could cause large numbers to migrate into adjacent cotton fields. “But most often the insects are long gone when the injury is observed.”

Clouded plant bugs feed on squares, flowers, and bolls and can cause significant damage when numbers are high. “Most states use stink bug thresholds for clouded plant bugs,” Catchot said.

Saltmarsh caterpillars are defoliating insects and can cause problems early and late in the season, according to Catchot. “Damage is usually confined to field borders because insects move out of alternate hosts to feed on cotton. Extremely high numbers of saltmarsh caterpillars are sometimes observed moving into cotton early in the season after alternate host plants are killed by herbicides.”

Late in the season, this pest can reach treatable levels but generally it is found in combination with loopers and is treated when premature defoliation is threatened, according to Catchot.

Garden fleahopper feeds on the underside of cotton leaves, puncturing individual leaf cells and disrupting the tissue, causing white splotches on the leaf surface. This resembles spider mite damage.

“This insect is of little importance economically now in the Mid-South and no economic thresholds have been established,” Catchot said.

Southeast pests

Hopping over to the Southeast, grasshoppers have emerged as pests of seedling cotton in recent years, primarily in conservation tillage systems, said Extension entomologist Ron Smith. “Cotton is most susceptible to grasshopper injury from the time it begins to emerge in the crook stage until the plants have about six true leaves. Both the immature and the adult stages may cause injury.

“As much as 50 percent or more of the acreage in certain regions of Alabama have been treated for grasshoppers in recent years,” Smith said. “Some acreage is treated every season and some fields require multiple applications due to additional emergence or migration from field borders.

“The most effective controls of adults have been obtained with high rates of pyrethroids or phosphate insecticides. Delayed, but good long-term suppression has been achieved with IGR's such as Dimilin.”

Southern armyworms have been occurring throughout the coastal plains area of the southeastern United States for about six years, according to Smith. “Controls have been warranted in some fields where the egg masses have occurred on stalks every few feet down each row. The plant where an egg mass is deposited usually receives near complete defoliation.”

The southern armyworm is very sensitive to most cotton insecticides, according to Smith. “Pyrethroids at low to medium rates are very effective. Most southern armyworm infestations occur in fields that have not been treated, or not been treated for caterpillar pests for several weeks.”

The leaf-footed plant bug has emerged as a significant economic boll feeder in the low spray cotton environment that has developed in the coastal plains of the southeastern United States, Smith said.

Smith noted that the insect was listed in a USDA Farmers Bulletin published in 1905 and in another printed in 1932. The damage was described in earlier days as “small, round, blackened spots appearing on the surface of young bolls.”

“This description is very accurate. It is similar to stink bug injury. In fact, when both are present in fields, it is impossible to separate the damage of the two. Their seasonal occurrence is also similar to stink bugs in that they are mid- to late-season boll feeders.”

Adult leaf-footed bug is brown, oblong, and almost 1 inch long. The common species has a white band extending across the front wings. According to Smith, lab research supports field observations that pyrethroid chemistry is not very effective on this pest. Phosphates such as Bidrin and methyl parathion at 0.5 pound active ingredient per acre are highly effective.

“Since leaf-footed bug damage is similar to that of stink bugs, surveys of internal boll damage to 10- 12-day-old bolls would capture damage caused by this species.”

Vegetable weevil has caused damage to cotton fields similar to that of cutworms. “Another sporadic seedling pest is white fringed beetle larvae. The larva is slightly curved, yellowish-white, and legless with a light brown head. An adult is dark gray with a marginal band of white hairs and is a foliage feeder. “The larvae are very destructive to tap roots and underground stems of many plant species during the spring season. Digging with a knife will uncover the larvae. No controls are available.”

Cotton leaf worm is an old cotton pest that is also sometimes observed in the low insecticide input cotton of the Southeast. This species is a voracious leaf feeder but very sensitive to most cotton insecticides.

Fall armyworms have occurred in the Gulf Coast areas in recent years, according to Smith, usually when the caterpillars were feeding on in-field weed species prior to burn down with herbicides. Sporadic pests that do not cause economic damage in cotton include true armyworms, Japanese beetles and false chinch bugs.

True armyworms have been observed in high numbers in April and May in cotton fields planted no-till into burned-down rye. “These armyworms are similar in appearance to fall armyworms but will feed only on rye stubble and not cotton. However, their numbers could make a grower or consultant nervous.”

Japanese beetles have occurred in recent years in localized spots of fields in the Tennessee Valley and other areas of northern Alabama, according to Smith. “Five to 20 or more adult beetles can cause severe ragging to infected plants. However, usually not enough plants are affected to justify fieldwide or even localized insecticide applications.”

False chinch bugs occur in cotton between emergence and pre-bloom, usually in dry seasons. “They migrate from wild hosts and occur on cotton in numbers of 25 or more per plant. The only damage ever observed is when they were present in high numbers and cotton was in the early seedling stage. Excessive plant feeding on young plants may cause death by desiccation.”

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