Melissa M. Willrich and B. Rogers Leonard
The abundance of stink bugs, including southern green stink bug, brown stink bug (Figure 1) and green stink bug, has increased in Mid-South and Southeastern cotton-producing states in the last six years. Stink bugs have become more common cotton pests because of a number of changes in Louisiana’s agricultural environment that have made crop and noncrop hosts available year-round. These include widespread adoption of conservation tillage, crop rotation and conservation and wetland reserve programs. In addition, eradication of the boll weevil and use of Bollgard cotton, which contains resistance to caterpillar pests, have both resulted in less use of broad-spectrum insecticides, which has turned pests once considered secondary, such as stink bugs, into significant insect problems. Mild winters also contribute to proliferation of stink bugs.
Stink bugs may be found in cotton fields from seedling emergence until harvest, but they generally prefer to feed on those parts of the plant that produce seed. Economic injury to cotton plants usually begins during boll (the fruit containing developing seed) formation. Young bolls, less than12 days after opening (anthesis), damaged by stink bugs have a high probability of dropping off (abscising) from the plant (Figure 2).
Older bolls, more than 12 days after opening, when fed upon by stink bugs, will remain on the plant. Direct injury to seed and lint can affect ultimate yield and quality until the bolls have been opened at least 20 days. Penetration of bolls by stink bugs causes discolored, yellowed lint (Figure 3).
Injured bolls may display feeding symptoms on the exocarp (exterior boll wall) and internally on the endocarp (interior boll wall), seed and lint. Indications of feeding on the exocarp may be best described as dark, circular indentations on the boll wall. These symptoms are difficult to identify and similar injury may be caused by tarnished plant bugs. Injury observed in bolls approximately 12 days after anthesis should be attributed to stink bugs because the tarnished plant bug, another cotton pest, does not have the ability to penetrate bolls of this age or older.
Dark feeding punctures or wart-like cellular growths are found on the internal carpel wall (Figure 4). This symptom can be visible on carpel walls within 72 hours of feeding. One or all carpels within a boll may be injured, and it is not uncommon to observe internal injury without signs of external injury.
In addition to internal warts and external punctures, stink bug damage to cotton bolls may be exhibited in other ways. At harvest, fields that have sustained stink bug infestations have been associated with “hard locked” bolls (Figure 5). Hard lock is a physiological condition that can be caused by pathogens, insects or adverse weather. Hard lock symptoms include bolls that partially open and lint that fails to fluff. These bolls usually contribute little to final yield because they are difficult to harvest with mechanical pickers. Finally, germination rates for seed may be reduced in bolls previously punctured by stink bugs.
Producers and crop managers should become familiar with cotton boll injury symptoms caused by stink bugs. The Louisiana Cooperative Extension Service recommends insecticide treatments be initiated against stink bugs at one bug per 6 row-feet or when 20 percent of quarter-sized bolls (12-16 days after anthesis) exhibit internal injury symptoms on lint, seed or endocarp. Sampling bolls is the most reliable predictor of a stink bug infestation in cotton because of this pest’s extreme mobility and the cumbersome task of estimating insect densities in late-season cotton.
Melissa M. Willrich, Graduate Assistant, and B. Rogers Leonard, Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.
(This article was published in 2003 spring issues of Louisiana Agriculture magazine.)