Padma Latha Bommireddy and B. Rogers Leonard
Stink bugs were first recorded as pests of cotton during the early 1900s. The most common species of stink bug found in cotton fields across the southern United States are the southern green stink bug, green stink bug and brown stink bug. The general reduction in the frequency of broad-spectrum insecticides for cotton pest management has increased the incidence of this pest complex. Stink bug infestations in Louisiana cotton fields increased from 8 percent to 69 percent of total cotton acreage from 1999 to 2006. This same group of stink bugs also can be found as pests of other Louisiana row crops including field corn, soybean, wheat and grain sorghum.
Numerous factors have been associated with an increase in insects with sucking mouthparts, such as stink bugs, during the past decade. The most important factor has been a reduction in the use of broad-spectrum insecticides, which target one pest but inadvertently control everything in the field. The frequency of use of these products against boll weevils and the target pests of Bt cotton have decreased. The void created by the absence of these pests allowed others such as the stink bug complex to become more important.
In cotton fields, stink bugs primarily damage immature bolls by piercing the fruit wall and feeding near or on immature seeds. Small bolls may fall from the plant, but larger, more mature bolls that are injured will form a rough, warty growth on the inner carpel wall at the site of injury and remain attached to the plant. These insects significantly reduce yield by affecting cotton weight and reducing seed quality. Both adult and late-instar stages of stink bugs can injure cotton bolls and reduce seedcotton yields.
Limited information is available on the effects of stink bug adult and nymph stages on cotton lint quality. The evolving change in the pest status of stink bugs and an emphasis on improving the value of cotton fiber properties has prompted LSU AgCenter researchers to examine the influence of southern green stink bug adults and fourth-fifth instar nymphs on cotton bolls and physical fiber quality.
These studies were performed at the LSU AgCenter’s Macon Ridge Research Station near Winnsboro in Franklin Parish during 2005 and 2006. Southern green stink bug adults and nymphs were collected from nontreated fields of corn and soybeans during both seasons. Insects were maintained in the laboratory on green beans for 24 hours to eliminate any insects in poor physical health. Cotton bolls were caged either with a stink bug adult or fourth-fifth instar nymph. Control noninfested bolls were caged but with no insects.
After 72 hours, all cages and insects were removed. All bolls were marked with a tag that included treatment information. Harvest aids were applied to the entire test area at crop maturity, and all tagged bolls were manually harvested 10 to 14 days after defoliation. Cotton was separated into lint and seed fractions with a tabletop gin in the laboratory.
Physical fiber properties were measured by processing lint samples through the high-volume instrumentation system in the LSU AgCenter’s Cotton Fiber Laboratory in Baton Rouge. The cotton fiber properties examined in this study included length, micronaire (measure of fiber fineness or maturity), strength, uniformity and discoloration or brightness (determined by reflectance and referred to as yellowness).
Stink bugs, regardless of life stage, significantly affected fiber quality. Fiber length was significantly shorter, and micronaire was significantly lower in stink bug-infested bolls compared to noninfested bolls. Both life stages significantly reduced fiber strength. Fiber uniformity was lower in stink buginfested bolls compared to that in non-infested bolls. Fiber discoloration was significantly higher in stink bug-infested bolls compared to that in non-infested bolls.
The value of cotton is influenced by fiber quality in addition to lint yield. The significant differences in fiber quality associated with stink bug-infested bolls will affect crop value. Fiber micronaire, strength, uniformity and length were significantly lower, whereas fiber discoloration was higher in bolls infested with southern green stink bugs compared to fiber harvested from noninfested bolls.
There were no significant differences in cotton fiber quality among southern green stink bug life stages. These results are important in developing and validating sampling protocols for action threshholds to initiate insecticide treatments for stink bug management. The lack of differences among life stages allows a cumulative number of southern green stink bugs to be considered rather than develop individual threshholds for different life stages.
These results indicate that the relationship of stink bug injury and cotton fiber quality must be considered in the development of action threshholds. Future studies should consider not only yield losses but also effects on fiber properties and harvested seed viability when estimating economic injury and developing action levels for insect pests that attack cotton bolls.
(This article was published in the winter 2008 issue of Louisiana Agriculture.)