Cotton fiber length can be preserved and short fiber content can be reduced through control of stink bugs, according to research conducted by entomologists at the University of Georgia. The studies also showed an increase in lint yield in plots treated for stink bugs versus untreated plots.
The stink bug has come a long way over the last eight years — from a relative unknown to a primary pest for cotton producers, noted Phillip Roberts, Extension entomologist, University of Georgia, speaking at the Engineered Fiber Selection Systems Conference in Memphis.
According to a University of Georgia summary of estimated losses to stink bugs, the pest wasn't even on the list as a predator for yield losses in the early 1990s. “We became boll weevil-free in Georgia in 1992, and started seeing increases in stink bug damage when we decreased our insecticide sprays for boll weevil.”
Stink bug and other boll-feeding pest numbers remained relatively low through 1997, “when we really started planting a lot of Bt cotton and again reduced the number of insecticides we used on an annual basis. In 2003, we had a very severe outbreak of stink bugs in Georgia. In 2004, growers responded and we did a much better job of responding to them.”
Stink bugs have a needle-like mouth part, which they insert through the boll wall to feed on a developing seed. Once they feed on the seed, “we can see symptoms of injury on the internal parts of the developing boll. The injury shows up as either stained lint or a callous on the inside of the boll.”
The physical damage “impacts the development of lint, but even more importantly, when stink bugs feed on these developing bolls, they introduce a pathogen or an organism which can enter the boll and cause varying degrees of yield loss.”
On the other hand, there is a lot of variation in how the damage manifests itself in the boll, according to Roberts. “Sometimes, we may see very little damage. Other times, we may see an individual lock fail to fluff. In severe cases, we may see the entire boll rot.”
Roberts said stink bugs will feed on bolls until bolls are 27 days old. “We believe they prefer to be on bolls about 12 to 14 days old. If the stink bug is feeding on the developing seed during fiber elongation or thickening, it makes sense that this would impact quality.”
In one study, University of Georgia researchers hand-picked first position bolls at a common node on all plants in research plots prior to commercial harvest. The bolls were sorted by how many locks were damaged, from no damage to four to five locks damaged. Damage was defined as localized discoloration on a lock.
The bolls were ginned on a table-top gin without a lint cleaner. HVI information was obtained on all the samples through Cotton Incorporated.
The results were not all that surprising. Researchers observed that micronaire was lowered at higher levels of stink bug damage. “We also see a trend that as we go from one damaged boll up, we see lower length. There were also correlations to more yellowness with higher levels of stink bug damage.”
The researchers also conducted more detailed studies which showed that even a small amount of stink bug damage could impact fiber quality. The research looked at four locations with varying degrees of stink bug infestation.
The researchers picked first position fruit by node in both untreated and treated plots. After ginning each position by node, they sent lint samples to Cotton Incorporated and quality was measured.
In the location where infestations of stink bugs were the highest, there was an increase in yield of 33 percent. “On the upper half of the plant, we saw significant increase in the mean length of fibers in the treated versus the untreated plots. We were able to preserve fiber length by treating for stink bugs.”
In the location with the second highest infestations, the researchers recorded a 16 percent increase in yield in the treated plots versus the untreated. There were also consistently longer fibers where stink bugs were controlled versus untreated.
In the locations with the third highest infestations of stink bugs, “the pressure from stink bugs is getting less and we don't see significant differences. However, there is still a trend for fibers to be longer on the treated versus the untreated.”
In the fourth location, which had the lightest infestations, “we saw a 2 percent increase in yield. There was not a lot of stink bug pressure, but there still is a trend for fiber lengths to be longer in plots which were treated with stink bugs and plots that were not.
“There were very few nodes at any of the locations where we did not increase fiber length in the treated plots versus the untreated plots,” Roberts said. “The study also indicates that where we can control stink bugs, we reduce the occurrence of short fiber content.”