Newly emerging insect pests in the Mid-South are giving new meaning to the phrase “Nature abhors a vacuum.” The rapid adoption of transgenic cotton varieties combined with a successful boll weevil eradication program, have allowed many growers the luxury of spraying fewer insecticides. Unfortunately, today's more environmentally friendly production practices also may be enabling aphid, plant bug and stinkbug populations to flourish in the Delta's cotton fields.
The low insecticide regimes that often go hand-in-hand with transgenic varieties and boll weevil eradication can flair the populations of secondary insects, says Aubrey Harris, a Mississippi State University entomologist at the Delta Research and Extension Center in Stoneville, Miss.
“The aggravating thing is that tarnished plant bugs and stinkbugs normally would be considered occasional pests, because they are not adequately controlled by natural enemies,” Harris says. “Spraying insecticides actually would help control these insects. Insecticide treatments targeted for the control of other pests were killing them before growers reduced treatments for worms and weevils.
“We truly could have very low insecticide use without stinkbugs and plant bugs.”
While plant bugs often made early-season appearances in many cotton fields, they generally were not considered a season-long pest. Because the insecticides that were killing them are not being used as much anymore — plus the fact that tarnished plant bugs have developed resistance to pyrethroids — grower opportunities to fully practice integrated pest management have been limited.
Scouting guidelines for plant bugs recommend that before cotton is squaring, growers should walk either along or across rows and scout visually for the presence of the insects. Adults are the primary concern at this time. Record counts as the average number of plant bugs per 10 feet of row.
After squaring, adult plant bugs are best quantified by using a 15-inch diameter sweep net. Take several 25-sweep samples per field, and record results as average number of plant bugs per 100 sweeps.
“Because adult plant bugs are very mobile and easily flushed, it is important to space sweeps widely apart and move quickly down the row while sweeping,” advises the Mississippi State University insect cotton guidelines.
Plant bug nymphs can be sampled using a 3-foot ground cloth spread between two adjacent rows, vigorously beating plants over the cloth, and counting the number of nymphs that are dislodged onto the cloth. “Take several such samples per field, and record counts as the average number of plant bugs per 6 feet of row. During the period between square initiation and early bloom it is also important to monitor percent square retention,” Harris says.
Growers can conduct insect counts by examining 100 potential first and/or second position square sites and counting the number of missing squares, as indicated by the presence of an abscission scar. On plants with more than five fruiting branches, limit counts to potential fruiting positions on the five uppermost fruiting branches.
Stinkbugs were being controlled coincidentally by insecticide treatments targeted for other pests, says Harris. “But they are emerging as a significant pest under the current insect management systems of lower insecticide use for bollworm and tobacco budworm, and no insecticide use evolving for weevils. As a result, stinkbugs are becoming elevated in their importance as a cotton pest.”
The stinkbug is an “opportunistic” type of insect, explains Harris, and fewer insecticide applications have created an opening for it to move into cotton. Stinkbugs prefer other plants, including soybeans, he says. But when soybeans get past the stage desirable for stinkbugs, they move into cotton, damaging young bolls.
“There has been some difficulty in coming to terms with the threshold for stinkbugs, probably because of problems with how to scout them,” says Harris. “The damage they cause, however, is something that's not going to fly away and is more easily observed. Treatments can be economical because stinkbugs can penetrate fairly mature bolls. Look for scarring inside the bolls as a sign of stinkbug damage.”
In Alabama, stinkbug thresholds were developed from scratch, says Ron Smith, Auburn University Extension entomologist.
“We started with a certain number of stinkbugs per row feet. But, eventually we saw that you can't catch stinkbugs in waist-high cotton. Now we've identified the exact size of the boll they prefer. We slice those bolls and look for internal damage — about a quarter-size in diameter.”
Scouting techniques, says Smith, have been modified to meet the demands of sporadic and emerging insect pests. “We've modified our thresholds for when growers should treat for certain pests,” he says.
Secondary cotton pests, like aphids, normally would be controlled by natural enemies if it weren't for insecticides controlling those natural enemies, says Harris. “Aphids have a lot of natural enemies that would normally reduce their populations. The less you spray cotton, the less likely aphid populations will explode.”
When scouting for aphids, entomologists stress the importance of noting any additional stress factors that may be affecting the crop. In addition, growers need to be aware of predators, parasites, and pathogens that may be affecting the aphid population.
Growers can scout for aphids by randomly selecting fully expanded leaves, and counting the number of aphids present. Record counts as the average number of aphids per leaf. Also, note the general distribution of the aphid infestations within the field and the degree of honeydew present.
Cotton scouting has evolved as the number and type of insects has evolved. It used to be that insect scouts could focus on one or two pests, like bollworms and weevils. Now, however, there may be 15 different potentially economically damaging pests lurking in your cotton field.
“Rather than having broad-spectrum, dominant insect pests requiring multiple pesticide applications, growers are now confronted with sporadic pests,” Smith says. “We're into a stage where we do more monitoring than treating.”