Stinkbugs are invading some Delta area soybean fields in population numbers far exceeding treatment threshold levels. Case in point. Scouting reports from the soybean fields at one Sunflower County farm, near Indianola, Miss., found 60 stinkbugs per 100 sweeps. That's more than four times the number required to trigger an insecticide spray.
Jim Hamer, an entomologist with the Mississippi Soybean Management by Applied Research and Technology (SMART) program, recommends soybean producers closely monitor their fields for stinkbugs. “Populations of this pest are growing and, left untreated, can cause quality and yield losses, especially in soybeans that have not yet reached the maturity stage where the soybeans are filling the pod cavity,” he says.
Hamer, who makes weekly visits to farms participating in the SMART program, says he is picking up an increase in stinkbug activity “from the hills to the Delta.” And, while only two SMART fields have been treated for stinkbug control so far, others are quickly reaching threshold levels, he says.
The reason, he says, is partly due to the maturity of much of the state's soybean crop.
“Many of the Group IVs were planted early and, from the fifth node down, are already at the R4 or R5 stage, which means they have pods that are maturing out rapidly. The synchrony with these early-maturing soybeans is resulting in stinkbugs moving into soybean fields a little bit earlier.”
“We're picking up an increase in the number of nymphs we're catching in our sweeps. We're also seeing a hatch-out of stinkbugs in many soybean fields,” Hamer says. While the nymphs are not as damaging as adults are, the egg mass cluster often contains many eggs. After hatching, the nymphs can feed heavily in one concentrated area. Then, once the nymphs mature into adults and they get wings, plant damage often increases as the stinkbugs move across a field to feed on developing pods.
The high number of stinkbugs being reported now could spell trouble down the road for later-maturing Group V and Group VI soybeans. Hamer says he expects even greater numbers of stinkbugs will be reported in later-maturity soybeans further into the season due to the large number of nymphs currently hatching out. “This later generation of adult stinkbugs will be especially troublesome for later-maturing varieties and may cause serious injury and yield loss in Group V and Group VI varieties,” he says.
Treatment thresholds for stinkbugs vary according to the maturity stage of the soybeans being invaded by the pest. When the developing soybean seed is filling less than half of the pod cavity it is much more susceptible to blanking caused by insect damage, and as a result, has a lower treatment threshold. Then, as the soybean seed nears filling the pod cavity, the threshold numbers go up.
According to Hamer, the treatment threshold for immature soybeans, where the pod is less than half full, is one stinkbug per foot of row or three stinkbugs per 25 sweeps. After the pod fills to the point that the beans are touching, the threshold increases to one stinkbug per 3 feet of row or nine stinkbugs per 25 sweeps. For those soybeans being grown for seed, the treatment threshold is one stinkbug per 6 feet of row.
When stinkbugs feed on soybean plants that have reached that later growth stage, the soybean seed will continue to mature out, although it may be of lower quality. Such resulting quality losses include a black discoloration of the seed and a lower soybean oil content.
Stinkbug damage to younger soybean plants, however, can cause significant yield loss, according to Hamer. “Stinkbugs can feed on that immature seed and blank that seed completely out. And because the pods continue to look normal but are simply empty, the soybean plant can stay green trying to fill out that pod, which can be one cause of green bean syndrome.”
Treatment options available to growers for the control of stinkbugs include Karate Z at 0.025 pound active ingredient per acre; Asana at 0.03 pound active ingredient per acre; methyl parathion at 0.25 pound active ingredient per acre; and Scout X-tra at 0.018 pound active ingredient per acre.
The SMART program is a partnership between soybean producers and Mississippi State University, and is supported by the Mississippi Soybean Promotion Board.