If left uncontrolled, stinkbugs can cause substantial economic damage in Midsouth soybean fields. But if you try to control the insects before they build to treatable levels, you could do more harm than good.
Insecticide applications made prior to bloom can kill off beneficial insects and, as a result, increase potential stinkbug populations later in the season, according to entomologist Gordon Andrews at the Delta Research and Extension Center in Stoneville, MS.
On the other hand, he says treatments made after bloom, with reproductive adults and nymphs both present in soybean fields, are more apt to affect the season-long population of stinkbugs.
Andrews has found that producers are likely raising more than one generation of stinkbugs in early maturing soybeans. A stinkbug's growth cycle lasts anywhere from 35 to 40 days and emerging nymphs complete five life stages before becoming adults. All stages feed on soybeans, he says, but the fourth and fifth instar nymphs can cause as much damage as the adult stage.
“Stinkbugs have approximately two months to infest Group IV soybeans between bloom and drydown. During that time they have sufficient food to feed and reproduce, and could cause economic damage,” says Andrews. “We've learned that the timing of insecticide application is critical. Insecticides applied pre-bloom actually seem to increase stinkbug populations because of the decreased populations of beneficial insects after treatment.”
A more optimum time to spray, he advises, is between the R3 and R4 growth stages.
Mississippi State University guidelines recommend an insecticide treatment for brown stinkbugs, green stinkbugs or southern green stinkbugs when 25 sweeps across a field between first flower and the beginning of pod fill yield more than three stinkbugs. From the mid-pod stage until harvest, the treatment threshold is nine stinkbugs per 25 sweeps.
Stinkbug populations were suppressed in Andrews' study where plots were sprayed with pyrethroid insecticides two to three weeks post-bloom. They also saw a reduction in beneficial populations in all three pyrethroid-treated plots, compared to two untreated checks.
Another concern is flaring lepidoptera pests due to the reduction of beneficial insects, says Jeff Gore, an entomologist with the USDA Agricultural Research Service in Stoneville, MS. “It's always best to make sure insects are present at a treatable level before an insecticide is piggy-backed with another treatment.”
Gore says minimizing stinkbug populations may be possible with early planted, early maturing soybeans. Gore found that early planted, early maturing soybeans may escape most of the heavier stinkbug populations. In comparison, later-maturing varieties planted later in the spring experience much higher stinkbug populations.
Rarely have the study's early planted Group IV maturity soybeans planted in mid-March to late-April reached treatment thresholds. Instead, these soybeans seem to escape the larger pest populations.
The bottom line for growers, Gore says, is to plant soybeans early enough to avoid these stinkbug population peaks. The populations build, he says, due to the progression of their host plants.
“In Mississippi, a large percentage of our soybean acreage is planted early,” Gore says. “Early in the season, large populations are scattered over a large amount of acres. However, as the early soybeans are harvested, that large population of stinkbugs is funneled down to fewer number of fields that still have soybeans growing in them.”
Brown stinkbugs, Gore says, seem to stay at a more constant population than their green and southern green counterparts. They also tend to be more scattered in population clumps. While one field may have a large population of brown stinkbugs, another field may have few, if any, of the pests. Brown stinkbugs are also more difficult to control, often requiring an organophosphate treatment for adequate control.
When left uncontrolled, Mississippi State University researchers say stinkbugs can damage soybeans by piercing the pod hulls and sucking juices from the developing seeds. This type of feeding can result in unfilled pods, severely shrunken seeds or discolored seed around the puncture sites. Punctured seed can cause lower grades and lower germination, according to entomologists.