SDSU Extension: Grasshoppers cause concern for South Dakota crops
In 2020, grasshopper populations were an issue in parts of central and western South Dakota.
The populations this year are moving into crops earlier and appear to be as bad, if not worse, than the ones observed last year. A factor that is likely driving grasshoppers into crops earlier this year is haying of nearby grass.
With the recently signed state of emergency order for South Dakota that allows the immediate mowing of roadside ditches, the grasshopper populations in fields will likely increase rapidly.
One of the crops that is currently being targeted by grasshopper populations is winter wheat. However, corn, soybean, sunflowers and other crops that are still green will be the next targets and should be monitored closely for grasshopper feeding.
One of the biggest hurdles with managing any insect pests is the preharvest interval that is present on the label. Choosing an insecticide with a long preharvest interval will delay when the crop can be harvested and could lead to other sources of yield loss.
Most insecticide products that are labeled for grasshopper management in wheat have a preharvest interval that ranges from 7–30 days. However, the products containing the active ingredient chlorantraniliprole (i.e., Coragen and Prevathon) have a preharvest interval of one day. Grasshoppers feeding on wheat can cause extensive defoliation and will also feed on or clip heads as the leaves begin to dry down.
Scouting and management
The thresholds for grasshopper feeding in wheat are 21 to 40 grasshoppers per square yard in the field margins, or eight to 14 grasshoppers in the field. There isn’t a threshold for the amount of head clipping that can occur before spraying. If this is occurring, spraying an insecticide is going to be based on how much head clipping has occurred, if the grasshoppers are still active in the field and the estimated time until the wheat can be harvested.
It’s important to remember that a large population of grasshoppers can cause devastating amounts of defoliation within a seven-day period.
If applying an insecticide, apply at the highest labeled rate, since the grasshoppers vary from nymphs to adults, and the adult grasshoppers are difficult to kill.
Another concern is that once the infested wheat is harvested, the grasshoppers will move into any other nearby crops that are still green. We recommend scouting those fields to ensure that severe defoliation does not occur.