During the past few days, we have had a major increase of caterpillar defoliation reports in soybeans and sweet potato from southeastern NC counties. Field visits south of Highway 70 and east of I-95 confirmed that fall armyworm (Spodoptera frugiperda) infestations are widespread. In soybean, significant defoliation and pod feeding has occurred in multiple fields. In sweet potato, several growers have reported significant defoliation and direct root feeding.
In North Carolina, two different strains of fall armyworm can cause crop damage. The ‘corn strain’ is the most concerning and can cause economic damage in multiple crops (including soybeans). The ‘grass strain’ is adapted to common weedy grasses and typically causes issues in fields with poor weed control following herbicide applications.
We think most of the issues being reported now are from the corn strain. Unfortunately, the corn strain can be less sensitive to common insecticides, like pyrethroids, and you can’t tell the corn and grass strain apart from looking at the caterpillars.
Because these armyworms are widespread, growers should intensively scout fields. In soybeans and sweet potato, a sweep net will be the best way to determine the level of fall armyworm infestation (please see resources below). Although economic thresholds have been developed for soybeans (based on percent defoliation), if widespread infestations of numerous small larvae are detected, growers should consider an insecticide application.
Large fall armyworm larvae are often a challenge to control and may cycle out quickly, so an application might not pay. Also, once soybeans are at R7, defoliation shouldn’t lead to yield loss, but armyworms will move to pods if they stem a field. Armyworms might be tolerated in R7 soybeans with reasonable infestation levels, but keep an eye on them if levels get too high and especially in earlier growth stages.
In soybeans, we are recommending Blackhawk, chlorantraniliprole (either Besiege or Prevathon), Diamond (if larvae are small to medium sized), Lannate, or methoxyfenozide (Intrepid or Intrepid Edge).
In sweet potato, we are recommending an application of chlorantraniliprole (Coragen) or methoxyfenozide (Intrepid). Some registered materials for sweet potato have a 14-d pre-harvest interval. Please refer to the insecticide label for specific information regarding harvest timing.