Figure 1. Lepidopteran larvae on soybean: (A) soybean looper, (B) green cloverworm, (C) velvetbean caterpillar (D) corn earworm. (Photos by James Villegas)
As we approach late into the summer, we should be on the lookout for late-season lepidopteran pests of soybean such as the soybean looper, velvetbean caterpillar, green cloverworm, and corn earworm. Soybean loopers can build large populations in a short amount of time and can be exaggerated by the use of broad-spectrum insecticides for stink bugs and three-cornered alfalfa hoppers. The threshold for soybean loopers is eight half-inch worms per row foot or 150 worms in 100 sweeps. Soybean loopers initiate feeding in the lower portion of the canopy. This behavior allows them to be protected from predators and insecticide application in a very dense canopy. Good insecticide coverage is crucial for optimal control of soybean looper. Several insecticides used against loopers include Prevathon or Vantacor, Besiege, Intrepid Edge, Steward, and Elevest. Once soybean plants reach R6.5 developmental stage, protection from loopers is no longer critical.
Velvetbean caterpillars and green cloverworms, like soybean loopers, can build a population quickly and can defoliate soybeans in a short amount of time. The threshold for Louisiana is eight half-inch worms per row-foot or 300 worms in 100 sweeps. Unlike loopers, velvetbean caterpillars and green cloverworms can be easily controlled with pyrethroids.
Although not a defoliator, corn earworms can damage soybeans by feeding on fruiting structures. The threshold for corn earworms is three worms per row plot or 38 worms in 100 sweeps after bloom. Corn earworms in Louisiana have developed high levels of resistance to pyrethroids; thus, the use of pyrethroid alone for this pest is highly discouraged. Recommended products include Prevathon or Vantacor, Besiege, Intrepid Edge, Elevest, and acephate plus a pyrethroid.
Before making any insecticide application for lepidopteran pests in soybeans, consider the insect species present, the insect numbers, and the defoliation percentage. Post-bloom soybeans can tolerate 20 percent defoliation without any significant yield losses.
The LSU AgCenter and the LSU College of Agriculture