Soybean Loopers Threaten Alabama's Soybean Yields

Soybean loopers became an economic threat to soybean growers in Baldwin County (on the Gulf  Coast) this year during the first week of August. Soybean loopers were treated in many Blackbelt (west central Alabama) soybean fields this week as looper numbers increased and defoliation levels rose. Now farmers on both sides of the Tennessee River are preparing to spray numerous soybean fields for this pest for the first time since 1988. Densities of soybean loopers at the Fairhope Research station across the bay from Mobile were an average of 8 per sweep (all sizes) in test plots that were averaging an estimated 10 to 15% defoliation when an insecticide was applied on August 7. Eight days later defoliation in untreated plots ranged from 25 to 40%. Farmers in the Tennessee Valley have found as many as 3 loopers per sweep this week and defoliation levels are steadily heading toward the 20% threshold level in many fields.  Farmers in north Alabama west of I-65 are watching the weather hoping that they can find  an opportune time to spray the loopers during  a projected 5 day period of scattered showers. Most of the soybeans in the Tennessee Valley were planted behind wheat and yield potential is good presently. These later planted soybeans are mostly in the R4 to R5 stage of development and need another good rain to produce a strong yield.  One such field that is being monitored in Franklin county was running one looper per sweep on August 25 and had 5% defoliation. This field had 2 loopers per sweep (all sizes) on August 30 and 10% defoliation.  A few loopers had pupated in the field and had attached themselves with silk to the undersides of leaves. No diseased worms were observed while sampling. Stink bugs are also present in these fields but numbers are mostly below the threshold of 2 per 15 sweeps from bloom to mid-pod fill and 3 per 15 sweeps after mid-pod. Three-cornered alfalfa hopper (3-CAH) numbers are usually running from less than one to two per sweep .  Pod worm numbers are very low in most fields. Chemicals currently listed in the Extension Soybean IPM guide for soybean looper control are Belt 4SC, Intrepid 2F, Steward 1.25 SC and Tracer 4 SC. None of these products will control stink bugs or 3CAH’s. Belt, Steward and Tracer are labeled for control of pod worms in soybean. Hopefully, one application will be sufficient this year to manage soybean loopers but growers should continue to monitor fields following looper sprays to insure that populations do not rebound.

By: Tim Reed- Extension Entomologist and Ron Smith- Entomologist and Professor Emeritus