Alabama Insect Situation: September 8, 2022

As the calendar turns to September, most of our cotton is past insects, some is beginning to receive defoliation applications, and all is hoping for a dry fall so we can get all the lint from the field to the basket. With the cotton season mostly behind us, the soybean insect situation is beginning to build.

VBC collected in 5 sweeps near Prattville, AL.

Over the past week, we have gotten several reports and phone calls about defoliating caterpillars in soybeans across central and northeast Alabama. While a complex of worms is in fields, soybean loopers (SBL) and velvetbean caterpillars (VBC) are the dominate and most important species in the mix. In addition to a detection technique (i.e., sweep-net or drop cloth), looking at the defoliation within the plant canopy can give an idea as to which species is doing most of the damage.

VBC: 4 pair of prolegs, defoliation begins in the upper portion of the canopy

SBL: 2 pair of prolegs, defoliation begins in the lower portion of the canopy

We have a couple of thresholds to use based on scouting technique for defoliating caterpillars. Prevent 20% defoliation during peak podfill (R1-R6), from R6 to R6.5 (≈7-10 days), prevent 30% defoliation. Alternatively, treat when 38 VBC or 19 SBL are found per 25 sweeps. As mentioned above, most fields will have a complex of worms, not just one or the other. Thus, detection techniques may be more useful for determining pest complex than treatment levels. Species composition is important for insecticide decisions. Another important reason for detection techniques is to get an idea of the size of the worms in the field. Like most things, VBCs and SBLs consume more food as they grow. In other words, larger worms eat more leaves (in a shorter amount of time) than smaller worms. If the majority of the population is made up of worms nearly an inch long or bigger, then action should be taken immediately. If most of the worms are small (less than 0.25 inch), then there is more time to make a spray.

Once the decision is made to treat, what are our options? There are several good materials to choose from, considerations for what to choose involve pest complex, economics and weather.

If soybean looper is the dominate pest, pyrethroids will do more harm than good. Our options for soybean loopers include diamides (e.g. Besiege, Vantacor, Elevest) or Intrepid/Intrepid Edge. These products provide good initial and residual control of the spectrum of the defoliating caterpillar complex. While the mentioned options provide excellent control of VBC, most labeled pyrethroids and IGRs also provide control. One issue with pyrethroids is the lack of residual control. We have seen instances in the past where multiple applications were required to manage VBC populations because of an extended migration over several weeks that would have likely been alleviated by a single initial application of a diamide.

If you would like to provide reports or observations on the insect situation from your region, please let us know. You can reach Scott Graham at 662-809-3368 or scottg@auburn.edu or Ron Smith at 334-332-9501 or smithrh@auburn.edu.

As always, if we can ever be of any help, please let us know.