Despite a relatively lack luster moth flight, corn earworms (a.k.a. bollworm or soybean podworm) are showing up in some late maturing soybean fields, and as usual the action in centered in the river bottom. The treatment threshold for corn earworm is based on sweep net sampling and using the table below.
Moths are most attracted to late maturing (flowering) fields and especially those with a fairly open canopy. The good news is that these fields are easy to sweep, and getting good insecticide coverage is usually not a problem. The bad news is we have to make some decisions about what insecticide should be applied.
Although pyrethroid insecticides are not as effective as they once were in controlling corn earworm, adding 1/2 lb per acre of acephate often provides acceptable control. I’m not too afraid of this approach when number are a little above threshold. However, the field should be checked 3-4 days after application to determine if adequate control was achieved. You should also consider that this combination might flare other pests such as loopers or spider mites. If numbers are well above threshold, the safer, albeit more expensive approach is to use products like Prevathon (14 oz), Beseige (7 oz), Blackhawk (1.7-2 oz), or Intrepid Edge (4 oz). Prevathon and Besiege will offer the most residual control, although in Tennessee, my experience has been that corn earworm control is usually a one-and-done thing in soybean.
Another option that has been used with success in recent years, and at an economical price, as been the application of Heligen (1 – 1.6 oz/acre). This is a good approach on high risk fields when the application is made when most the larvae are small. Thus, treatment is suggested a little earlier than with other insecticides (e.g., when 3-6 larvae are found per 25 sweeps). Heligen is a virus specific to corn earworm and tobacco budworm, and the goal is to get an infection started that spreads throughout the population. It requires a little patience and training to assess how well it is working. I encourage folks to at least take a look at Heligen as in alternative in some soybean fields, but this is not the product of choice when facing a raging infestation of corn earworms. I also do not suggest using this product in cotton.