I’ve had the first report of treatment level infestations of soybean loopers in West Tennessee, and I’ve also seen an upsurge in larval numbers this past week. I am not surprised as infestations have been migrating northward through Mississippi and Arkansas the past few weeks.
Loopers have two pair of prolegs. This separates them from other caterpillars that may also be present. I previously wrote and article about distinguishing loopers and green cloverworms, another common defoliator that is typically less damaging and easier to control. Loopers do not feed on pods. They are foliage feeders. It is important to not let defoliation exceed 25% prior to R6. The impact of defoliation on yield will be very minor after R7. Looper populations may be composed of both the soybean looper and cabbage looper, but soybean loopers often compose the majority of the population during late season. Unfortunately, they are more difficult to control with insecticides. Below are some tips for managing infestations of soybean loopers.
- Treatment is recommended anytime before R6 if defolation exceeds 20-25% or if looper populations exceed an average of 19 larvae per 25 sweeps. Small larvae cause little defolaition, so concentrate your scouting on larvae bigger than 1/2 inch in length.
- Do NOT use a pyrethroid insecticide to control loopers. Pyrethroid insecticides typically provide no better than 0-50% control, and populations sometimes rebound to even higher levels following treatment. Indeed, you are more likely to have problems with soybean loopers if a pyrethroid insecticide was used within the last several weeks.
- Use one of the recommended insecticides below. They all should provide excellent control of loopers and other defoliating caterpillars that occur in Tennessee. Unfortunately, you will have to tank mix with another insecticide if stink bugs are also present in treatable numbers.
Recommended Insecticide for Control of Soybean Loopers:
- Intrepid (4-6 oz/acre) – Be prepared for a slow death. Some larvae will persist for 5-6 days after treatment, but little feeding will occur after the first few days.
- Steward (6-8 oz/acre)
- Tracer (1.5 – 2 oz/acre)
- Belt SC (2 oz/acre) - A relatively new product that appears to provide more residual than the other choices. Thus, it would be my choice if loopers get an early start. It is a little more expensive.
Can I Cut Rates?
Well it is legal, although companies will not stand behind lower than labeled use rates if a failure occurs. I would not consider cutting rates unless loopers arrived late (R6 or later). At this time, you only need to protect plants from excessive defoliation for another 14-21 days, and the plants are getting progressively less sensitive to defoliation. I’ve done several tests looking at reduced rates of Intrepid at 2-3 oz/acre in combination with a pyrethroid insecticide or Belt at 1 - 1.5 oz/acre. I’ve seen good control in these tests, but you should expect less residual activity. Using a slightly lower rates of the above products in combination with a pyrethroid insecticide is something to consider if stink bugs are also in the mix (which looks to be a likely scenario this year). However, you do so at your risk, especially if looper populations are well above the treatment threshold.