Thursday, September 9, 2010

Soybean Loopers: When Can You Stop Spraying? Rules Of Thumb.

From Ames Herbert, Virginia Tech Extension Entomologist

Soybean looper calls are pouring in from all over the eastern half of the state. It is adding insult to injury, having to treat fields for the second, third, or even a fourth time in the most extreme cases. On top of that, many fields are so drought stressed they are struggling to stay alive.

When helping growers make the decision on whether to treat a field for loopers we have to take the time to consider several components that influence the decision: the maturity of the crop, the health of the leaf canopy, and the number of loopers present.

Let's take them one at the time.

How susceptible? In terms of crop susceptibility, after some long discussions with soybean agronomists (David Holshouser at VT and Jim Dumphy at NCSU), we came up with a rule-of-thumb as to when fields are safe, that is, worms can be left untreated with no fear of lost yields.

We suggest that fields will need to be protected as long as the pods are still green and until the lower leaves are just beginning to yellow. This should correspond, more or less, with the R6.5 stage (10 days after R6.0 = full green seed). If leaves are beginning to yellow up the stem, not from drought but from the maturity process, and there are any pods on the plant that are beginning to yellow, the field is safe, no need to treat.

How much canopy have you got? Next we have to determine the health of the leaf canopy -- robust, average, or thin. Each can tolerate different amounts of leaf loss before reducing yield potential.

  • Robust fields (mid chest or higher) can tolerate a lot of feeding.
  • Average fields (upper thigh to mid chest) can tolerate normal amounts of feeding.
  • Thin canopy fields (mid thigh or below) cannot tolerate additional leaf loss.

How much defoliation have you sustained? In this canopy assessment, we need to take a stab at estimating the current percent defoliation. This is not an exact measure, but your best estimate looking over the entire canopy top to bottom. The eyes tend to focus on those badly defoliated top leaves. Look beyond those and try to come up with an overall average.

One thing we (and others) have noticed about soybean loopers is that their feeding is often in mid-canopy, not at the top like most other defoliators. All the more reason to inspect the entire canopy.

How many loopers are present? There is not a single threshold because of all the factors we have just discussed, but a very general rule of thumb is that 15-20 or greater/15 sweeps constitutes a potential threat, depending on the maturity and canopy health.

Thresholds vary quite a bit from state to state but this one falls pretty well in line with the other states.

In considering these components, some fields will be no-brainers. Mature fields (late R6 or older) or fields with robust canopies and just a few loopers (10 or less/15 sweeps) can be left alone.

On the other end of the spectrum, early R6 stage or younger fields with stressed, thin canopies and 15-20 or more loopers/15 sweeps need to be treated. This week in Virginia, there are a lot of fields in the grey zone (plants are in the mid to late R5 to early R6 stage, the canopy is average, the looper number is in the 12-18 range, and defoliation is less than 20%).

I tell folks, if they can do this:

  • Take a close look at these fields and make a mental image of the extent of defoliation.
  • Revisit in 2-3 days (no longer) to see if it has greatly increased.
  • If the percent defoliation has increased and loopers are still present at or near the threshold, treat it.
  • If the level of defoliation has not increased much and/or the looper numbers have decreased, don't treat.

It all sounds pretty complicated and it is, but taking the time to consider these components should help determine if a field needs to be treated. Some will and some won't, and allow growers to protect at-risk fields but save money on safe fields.

When will this looper flight end and what will stop it? I have been asked. I wish I knew. We need rain, badly, which will move fields to maturity and get us out of this mess.

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