Is This The Last Week To Spray Cotton?

JACKSON, TENN.
   The short answer to this question is yes for the vast majority of fields! Terminating insecticide application is always a tough call and even harder this year because so many fields are late maturing, just now reaching NAWF 4 or 5. Below are a few points to consider.
   Even for the latest cotton, we should be using the effective cutoff date of August 15th, representing the date where a white bloom has a 50 percent chance of maturing before a frost. Based on the current weather forecast, we will accumulate approximately 310-320 DD60s by September 1st.
   You should remember that the above date indicates the chances that a boll could accumulate enough heat units to beat the frost. However, upper position bolls typically contribute less to yield compared with bolls in the bottom two-thirds of the plant. This is one reason “relaxing” treatment thresholds is recommended as we approach NAWF5 + 350 DD60s.
   Research indicates that tarnished plant bugs cause little damage to bolls once they accumulate 250 DD60s. It is true that both clouded plant bug and stink bugs can damage older fruit, but it is unlikely that fields well below any treatment thresholds will have populations rebound to economically damaging levels levels once they accumulate 300 or more DD60s past cutout.
   There is a moderate flurry of bollworm activity in some areas. This is one case where I would consider treating during the first week of September in late maturing cotton if larval counts exceed 8 or more larvae per 100 plants. My concern is that these larvae will move down to feed on higher value fruit in the middle of the plant.
   I’ve had a number of questions about whether fields should be treated this week or wait until pest populations increase and hit them next week. My suggestions for most scenarios is, that if field truly requires a treatment, it is best to make it this week. There are a few reasons why. First, a good treatment this week will likely buy you a week of protection, after which treatments should not be necessary. Fortunately, Bt cotton makes this statement less risky. Second, the fruit that will be fed upon this week are more valuable. A square or boll that is currently less than 10 days old has a low chance of being harvested. Sprays made next week would primarily be wasted on protecting these low value fruit. Third, you may miss the opportunity to treat next week should a big rain occur, at which time any treatment decision will be a painful call and potentially too late. Δ
   DR. SCOTT STEWART: IPM Extension Specialist, University of Tennessee