This will likely be my last insect update for cotton this year. Reports are that 80% or more of the fields have reached the point where pest management is no longer needed. Plant bugs are still common in less mature fields. Stink bugs and a surge in bollworm eggs and larvae have also been reported this week. Spider mites are also lingering in some fields. Below are a couple of thoughts to consider when making decisions to treat for insects in late maturing fields.
- August 10-15th represents the date when a white bloom has a 50% chance of making a mature boll (based on the historical average frost date for Tennessee). Thus, for fields that are at NAWF=5 or higher, I suggest setting August 15th as the default “cutout” date.
- Given the above, you should terminate applications for insect pests no later than 350 DD60s after August 15th. This will typically take 15-18 days at normal temperatures. Insecticide applications after the first of September have dubious value. Treatment thresholds for insect pests should be relaxed once you’ve accumulated 200-250 DD60s. Once you’ve accumulated 200 DD60s past the August 15th deadline, it is very questionable whether plant bugs should be treated. The threshold for bollworm larvae should be doubled to 8 larvae/100 plants at the same time (approximately August 25-28).
There have been several questions about whether we should be spraying cotton that has open bolls at the bottom. This is pretty unusual but not impossible, especially considering the early dry weather and a second effort by the plants to make more fruit in some fields. First, you must remember there can be a big difference in maturity across many fields in Tennessee …. hills vs. bottoms, sandy soil vs. good dirt, etc. It is possible parts of the field may be in extreme cutout, and other parts may be very green. Treatment decision really should be made, not based of the maturity of the first bolls, but on the maturity of last harvestable bolls (those present at NAWF=5). You’ll need to use some judgement in fields with uneven maturity. You probably shouldn’t make a special trip if less than 50% the field is at risk, especially if you have to drive through the whole crop to just spray the less mature spots. You may cause more mechanical damage than it is worth.
Of course, you also have to consider pest pressure. Are pests populations very high in the susceptible portion of the field, or are you just a little above threshold? Decisions should be made based on averages of insect samples taken from representative areas of the field (not just the green parts). It is very helpful to have documented the date when the average plant in the field reached NAWF=5. Otherwise, you end up guessing how much longer the crop needs protection from insects.
