Crop Maturity Based Insecticide Termination Rules. For a normally maturing crop, the standard suggestion is to terminate insecticide applications for plant bugs, stink bugs and bollworm at NAWF5 + 350 DD60s. The reality is that it is probably closer to 250-300 DD60s for tarnished plant bugs and 350-400 DD60s for stink bugs and bollworm. We would typically suggest managing for defoliators (including spider mites) up to about NAWF5 + 600 DD60s. However, spider mite infestations that are not apparent at NAWF5 + 350-400 DD60s rarely increase to levels that would cause substantial economic injury. There are a few other points to make.
- Obviously, it important to document when a field reaches or falls just below having 5 or fewer nodes above the uppermost (first position) white flower.
- It takes 18-24 days under normal temperature conditions to accumulate 350 DD60s. Remember, daily accumulations are calculated by averaging the average daily temperature (max + min/2) and subtracting 60. So for a day with a 90o high and a 60o low, you accumulate 15 DD60s = (90+60)/2 – 60 = 15.
- All this assumes that the field has been properly managed for insects. You should not walk away from a heavily infested field just because you have accumulated the magic number of DD60s.
- It is sometimes suggested to relax treatment thresholds as the crop approaches NAWF5 + 350 DD60s. For example, treating for bollworm only when populations reach or exceed 8 larvae per 100 plants (as opposed to 4 larvae). This is partly because the bolls at risk will generally be smaller and contribute less to the overall yield. However, I would not consider doing this until you’ve accumulated 200 DD60s after NAWF5.
Weather Based Insecticide Termination Rules. For a late maturing crop, weather based termination rules come into play. These are based on the long-term average frost date, which is approximately October 15th for West Tennessee. In a normal season, the last effective bloom date is August 10-15. This is the date when there is a 50% likelihood that a white flower can accumulate enough heat units to become a mature boll before the first frost. If you’re a risk taker, you might push this an extra week, but you do so at the mercy of Mother Nature.
- I’m defining a late crop as one that has not reached NAWF5 by August 15th. This is the scenario when weather based termination rules should be implemented.
- Considering the above, my best recommendation is to consider August 15 as your last effective bloom date. You need to protect the bolls developing from these flowers for another 350 DD60s. Any insecticide applications made after the first week of September have highly questionable value.
Defoliation and Weather Rules. Timing the application of harvest aids is normally based on the maturity of the last, uppermost harvestable bolls. Common timing mechanisms are to defoliate when the crop has 1) 60% open bolls, 2) there is an average of 4 or fewer nodes above the uppermost, first position cracked bolls, or 3) when the last harvestable bolls have a black layer around the developing seed. However, weather rules also come into play for a late-maturing crop. It is generally suggested that harvest aids should be applied no later than October 1-4, allowing them time to work before a frost occurs. You may want to consider and slightly earlier start for green, rank and late cotton where a two-pass approach may be needed. This topic will be addressed in future articles.