Cotton Insecticide Termination Timing
DR. SEBE BROWN
JACKSON, TENNESSEE
With many of our cotton acres at or past cutout, wrapping up insect control is coming quickly. We’re beginning to see a general decline in plant bug populations but stink bugs continue to linger with numbers increasing.
Keep in mind that the average last effective bloom date is right around the corner (August 15-20th). This is the average date that a white flower has a 50% chance of making a harvestable boll. It is a roll of the dice whether bolls set after this date have time to mature before a frost. For late maturing fields that are just now or have not reached NAWF5, it is still advisable to continue to good insect management practices for 2-3 weeks to allow these last bolls to accumulate enough heat units to be relatively safe from insect attack.
Once cotton reaches NAWF 5, accumulated heat units (DD60s) determine when individual insect pests no longer pose an economic threat. Recommended termination thresholds are:
- Tarnished plant bug: NAWF 5 + 350 DD60s
- Bollworm: NAWF 5 + 400 DD60s
- Spider mites: NAWF 5 + 350 DD60s
- Soybean looper: NAWF 5 + 350–400 DD60s
- Stink bugs: NAWF 5 + 450 DD60s
- Fall armyworm: NAWF 5 + 500–550 DD60s
With the above considerations in mind, producers can potentially relax thresholds once we’ve reached the last effective bloom date.
- The last effective bloom date mentioned above represent a 50% chance that a new boll will make it into the picker. Thus, there is a pretty good chance these bolls will not contribute to yield.
- The upper, less mature bolls typically are smaller in size and number and represent a relatively small proportion of the overall yield. This assumes average or better boll retention on the bottom two-thirds of the plants.
My suggestion from this point forward is to pay better attention to stink bug and active bollworm infestations than to plant bugs. Stink bugs and bollworms are more likely to injure bolls. Spending a lot of money to protect squares and small bolls from plant bugs as the season gets later becomes harder to justify. Of course, we have some built in protection from bollworm because almost everything is Bt Cotton. Bollgard 3 and Widestrike 3 cotton varieties have held up well this year, however; Bt expression can also decrease at cutout and beyond. Keep an eye on fruit damaged potentially damaged by bollworms out to NAWF5 + 400 DD60’s. Final clean up shots need to have a pyrethroid, OP or combination of the two to take care of stink bugs, plant bugs and potentially catch a stray worm in the top. ∆
DR. SEBE BROWN
UNIVERSITY OF TENNESSEE