When Should You End Insecticide Sprays for Tarnished Plant Bugs in Cotton?
Article by Reisig and Collins
To answer this question, you need to know when the field first reached cutout, which is at 4 nodes above the first position white flower. Keep in mind that the last effective bloom date (the point at which there is a 50% chance the bloom will set a harvestable boll) for North Carolina is generally between August 20 – 25, but can range from August 15 – September 1 depending on fall weather and location within our state. So generally, any squares that have not bloomed by this point are likely not going to matter significantly for yield. Even if they do make it to harvest they will likely contribute little to overall yield if the bottom and middle crop is good. Please consider this before investing in the cost of an insecticide application that may not be 100% effective (see insecticide trial results below).
Plant bug management after cutout really needs to focus on keeping the internal boll injury to a minimum. Like stink bugs, plant bugs can feed on bolls and, like stink bugs, plant bugs transmit pathogens that can stain lint or cause boll rot and they can interfere with locule development. Occasionally bolls can abscise from plant bug feeding. While most stink bugs in cotton (brown and green) prefer to feed on medium (just under an inch in diameter) and small bolls (~1/2 inch diameter), plant bugs prefer to feed on small bolls. Internal boll injury from stink bugs and plant bugs is indistinguishable, so it is important to scout!
Bolls are safe from plant bug feeding once they accumulate 300 DD60s. During this time of the year, we typically accumulate around 18 DD60s a day. So it takes around 17 days for a boll to be completely safe from plant bugs. More recent data from the Midsouth suggest that once cutout is reached a boll is safe from tarnished plant bugs once it accumulates 250 DD60s, or around two weeks. Therefore, only bolls that formed before August 25 should be protected from tarnished plant bugs, and they should only be protected for two weeks following cutout. Scott Stewart, University of Tennessee, recommends increasing the threshold from 2-3 plant bugs per drop cloth sample to 3-5 plant bugs per drop cloth sample after cutout. Both stink bugs and bollworm should be managed until bolls reach 350 DD60s.
Be sure to rotate insecticides, especially if you’ve used multiple sprays. Results from some previous plant bug insecticide trials can be found on the Insecticide Recommendations for Plant Bugs page and results from this year’s trial are below.