• By Dominic Reisig •
As mentioned in the 2019 North Carolina Cotton Information Guide, “The key to successful cotton production in North Carolina is the adoption of a short-season management strategy.”
Tarnished plant bugs can throw a wrench in all these areas and should be scouted and managed from cotton time it squares until the last harvestable boll is formed. Scouting cotton for tarnished plant bugs is intensive, and most growers will opt (and should in my opinion) to use a trained crop consultant.
You can visit the cotton scouting guide for detailed information, and these pages for quick recommendations for scouting and treating plant bugs prebloom and postbloom in cotton. This article focuses on the reason why we recommend two specific scouting methods. It is critical that you split scouting into pre and post bloom because our thresholds change throughout time.
Two different thresholds
Prior to blooming our threshold is BOTH 8 plant bugs in 100 sweeps AND 80% square retention. Once cotton blooms, our threshold is 2-3 plant bugs per drop cloth sample (representing 5 feet of row). We recommend this change to take advantage of pest biology and cotton biology.
Adult plant bugs tend to migrate into the field during squaring, producing few nymphs until bloom. Nymphs tend to dominate after bloom compared to adults. Sweep nets are good at sampling adults, but poor at sampling nymphs. Drop cloths are good at sampling nymphs, but poor at sampling adults.
The data set below demonstrates this principle. This was timely planted cotton at Plymouth, North Carolina, during 2017. The threshold for both the sweep net and drop cloth is indicated by the purple dotted line.
Note that while the sweep net threshold was exceeded on July 6 and 13 (more than 8 total plant bugs in 100 sweeps, ignoring square retention for the purpose of this example), it was not exceeded on July 21 (only an average of 5.5 total plant bugs per 100 sweeps).
However, the cotton was blooming by this point and, using the drop cloth numbers from July 21, the threshold was exceeded by about 2x. In this case, if the sweep net were used instead of the drop cloth, it would have resulted in a missed spray.
Therefore, to properly protect cotton from tarnished plant bug and to maximize yield, the sweep net and square retention should be used prior to bloom, while the drop cloth should be used postbloom. Finally, we recommend using a black drop cloth over a white drop cloth. Small plant bug nymphs are easy to miss on a white background, but contrast nicely against a black background.
Dr. Dominic Reisig is an associate professor and Extension specialist, entomology & plant pathology, at North Carolina State University. He may be reached at firstname.lastname@example.org.