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I will say up front that no two seasons are exactly alike when considering the weather, crop conditions, and potential insect damage. 2017 has already been uniquely different than any season I remember. Delayed planting, excessive rainfall, mild winter, and warm spring are just a few of the factors that could affect the movement, numbers, and level of damage from plant bugs to this year's cotton crop. Only by close monitoring of plant bug numbers and their damage over the next 4-6 weeks will we know how to deal with this pest.
We need to begin sampling the oldest and largest cotton immediately with sweep nets for adult tarnished plant bugs. In addition, we should keep check on our square set by making pinhead square retention counts. 80% retention is considered our treatment threshold. In other words, we do not want to lose more than 20% of our small squares to plant bug damage.
If past history is any help, abundant to excessive rainfall in June tends to keep wild host plants like fleabane fresh a little longer. This slows or prolongs the movement of plant bugs into cotton. After fleabane dries down, no other wild host is attractive to adult plant bugs, therefore cotton is the best host they have. The movement of plant bugs into cotton has already begun. This migration could continue for several weeks. This slow extended migration may not reach what we would call a threshold or treatable level, making treatment decisions difficult. In hot, dry springs, these adult plant bugs leave fleabane in high numbers over a relatively short period of time, say 7-14 days. This sort of rapid movement into cotton is easier to detect and to make treatment decisions for.
There are a number of other factors that could influence the severity of the plant bug issue in cotton this season. Overall, I would estimate that the maturity of our cotton is a little behind where it normally is this time of year. Will plant bugs move past these fields in search of the oldest most mature cotton? April cotton may serve as a trap crop for plant bugs in 2017. That would be to our advantage. Since early June, we have had a lot of cloudy days with temperatures in the 70s or 80s. Plant bug survival on cotton has likely been higher under these conditions. This could spell higher damage levels in coming weeks. Plant bug adults, and especially their immature offspring, do not fare well under drought conditions and high temperatures (over 95°F), leaving cotton plants under a stressed and wilted condition. Water logged spoils, which many fields have had during the month of June, can result in some pinhead square abortion. This effect would be very difficult to separate from plant bug injury. How can we distinguish this square loss from plant bug injury? The easiest way would be to use a sweep net to document that some level of plant bugs are actually in the field. One additional little trick that could help us answer this question was developed by our Arkansas entomology friends several years ago. This technique takes a little time, precision, and magnification. The tool needed would be a "pinhead square slicer." Take a damaged pinhead square from the plant (square will be brown or black in color), use a sharp razor, and slice the square in half. If it aborted due to weather, the inside content will still be present but deteriorating, whereas if it has been damaged by a plant bug, the interior will be hollow. Plant bugs tend to dissolve the contents and suck the interior of the square as food. Plant bugs will likely be the primary cotton insect focus of entomologists, consultants, scouts, farmers, and other interested parties until well into July this season. We need to recognize this and not allow plant bugs to further limit our maturity and potential yield in 2017.
Ron Smith, Extension Entomologist
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