The following report is based on data from the week of July 12, 2021.
Cotton insect pests are posing risks for producers as they do each growing season. However, with different cotton maturity levels and difficult weather conditions, management during the 2021 growing season will be different than a traditional year.
While the peak migration of tarnished plant bugs in central and south Alabama seems to be over, it appears to be beginning in the Tennessee Valley. The Alabama Extension crops team received reports and saw fields with one to three times the threshold populations (2 per 25 sweeps) of adult tarnished plant bugs the week of July 7. In almost every case, square retention was still high, which likely means that the plant bugs had just started moving in. Square retention will likely be significantly less if those fields were not treated. Because of delays in finishing planting and differences in cotton maturity across the state, producers will likely be managing plant bugs (and many insects) differently in individual fields this year.
In the most mature fields (those in bloom) field advisors need to be sampling for immature plant bugs. This is done by placing a black drop cloth between two rows of cotton and vigorously shaking the plants on either side of the cloth to dislodge any insects from the plants onto the cloth. Count the number of immature plant bugs on each sample. The plant bug threshold is an average of 3 bugs per 5 row feet (one drop cloth sample).
In later planted cotton (prior to bloom), field advisors are still looking for adult plant bugs migrating in. To scout for adults, the most efficient methods are to use a sweep-net and monitor square retention. To monitor square retention, look for the presence (or absence) of first position squares on the upper two or three nodes of the canopy. Threshold is an average of 2 adults per 25 sweeps, or if square retention falls below 80 percent. One thing to keep in mind when scouting for adult plant bugs is that no product will provide much, if any, residual control. New adults may reinfest fields after sprays, so if scouting a field seven days after an application, it may be possible to find the same number – or more – adult plant bugs than were found prior to treatment. This does not necessarily mean a control failure, just that more adults have moved into the field. As long as fields are setting squares and square retention is above 80 percent, then the application did its job.
As some of the most mature fields in Alabama have reached the second or third week of bloom, field advisors must start thinking about stink bugs as well. Stink bugs are seed feeders and prefer bolls that are approximately 10 days old (about the diameter of a quarter), however in a situation with few of these bolls present, stink bugs will feed on even smaller bolls. Although not common in cotton at this age, field advisors should be sampling bolls for damage in these earliest fields.
Remember that sweep nets and drop cloths are not an effective way to scout for stink bugs. In order to scout for stink bugs, look for internal boll damage by cracking bolls open and looking for signs of stink bug feeding (warts, pin-prick marks, or stained lint). The threshold during the first two weeks of bloom is 30 to 50 percent internal damage and 10 percent for weeks three through six.
In a “normal” year, producers don’t worry as much about damage during the first two weeks of bloom because there are fewer bolls at risk during this time. However, this year with fewer acres in the area blooming and setting bolls at this point, there are the same number of stink bugs with less acres of cotton to dilute the population. Thus, stink bugs may be in more concentrated numbers in fields in our most mature cotton than “normal” this year. Be sure to look at small bolls during this time while still scouting for plant bugs. Stink bugs tend to infest field borders (approximately 50 feet) and smaller fields (approximately 20 acres) more heavily so keep that in mind when scouting.
Aphids and Spider Mites
The crops team has received reports of the aphid fungus killing aphids in parts of south Alabama over the past week. Hopefully the fungus will spread statewide quickly and knockout any lingering populations producers are dealing with.
There have also been reports of spider mites in fields in the east central and southeast areas of the state. For now, producers should watch these populations and wait until the weather forecast shows several consecutive hot, dry days before spraying. This will help to maximize the effectiveness of the application and reduce the chances of a rain event reducing efficacy.
Take Home Message for the Current Cotton Insect Situation
With the varying stages of cotton (from first true leaf to the third week of bloom) across the state, as well as within individual farms, there will be a lot of insect management on a field-to-field basis in 2021. Proper scouting and keying in on the correct pest for the stage of crop and time of year will be critical.
For up-to-date thresholds and insecticide recommendations, see the Alabama Extension Cotton IPM Guide, IPM-0415. Producers can also access information through Twitter, the Alabama Crops Report Newsletter and Podcast, and on the Pest Patrol Hotline. Producers can also reach out directly to Extension professionals.