Monday, July 12, 2021

Alabama Cotton Insect Situation July 12, 2021

 Tarnished Plant Bugs

Immature Tarnished Plant Bug
While the peak migration of tarnished plant bugs in central and south Alabama seems to be over, it appears to just be starting in the Tennessee Valley. We received reports and saw fields with 1-3x threshold populations (2 per 25 sweeps) of adult tarnished plant bugs last week (July 7). In almost every case, square retention was still high, which tells us that the plant bugs had just started moving in. As this newsletter is coming out, we suspect square retention will have significantly fallen if those fields were not treated. With the delays in finishing planting and differences in cotton maturity across the state, we will likely be managing plant bugs differently in individual fields this year.

In our most mature fields (those in bloom) we need to be sampling for immature plant bugs. This is done by placing a black drop cloth between two rows of cotton and shaking the vigorously the plants on either side of the cloth to dislodge any insects on the plants onto the cloth. Count the number of immature plant bugs on each sample. Threshold is when you find an average of 3 bugs per 5 row feet (one drop cloth sample).

Adult Tarnished Plant Bug
In our later planted cotton (prior to bloom), we are still looking for adult plant bugs migrating in. To scout for adults, the most efficient methods are to use a sweep-net and/or monitor square retention. To monitor square retention, look for the presence (or absence) of first position squares on the upper 2 or 3 nodes of the canopy. Threshold is when you find an average of 2 adults per 25 sweeps OR if square retention falls below 80%.  One thing to keep in mind when dealing with adult plant bugs is that NO product will provide much, if any, residual control. Adults may continue to infest fields after sprays, so if checking a field 7 days after an application, it may be possible to find the same number – or more – adult plant bugs than the previous week. This does not necessarily mean a control failure, just that more adults have moved into the field. As long as we are setting squares and square retention is above 80% then we know our application did its job.

Depending on the targeted populations, we have several options for control of plant bugs. When going after adults in pre-bloom cotton, the neonicotinoids provide adequate control of both plant bugs and aphids (e.g. imidacloprid, Centric). When primarily dealing with immature plant bugs in blooming cotton, we tend to shift away from the neonics to other chemistries. The insect growth regulator, Diamond, is a good option that provides 2-3 weeks residual control when used at a rate of 6-9 oz, respectively. Diamond should be combined with a knockdown insecticide such as Transform, acephate, Bidrin or pyrethroids (depending on resistance levels in your area) to provide more immediate control.

Stink Bugs

Smaller boll damaged
by stink bugs
As some of our most mature fields are at the first couple of weeks of bloom, it is time to start thinking about stink bugs as well. We know that stink bugs are seed feeders and prefer bolls that are around 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. Don’t slack up on 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 we have to sample for internal boll damage by cracking bolls open and looking for signs of stink bug feeding (warts, pin-prick marks, stained lint). The threshold during the first two weeks of bloom is 30-50% internal damage and 10% for weeks 3-6. In a “normal” year, we don’t worry as much about damage during the first 2 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, we have 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 (50 feet) and smaller fields (20 acres) more heavily so keep that in mind when scouting.

Aphids and Spider Mites

We have 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 we are dealing with from now on. We have also gotten reports of spider mites in fields in the east central and southeast areas of the state. For now, we are advising folks to watch the populations and wait until the forecast shows a several of hot, dry days in a row before spraying. We want 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 1st true leaf to 3rd week of bloom) across the state (and within an individual farm in some cases) we are going to have to do 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. We will continue to put out information through twitter, this newsletter and the Alabama Crops Report Newsletter, Podcast and on the Pest Patrol Hotline. As always, if we can ever be of any help please don’t hesitate to let reach out.