Cotton Insect Control for Remainder of 2019 Season

In most seasons, we would be entering the home stretch for cotton insect control by early August. While this may be true for fields planted on time in 2019, it is not the case for a lot of late planted, late emerging, late maturing fields this season. I hope we can narrow our focus to just a few species of insects for the remainder of this season. The primary group would be the bug complex (plant bugs and stink bugs) and escape bollworms on 2 gene cotton. However, there are several other insects that could arise if weather conditions or other circumstances permit. These are spider mites, late season aphids or even silverleaf whiteflies.

The insects of focus for the remainder of the 2019 season may not be as concerning as how long our management and control programs should extend into September and even October on our later maturing cotton. Let’s look at some general guidelines. If past seasons give us any trends, our escape bollworm issues should end by Labor Day. However, we should continue our monitoring as long as our late maturing fields have squares in the top of plants that would serve as a food source for a one-day old bollworm. Once all the squares are gone, escape bollworms will have a difficult time becoming established. The bug complex should eventually be dominated by stink bugs here in Alabama. This may consist of several species; the brown, southern green and the brown marmorated (BMSB), which can now be found in many cotton growing counties. How long should we continue stink bug controls on late maturing cotton? Our general rule with our traditional stink bug species is to continue controls until the top bolls we hope to harvest are about 25 days old. When the BMSB is in the mix, we may need to protect even longer since this species will attack bolls from thumb nail size, up until they begin cracking. With our late maturing fields this season, we will need stink bug controls through at least the month of October. An application for stink bugs usually gives us 10-14 days of boll protection. However, as some fields mature out, just like with corn, stink bugs will move to younger cotton, or swag areas of fields that are still producing bolls or to late maturing soybeans. In other words, as our crops mature in September and October we will get field to field and crop to crop movement of stink bugs. As far as insecticide choices—Bidrin, bifenthrin or any other pyrethroid at a mild to high labelled rate should give adequate control. The best way to scout for stink bugs will be to examine bolls for internal injury. Just observing or using a sweep net or drop cloth for stink bugs is not very effective and often leads to underestimating the number present.

Red-Banded Stink Bug Found in Central Alabama

Following a mild winter, red-banded stink bugs (RBSB) are a soybean pest that we should always have on our mind. Last Tuesday, 7/16/19, we found treatment level of adult RBSB at the Prattville experiment station. We found these in relatively early planted soybeans at R4/R5, and although we don’t have very large plots, we were exceeding 5 RBSB on 10 sweeps. That works out to about 3 times threshold and is a cause for concern for soybean growers in Alabama. This is a pest that we haven’t dealt with as much as the mid-south growers in Arkansas, Louisiana, and Mississippi, but they’ve put out some excellent info for control tactics that we can draw from.

RBSB’s are a tropical insect and behave and look different in many ways to the native stink bug pests we often deal with in soybeans, and the control tactics differ as well. RBSB are harder to capture with sweep net samples and time of day can have a large impact on the number you catch in the same field. In our plots in Prattville we caught 3 times threshold on 7/16 and 7/22 on warm mornings when the dew was still wetting the plants, but caught 0 on 7/18 during a hot mid-day. These stink bugs will drop off the plant very quickly when disturbed, and it may be hard to sample them in the heat of the day when they can evade capture or even on very cool mornings when they may be seeking shelter from the cooler temperatures.

RBSB’s should be controlled when populations average 4 adults or nymphs per 25 sweeps through R6.5, 10 adults or nymphs from R6.5 to R7, and should only be controlled after R7 if weather conditions are unfavorable. Cool and wet conditions can make mature soybeans more susceptible to feeding from RBSB, but if conditions will be favorable, no control is necessary after R7. RBSB are harder to control than our normal soybean stink bug complex and generally require a tank-mix to get adequate control and residual to keep them knocked back. Bifenthrin at 5.8–6.4oz with a half-pound of acephate is one of the most reliable control options. Bifenthrin can also be tank-mixed with imidacloprid at 3oz or Belay at 4oz and provide reliable control. Endigo at 4–4.5oz is also a good control option, but may not be available for many producers.

Aaron Cato, Post-Doctoral Fellow in Entomology
AJC0108@auburn.edu
479-249-7352

If you have any questions or concerns, please contact Ron Smith at 334-332-9501 or Smithrh@auburn.edu.



Adults are identified by their cigar shape when compared to normal stink bugs, the red band behind their head, and the clear/white spine extended forward between the legs as picture on the left.

Current Alabama Cotton Insect Situation

During the past 7 days, I have traveled from Auburn south thru Troy, Elba and Samson; west to Tallassee, Prattville and Autaugaville; north to Belle Mina in the Tennessee Valley and northeast to Centre and Gadsden. We have some cotton fields that are growing, beginning to bloom and look promising.  However, we have many fields that are late(about 2-3 weeks behind where we would like to be maturity wise), with very poor stands or either with cotton in the same row 2-3 weeks different in maturity. These fields have a long way to go to produce a decent yield. Furthermore, they are going to be difficult to manage from many aspects: weed, PGR and insect management, defoliation and harvest. No two fields on many farms will need the same input at the same time. I am sorry to say that we are going to have to make this crop one field and one week at a time.

Insect wise, we still have plants with heavy thrips damage that may be as mature as the 9th node. However, these plants are growing and I would suggest we focus on other pests now, such as plant bugs. We have anywhere from about a half treatment level to more than a threshold of adult plant bugs in many fields. Many of these fields are setting less than 80% of the fruiting sites. However, after doing some plant mapping and hands on observations last week, I do not believe all of this square loss is from plant bugs. I believe some of these fruiting sites were lost due to the weather and physiological shed. However, we know that these plant bugs are depositing eggs that will produce immature plant bugs in a couple of weeks. Therefore, I would treat fields that have one-half or more of the threshold level of plant bugs and less than 80% square set. Our threshold for adult plant bugs in 8/100 sweeps or 2/25 sweeps. If no sweep net is available, treat with the pinhead square set is less than about 80%.

We have several chemical choices or combinations to choose from. The presence of aphids, which are beginning to appear in some fields, may dictate the chemical choices. Centric is a good choice for both tarnished plant bug(tpb) and aphids. An option would be bifenthrin and imidacloprid(Admire Pro). After first bloom, Bidrin comes back as an option for adult plant bugs and can also be tank mixed with imidacloprid. After first bloom, we may also begin to see immature plant bugs, that have hatched in the field. In fact, we have already observed immature plant bugs in some of our oldest cotton. Under high tpb pressure, Diamond is an excellent tool as a tank mix with products we have mentioned already for adults. Diamond at 6-9 g/acre will give 2-3 weeks control of the hatching tpb nymphs.

Several fieldmen have now reported clouded plant bugs in the mix. Both brown and green stink bugs are also present in many fields, along with the plant bugs. Stink bugs prefer bolls about 10 days old, but when they are not available they will feed on small bolls as soon as the bloom drops off. Stink bugs will need to be controlled along with plant bugs once cotton begins to bloom. Stink bugs do not feed on squares.

Spider mites are also present in many fields at low levels. As long as we are in the thunderstorm weather pattern, I would not worry about spider mites, but instead focus on plant bugs and stink bugs. Remember that no two fields may need the same management or inputs at the same time.

Cotton Insect Management Tips for 2019

Cotton insect management is constantly evolving or changing. It may be in the area of: genetic trait or insecticide resistance; new economic insects such as the silverleaf whitefly or the brown marmorated stink bug; the introduction of new chemistry or new traits such as the lygus-thrips trait that will likely be available about 2021 or 2022.

Our current production system in Alabama is good at present as it relates to insects. This is evidenced by the good to great yields we have produced (when not impacted by tropical storms) in recent years. We do not need to make major changes in our production system based on 2018. Let's don't let one weather related disaster change the good job growers are currently doing. Maybe tweak a few points. One of those being a major focus on earliness. Earliness would be beneficial in a wet harvest season, help in reducing whitefly damage in southeast Alabama and minimize the new cottonleaf dwarf virus disease concentrated  in southwest Alabama.

In this article I want to point out where we are with the management and control of some of our major insect pests and highlight some things we need to be aware of in 2019 and the next few years.

Beginning with thrips, where do we stand? We have documented resistance to both seed treatments - thiamethoxam (Avicta) and imidacloprid (Aeris). Resistance to thiamethoxam is higher and more widespread throughout most of the cotton belt. Imidacloprid has proven more consistent in our Alabama research trials. However, the resistance level of either chemistry did not seem to increase in 2018. This may have been due to a light and late thrips year. The peak movement of thrips from wild hosts to cotton in 2018 did not occur until about mid-late May, when early planted cotton was beyond the 5th true leaf stage.

Planting date and weather are the two most important factors influencing cotton thrips injury levels. Rainfall influences when thrips move from wild hosts to our crops. The more abundant the spring rainfall, the later the movement of thrips. The warmer the nights, the faster the cotton will grow up. Cotton is only susceptible to thrips injury from the first true leaf to about the fifth true leaf. The faster cotton is growing, the shorter the thrips susceptible window is in days.

We can use planting date, rainfall and temperature to determine when we need to help our seed treatments with a supplemental foliar spray for thrips control. In fact, a model has been developed by entomology researchers at North Carolina State that will project the level of thrips pressure to expect, based on your planting date and weather records from your nearest weather station. This model was developed for the entire southeastern cotton growing area, by utilizing thrips data generated on University research stations from throughout the southeast. This model can be accessed and utilized by growers, consultants, and fieldmen by going to http://www.climate.ncsu.edu/CottonTIP.

The second insect I wish to discuss is aphids. I am including an aphid discussion for the 2019 season because of their potential relationship to the new virus disease known as the cotton leafroll dwarf virus (CLRDV). Our usual management advice for aphids is to just wait for the naturally occurring fungus to take out the population. In the past we have recommended to control aphids with a foliar insecticide only when cotton was under extreme drought distress. In recent years we have also added a second exception for controlling aphids. It is advisable to control aphids when they are heavy and a bollworm moth flight is expected on 2 gene cotton varieties. Aphids and their resulting honeydew make it impossible for scouts and fieldmen to detect bollworm eggs or small larvae among the aphids and honeydew. Foliar sprays to control aphids would need to be done in advance of the expected moth flight.

Aphids begin to appear in June. First on select plants, maybe less than 10%, in early square cotton. The population will usually peak and die off from the natural fungus disease about mid July. The slower the population takes to build up, the longer it usually takes to crash or die off. This natural fungus usually takes about 10-14 days later than desired. The moth flight of corn earworms/cotton bollworms coming from corn depends on the maturity of corn, but usually occurs in Alabama between about July 10 in south Alabama to August 1 in the northern counties.

Aphids are vectors for the CLRDV disease that was found in several Alabama counties in 2017 and 2018. However, experts tell us that the disease can be transmitted in as little as 40 seconds. Therefore, aphid control will not eliminate this disease. In other parts of the world where the disease occurs it has been controlled primarily by developing resistant germoplasm.

Aphids may rebound in cotton in late season. This late season population is not as prone to die off from the fungal disease. Foliar sprays may be warranted if lower bolls are opening and are susceptible to being covered by honeydew which allows sooty mold to develop on the open lint. The late season aphid population could be more closely involved with the disease transmission since the disease seems to be much worse on late planted cotton.

In a follow up blog, if time permits, we will discuss management ideas for plant bugs, escape bollworms on 2 gene cotton, and stink bugs, we will mention the silverleaf whitefly (SLWF) but hopefully we will not see the pressure from this pest in the southeastern counties as we did in 2017.