Silverleaf Whitefly Control in Cotton

The silverleaf whitefly (SLWF) is a devastating economic pest of cotton. A multi-county area surrounding Tifton, GA was heavily infested in 2016. In 2017, this infested area has spread over much of the 1.3 million acres of cotton planted in Georgia. In early to mid August of 2017, these SLWF infestations have spread throughout the Wiregrass area of southeastern Alabama.

The SLWF was first observed on cotton in Alabama in Mobile county in 1997. This pest has historically been associated with the more arid regions of cotton production such as California, Arizona, and the Rio Grande Valley of southern Texas. Major outbreaks occurred in those areas in 1992 and 1993.

SLWF has sucking mouthparts and their feeding is similar to aphids. This feeding will stunt plants and reduce their vigor. However, a more serious problem is their secretion of honeydew, which falls in lower parts of the plant. This would be lower leaves during the growing season but open bolls as the plants mature. A sooty mold grows in this honeydew, which will reduce the quality of the lint once cotton begins to open.

Heavy infestations of SLWF can cause premature defoliation. SLWF are not known to die off from a naturally occuring fungus like aphids. SLWF populations continue to increase until cotton is defoliated or until the leaves drop from SLWF feeding.

The first sign of a SLWF infestation will be the presence of whiteflies clustering around the plant terminal or underneath the terminal leaves. The adults will fly when disturbed, Population increases will be observed about two plus weeks after the first presence of adults. SLWF adults deposit eggs underneath the leaves. These eggs hatch into a crawler stage which finds a place under the leaf to begin feeding. This immature stage is then immobile until it develops into an adult. The total life cycle of the SLWF is 15-18 days, depending on the temperature.

Treatment decisions for SLWF can be made by examining for the presence or absence of immatures on the 5th main stem leaf below the terminal. Controls are recommended when 50% of the plants have immatures on the lower surface of this leaf. Immatures will appear oval, flattened, and yellowish in color. They can be separated from aphids by their flattened shape and the absence of appendages and movement.

Points to remember about SLWF as expressed by Dr. Phillip Roberts, Extension entomologist, UGA: Do not overreact but be careful to not underreact to this pest. You cannot get behind with controls and ever catch up. Try not to do anything to make the situation worse. Do not treat for other pests unless necessary. When controlling other pests, use the most selective insecticides on beneficial insects. Cotton is not safe from SLWD damage until the day it is defoliated for harvest. Growers should try to get to defoliation time with green leaves in the plant terminal with no honeydew present.

Rainfall may reduce the number of adults but will have no effect on the immature stage underneath leaves. The damage potential from the SLWF is greatest on late planted cotton (late May-June). This pest prefers hairy leaf or semi-hairy leaf varieties over smooth leaf ones. Hot and dry conditions are favorable for more rapid SLWF reproduction.

Controlling SLWF in 2017 will be very expensive and challenging due to the unavailability or short supply of most recommended controls. The most effective control can be achieved with the insect growth regulator (IGR) type insecticides Knack or Courier. Both work on the immature stage. Their activity is slow but they have long residual. It is advisable to wait 10-14 days after treatment before making opinions about benefits.

Other products that have activity on the SLWF are acetamiprid (Assail/Intruder), Venom, and Sivanto. Centric at high label rates will suppress the adult stage.

Late July/Early August Cotton Insect Situation in Alabama

What are we currently finding in Alabama cotton fields? Plant bugs, both the tarnished and clouded species, have finally reached threshold or treatment levels in April planted fields that have not been sprayed. Some level of brown stink bugs can also be found in these same fields. A bug clean up spray would be advised for most cotton that is in the fourth or fifth week of bloom. Peak numbers of squares and bolls are currently at risk. Cotton planted after about May 10 missed most of the plant bugs this season. In central and south Alabama, there are very few plant bugs currently present in wild host or other crops. In other words, there are no more plant bugs in the landscape to migrate to cotton in 2017 in that region.

Between July 24 and 28, numerous field people from all areas of the state were contacted about escape bollworms on cotton with caterpillar technology. As of July 31, no none has reported any problems. We have conventional cotton on research stations in several areas of the state. Some level of bollworms and damage can be found on this cotton. The corn earworm flight from corn began about July 15 and has likely already peaked. Tobacco budworms will enter the mix during the month of August, but they will not be part of the "escape" situation in August. However, fieldmen should continue to monitor closely for escape bollworms. In Alabama, I would suggest we try the pyrethroid chemistry first if escapes are found.

Spider mites came into the picture about the third week of July in the Tenn. Valley area. Mites will likely show up in other fields statewide if we ever experience a 7-10 day period between rain events. Abamectin will provide the most economical control of mites. Whiteflies may occur in late season on our late maturing fields. If whiteflies are observed, I would recommend we use pyrethroid chemistry for stink bugs since Bidrin seems to aggravate the whitefly situation.

We will likely see stink bug numbers increase as we move into August and even September in our late maturing cotton. The southern green stink bug will make up a greater part of the population in coming weeks. This gives us the flexibility to choose either pyrethroid or phosphate chemistry. For weeks 3 through 6~7 of bloom, we recommend a 10% internal damage boll threshold for stink bugs.

Thus far in 2017, insects have not been a major limiting factor in Ala cotton production. Let's hope we can keep this trend going for several more weeks.

Mid-Late July Outlook for Cotton Insects

Tarnished plant bugs (TPB) have been hatching into the immature nymphal stage for about two weeks in April planted cotton. A few immatures are almost to the adult stage. In fact, the first 2017 cotton reared adults were observed in central Alabama today, July 17. Numerous more immature TPBs are still hatching from the egg stage. The point to remember is that the large immatures and adults do most of the fruit damage to cotton. In unsprayed fields, a second generation will begin to develop soon. The first generation was not at threshold or treatment levels in most fields. However, eventually you may have damaging or treatable numbers as the population continues to increase. At this point, you will have an embedded population that contains all stages from eggs to adults. When this situation develops, one insecticide application will not clean up a damaging level. There may even be a place in 2017 for a late July-early August application of the IGR Diamond added to an adulticide.

2017 is going to be different than recent years in controlling TPBs. Both 2015 and 2016 were hot and dry during July, and the plant bug population crashed on their own. Cloudy weather, abundant rainfall, and lush rapid growing cotton is a much better host for TPBs in 2017, resulting in a high level of survival this season. The numbers of TPBs will likely continue to increase until controls are applied. A sub-threshold level in July will lead to numbers and damage well above threshold by early to mid August if left untreated. A high number of TPBs of all ages is difficult to clean up. Don't blame the chemical or consultant or Extension agent when one application does not do the job. Other than Diamond on immatures, no product will give more than 3-5 days of plant bug control.

Part of the current population of plant bugs in central Alabama and as far north as Talladega is the clouded plant bug species (CPB). Both immatures and adults look different than tarnished plant bugs. Immature CPBs are not as green as TPBs. CPBs are more cylindrical in shape and have alternating dark and light bands on their antennae. Damage from both species are the same, and the same chemicals should work on both.


Attached is a picture of a clouded plant bug nymph.

Rambling Thoughts About the 2017 Weather and Tarnished Plant Bugs in Cotton

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.

Thrips and Grasshoppers Need Attention

Early season cotton seedling insect activity is heating up.  Both grasshoppers and thrips need attention this week (May 8-12).  Cool (cold) nights last week slowed down the growth of cotton seedling on our earliest planted cotton.  This will serve to keep cotton in the thrips susceptible stage longer.  At the same time, where we have had multiple rain events, we likely have lost some of our at-plant thrips protection chemicals to leaching.  This would be true of both seed treatments and in-furrow sprays.  Cotton planted on or after May 10^th should have reduced thrips pressure and less need of a foliar spray.

Cotton that is emerging in the next several weeks will be at some level of risk from grasshopper feeding.  Immature grasshoppers are abundant in fields statewide from the Florida line in the South to Tennessee line in the North.  These immature, up to thousands per acre, will jump when disturbed but cannot yet fly.  A few grasshoppers have already spent 30-50 days as immatures and are already in the adult stage.  The most susceptible stage of cotton to grasshopper damage is the “crook” stage just as it emerges.  Damage at this stage kills the plant leading to reduction in stands.  Seed are too expensive to plant a few extra for these grasshoppers.  This is the reason I would suggest that growers treat on damage potential or risk, and not on thresholds.  In fact, it would be difficult to establish a threshold since grasshopper feeding on cotton is unpredictable.  Immature grasshoppers can be controlled with low rates of several cotton insecticides.  These include acephate, Bidrin, Lorsban, pyrethroids and others.  A couple of ounces of Dimilin added to the spray would add to the residual control of late emerging or migrating grasshoppers.  Immature grasshoppers require 30-50 days to reach the adult stage.  Development may be somewhat quicker with warmer weather which we incurred in March and April.  Adult grasshoppers are very difficult to control.  Even the highest labelled rates of insecticides often do not give 100% control.  If growers observe a need for grasshopper control to minimize stand loss risk, sooner is better than later to apply these controls.

Times of Greatest Importance for Cotton Insects in Alabama

THRIPS             Thrips may delay maturity if heavy damage occurs at the 1 to 5th

                           true leaf stage.

 

Use seed treatments or insecticide applied in-furrow at planting.

 

Make foliar spray when the first true leaf is just emerging if cotton is planted before

May 10 and the minimum nighttime temperature is below 65⁰ F.

 

 

 

PLANT BUGS   Monitor cotton for pinhead square set when squaring begins (about

                          6th true leaf).

 

Continue checking pinhead square set until about first bloom. During this same time period use sweep net (can be purchased for about $75 from Gempler’s ,  www.Gempler’s.com) to sample for adults. Spray for adults if 5-8 per 100 row feet (33 sweeps) are found.

 

 

 

PLANT BUGS    After first bloom check for immature plant bugs (small, green with long antenna) with a drop cloth spread between two rows. Beat plants briskly to dislodge immatures from inside of the square bracts. Spray at threshold of one immature per row foot.

Continue surveys for about 3-4 weeks.

 

APHIDS             Only treat when over 50% of the plants have aphid clustering under leaves, honeydew is present, and cotton is under drought stress.

 

ESCAPE

BOLLWORMS  For about 10-14 days, beginning July 20 in Central Alabama monitor squares and white blooms for small bollworm larvae.  Move this date 10 days earlier for South Alabama and 10 days later for North Alabama.  If the field has not been sprayed multiple times for plant bugs, fire ants will also be searching white blooms for these small larvae. Spray pyrethroid at a high labelled rate if 5 to 10 or more larvae about quarter inch in

length are found per 100 plants.  You may look at fewer than 100 plants but express the number on a 100 percent basis. Tobacco budworms do not escape genetic cotton, therefore, even large bollworms can be controlled unless they are already imbedded in a boll.

 

 

STINK BUGS     Begin looking for stinkbug damage on field borders (first 5-10 rows) about the third week of bloom. This would be about July 20 in an average plant date year. Pull 10-25 bolls that are the size of a quarter in diameter and still soft to the touch. This boll would be about 10-12 days old. If you find internal damage on the border, search further into the field and collect more bolls.  Treat border and/or the entire field when 10% of these bolls have internal damage from stinkbugs. Damage will appear as warts in the boll wall or dark areas around the seed.  Continue scouting for stinkbugs until the top bolls you hope to harvest are about 25 days old and hard. Scout for 5-6 weeks beginning the third week of bloom.

 

SPIDER MITES  Observe for the presence of spider mites underneath leaves. Mites will move when the leaf is pulled, inverted, and the mites are exposed to the sun. Treat when mites are detected over much of the field and the weather outlook for the next 7-10 days is hot and dry.  Mites do not reproduce or spread as fast when weather conditions are not favorable.  Rainfall does not kill mites, they just do not reproduce ad spread as rapidly.

Update for the Week of August 4, 2014

The most significant insect event in the past week was the reports of plant bug tolerance to all labeled or recommended insecticides in the Tennessee Valley of North Alabama. Several of our most experienced consultants are reporting that all classes of chemistry are suppressing plant bugs but their presence and damage behind applications are still above threshold levels. This has required repeated sprays and now tank mixtures with two recommended insecticides. Under these conditions, input costs for cotton insects, particularly plant bugs, make cotton an unattractive crop option going into the 2015 season. Some areas of the mid-south have been dealing with this problem for several years. At this point, the Alabama situation appears to be confined to the Tennessee Valley area.

It has been my experience that once plant bugs get embedded in rank cotton in July, they have always been difficult to control, requiring multiple applications. So now, we have insecticide tolerance in addition to a coverage issue. Looking ahead I see a larger role for the IGR Diamond during the early bloom period in future years.

Now to a quick oversight of other insect problems on cotton and other crops; stink bugs, primarily brown, are requiring controls in some cotton fields. However, overall they are not as heavy and widespread as expected. Aphids are showing back up in some fields requiring controls. Spider mites are present in many fields. It seems like we get more calls about mite control following weeks of high temperature and dry conditions.

In soybeans we are beginning to see loopers as far north as Montgomery in older beans. So far they are only at about a third of a threshold level. Scouts are reporting an occasional corn earworm and velvetbean caterpillar in the soybean mix. The same holds true for peanuts.

Fall armyworms (grass strain) are on about every crop except cotton now. Hay, grasses, peanuts, soybeans are all experiencing the need for repeated applications for FAW. We could safely say that FAW’s have about worn out their welcome, even though they are easy to control. It seems like their preferred row crop is soybeans planted in wheat stubble. Some fields are now on their third infestation and replanting has been required due to near 100% defoliation.

In many ways, 2014 has been like “old times” to Extension entomologists with numerous phone calls about when to treat and what chemical to choose. We don’t feel like step-children to the weed resistance and disease issues this season.

Scouting crops is the first step in all insect management and control programs.