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.

Update for the Week of September 9

As we observed and stated last week, soybeans in Alabama are experiencing the most widespread outbreak of foliage feeding insects of the past 41 seasons. This outbreak has been going on for about three weeks or more. However, some growers are just now discovering the situation. This has led to the most calls and questions that Dr. Tim Reed and I have ever received during one week. I will attempt to summarize the questions and what our responses have been.

Most calls begin with – “I have worms in my soybeans, when do I need to treat and what insecticide do I need to use?”

First of all, in most all fields we are dealing with a mix of soybean loopers (SL), velvetbean caterpillars (VBC), and green cloverworms (GCW). Loopers are statewide, VBC are a significant part of the mix in south and central Alabama while GCW are a large part of the mix in northern areas of the state. Loopers are resistant to pyrethroids, Lannate and all of the older chemistry. Since they are part of the mix statewide, we must go to the newer chemistry (in alphabetical order): Belt 3 oz.; Blackhawk (Tracer) 2 oz.; Intrepid 4-8 oz.; or Steward 7 oz. Prices for Belt, Blackhawk and Steward will be somewhere between $15 and $20 per acre. Intrepid will range from about $7 to $14, depending on the rate used. The 4 oz. rate has been what most growers have chosen due to economics. However, where significant foliage loss has already occurred and worm numbers are high, this 4 oz. rate is not adequate. At least 5 or 6 oz. of Intrepid will be needed.

In addition to worms, most all soybean fields have some level of stink bugs present. None of the worm materials will control stink bugs. Therefore, a pyrethroid or acephate (Orthene or generic) at 0.75 lb./ai must be added. This rate of acephate makes pyrethroids the most economic choice. The majority of our stink bugs are the green or southern green species, which pyrethroids control very well.

Now back to what a threshold is on foliage loss. Depending on the stage of beans, our threshold is 20-30% foliage loss. That does not mean that we wait for that level before treating. We treat in advance when we find worm numbers that will give that level of loss. Also, we must allow 4-5 days for products like Intrepid to control the worms. However, foliage consumption will normally end much sooner. Big factors, which are seldom mentioned in discussing foliage loss thresholds in soybeans, are the amount of moisture available to the crop and the reproductive stage of the beans. Soybeans can compensate at a much higher level when soil moisture is available and we have that statewide at present. Also, soybeans will replace lost foliage when they are still in the blooming stage.

How many worms does it take to cause 20-30% foliage loss? In traditional row beans, where a drop cloth can be used, somewhere between 4 and 8 larvae per row foot will give 20-30% foliage loss. The size of the beans is also a variable here. Due to the good moisture in August, many of our beans are waist high and have lots of foliage. Many of our beans in 2012 are drilled. Therefore, a sweep net is needed to get a measure of worm numbers. I suggest taking 10 sweep samples. Some states recommend treatment when about one larva per sweep is found. I have found in 2012 that it takes about 3 or 4 worms per sweep to cause 20-30% foliage loss. Another factor to consider is how long the worm infestation has been ongoing. In fields I have monitored, the answer is three or more weeks. New worms just keep hatching, so where a subthreshold has been present for 15-20 days, much of the lower canopy has already been consumed by the worms. In this situation a 10% foliage loss can turn into a 30-40% loss in 7 days.

In summary, I feel that our Extension educational program on soybean insects has been very inadequate and we promise to do better beginning with winter meetings. However, growers must also do their part in managing insects in soybeans, especially when they are worth $17-18 per bushel. We do not know what level of insect pressure 2013 will present, but if we plan to stay on top of the insect situation, soybeans will need to be monitored by a trained or experienced person at least weekly during the season, just like we have done in cotton for the last 60 years.

Impact of Alabama Cotton Commission Research Support for Cotton Insect Management-30 Year Overview 1986-2011

Thirty years of cotton insect research in Alabama— from boll weevils to boll weevil eradication– to beet armyworms and pyrethroid resistant tobacco budworms– to genetically altered varieties– to boll feeding bugs (stink bugs). On the chemical arsenal side– from Azodrin, EPN + methyl parathion, Lannate, Bolstar, Curacron, Galecron, and Fundal and the original pyrethroids (Ambush, Pounce, and Pydrin) to generics of the third generation pyrethroids.
 
What an evolution and what a ride. Thanks to support by the Alabama Cotton Commission, we have survived and are better for it.

One could argue that the past 30 years have been the “worst of times and the best of times” in cotton insect control. I consider it to be the most exciting and revolutionary period in cotton insect control history. The worst of times was the period of 1987 to 1995 with uncontrollable beet armyworm outbreaks and the development of pyrethroid resistance to the tobacco budworm. The best of times was made possible by the eradication of the boll weevil and the introduction of genetically altered cotton varieties. The “best of the best” times were experienced by many growers in 2011, with one of the statewide highest yielding years in history, the best market prices in my career and the lowest insect loss year in history. Both Alabama and the U.S. reported total losses to insects in 2011 as 2-3%, the lowest ever recorded.
 
Two of the most important events in cotton insect control in the past 100 years happened during the past 30 years: boll weevil eradication and the introduction of genetically altered varieties. In 1996, the first year of Bollgard cotton, Alabama lead the cotton belt by planting 77% of our acreage to the new technology. I feel certain that this set a record in the adoption of new technology. This was the result of two factors, one much more significant than the other. The overriding factor was the pressure and damage by the tobacco budworm in 1994 and 1995. A secondary factor was impacted by funding by the ACC which allowed us to work with the Bt technology for four years before it was introduced. We felt very comfortable in recommended Bollgard varieties to growers based on four years of research experience.
 
Looking back at this 30 year period, we can see where research support by the ACC was involved at every turn and event during this historical period.
 
Let us look back now at some of the major insect events and advancements that happened during the past 30 years and discuss briefly how ACC projects gave answers to the most significant concerns and questions during this era. I will attempt to present these in chronological order.

 

A. Beet Armyworm Outbreaks

• No effective chemistry available
• Parasites and predators, if present, gave effective control-- but not possible during active eradication
• Experimental insecticide, Pirate, highly effective but not approved by the EPA for emergency use until too late (August 1995)
• The original efficacy work on BAW control with Pirate was done at Prattville Experimental Field with ACC support
• Meeting with EPA in Senator Heflin’s office- Washington D.C.

B. Pyrethroid Resistant Tobacco Budworms

•  Pyrethroid Resistant Tobacco Budworms
• No effective chemistry available
• 1994-1995 highest numbers in history, up to 5 larvae per plant
• Yield losses astronomical
• ACC funded research showed that a combination of Lannate + Larvin on 5-7 day schedule gave suppression

C. Transition from In-furrow to Seed Treatments for Thrips Control

• Grower adoption rapid due to convenience and safety
• Initially Cruiser and Gaucho
• Transition to complete-pak with nematode suppression, Avicta and Aeris
• ACC research showed that seed treatments were not as consistent as in-furrow, and often needed a supplemental foliar spray, especially if planting in the early season window. Also, that seed treatments were less effective than Temik for nematode and spider mite suppression

D. Introduction of New Caterpillar Chemistry

• First- Tracer (Spinosad)- soft on beneficials, great on Tobacco Budworm
• Later- Steward (indoxacarb), Prevathon (rynaxypyr)- long residual and rain fastness, Belt (flubendiamide)
• All much more effective on small worms and require good coverage (ground application superior to air)

E. Evolution of Genetically Altered Technology

• Bollgard
• Single gene
• Weak concentration of Bt in blooming zone of plant leading to mid season bollworm escapes under bloom tag
• Benefits of pyrethroids overspray in mid-late July for sub threshold level of multiple pests
• WideStrike
• Two gene
• Second gene effective on fall armywor
• Weak concentration of Bt in terminal (original work Beltwide was at Prattville
• Scouting technique must be different than Bollgar
• Under pressure, WideStrike required overspray with pyrethroid
• Bollgard II
• Two gene
• Reduced bollworm escapes by about 80-90% over Bollgar
• Broaden caterpillar spectrum over single gen
• Second gene has long term benefit in resistance managemen
• No refuge requirement
• Bayer Twin Link
• Two Bt insect genes and
• Caterpillar effectiveness between WideStrike and BGII
• Bollgard III
• Contains third insect gen
• Third gene not Bt so has different mode of actio
• Should provide long term caterpillar resistance managemen
• Lygus Genetics
• Early stages of developmen
• Evaluated 40 lines of genetically altered cotton for lygus in 2011, Prattville

F. New Generations of Pyrethroid Chemistry

• Early 1990’s documented that not all equal for fall armyworm control. Karate, bifenthrin (Capture) superior
• More recently developing data to show that bifenthrin not as effective on bollworm species
• Pyrethroids not effective on the brown stink bug; however, bifenthrin superior to other pyrethroids
• Some generic pyrethroids may not be as effective as the “brand” names

G. Evolution of the Shifting Insect Complex Following BWE and Bt Varieties

• Current low spray environment
• Development of boll feeding bugs and sucking pests (plant bug and stink bug complex, leaf footed bugs)
• Role of IGR Diamond for plant bug management
• Reemergence of spider mites as an economic pest of cotton (influenced by movement from Temik to seed treatments)

H. Stink Bug-Dominant Insect of Alabama Cotton

• Work to develop best scouting technique
• Now sample 10-12 day old bolls for internal injury
• Worked with other southeastern states to develop a “dynamic” threshold for stink bugs. One that considered the number of bolls at risk at a given point in the season

J. Insect Management in Conventional Cotton Systems

• Conducted research on which new insecticides provide the best caterpillar control
• How to best manage beneficials, plant bugs, and early mid-season pests
• Trials indicate that weed control costs similar in both conventional and generic systems
• Tobacco budworms are the budget buster in a conventional system
• Can make only one TBW spray per season with the savings from no insect technology fee
• Discovered role of fire ants in both conventional and generic systems

K. Role of Fire Ants in Cotton

• Are the dominant beneficial in Alabama cotton today
• Not only important in conventional systems but also reduces escapes in Bollgard and WideStrike (Example: Mississippi consultants spray BGII on % egg lay)
• Some chemicals suppress fire ants more than others (Centric, Steward, pyrethroids, imidacloprid)
• Dr. Tim Reed and I conducted a project at Prattville last season (Poster Presentation)
• Results show less boll damage on both conventional and genetic cotton with fire ants in system as opposed to no fire ants

Numbers on the side represent the number of worm damaged bolls per 45 feet.

Impact of Fire Ants On Bollworm
Damage in Alabama Cotton

L. Emergence of Sporatic Pests

• Grasshoppers have increased in importance in reduced tillage environment
• Pose risk to stands
• $$$ greater investment in front end of season production cost now compared to old days
• Immature G.H. easy to control, adults difficult
• Ideal timing is when “burning down” in early spring
• Most all labeled chemicals, including the IGR (Dimlin), work on immature at low labeled rates

M. Study of Bollworm Resistance to Bt

• Lab studies conducted by Dr. Bill Moar, who has since moved onto the Monsanto resistance management team
• His findings indicate that bollworms have the ability to develop resistance to Bt in the lab– but they usually are unfit for survival when they do so.

In Summary
The ACC did not fund all of every project completed over the past 20-30 years. However, your support often made it possible to be in a learning situation on insect outbreaks; new emerging pests/problems and on projects/trials that were not directly funded. Some of you may remember that my grants in earlier years were entitled “To look at New Chemistry and Technology and Determine Where They Best Fit in Alabama Cotton Production”. This allowed us to know more about the strengths and weaknesses of new chemistry/technology than even the companies bringing them to the market. Cotton insect research is unlike most of the other disciplines in that it requires much more hands on involvement. Often times, support from multiple sources had to be pooled to provide the labor and travel to conduct cotton insect trials. Fifteen to 20 trips to a research site are often required between planting and harvest. This is especially true with the genetic technology under development.

A final point I would like to make is that we in Extension have conducted basically all the cotton insect research since the early 1990’s without an Ag Research station counterpart. We feel that we have kept Alabama growers up-to-date and often ahead of the curve in cotton insect management, when compared to other states. Much of the credit goes to the support we have received from the Alabama Cotton Commission.