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Friday, September 7, 2018

Assessing the Value of Bt Corn When Insects Are Resistant - Part I: Corn Rootworm

We are entering a time when insects have become resistant to many of the Bt toxins in our GMO corn and cotton. This is the first in a series of articles where I will discuss what this might mean in terms of crop damage and the value of the Bt technologies. 

To start the series, I want to make it clear that Bt crops are more than just insect protection; they are herbicide tolerance, cold tolerance, and improved genetics for yield, drought and disease tolerance. These other virtues remain even when the insect control fails, although reduced insect control clearly can affect some of them, like yield potential and drought tolerance. 

Back in the early days of Bt crops, seed companies defined and charged a "technology fee" that was added to seed cost; growers knew what they were paying for insect protection. Those days are long gone, for various reasons, and now the seed cost is what it is and there is no way to know how much is being paid for the Bt traits. 

While we don't know the cost of the Bt component in seed, we can approximate the value under significant pest pressure. The situation is perhaps the most straightforward in corn rootworm where the larvae prune roots, reduce water and nutrient uptake, cause plant lodging, reduce plant biomass and direct grain yield and put pollination at risk through silk clipping. 

For corn rootworm, the commonly accepted approximation (in the Midwest) is that for each node of roots pruned there will be a 15% yield loss. Two nodes of root pruning in a 200 bushel field would equate to 60 bushels, or $216 at $3.60 corn. This dollar loss figure does not reflect harvest difficulties due to plant lodging. Direct bushel loss is likely to be significantly higher here on the High Plains where we often grow corn under more severe water deficit. (To my knowledge there are no similar regional studies on silage corn.) 

The photo below was taken this week near Hart, TX and shows two hybrids in the same family from the same seed company. The plant on the left had no Bt toxin effective against corn rootworm. The plant on the right had mCry3a, the toxin we believe is now compromised. The root masses on the mCry3a plants are mostly regrowth; the primary roots were heavily damaged and would have rated about a 2.0 on the formal 0-3 corn rootworm injury scale. The plant without any Bt would rate a 3.0, the maximum damage possible. (Corn in this field had plenty of water to promote root regrowth; it was irrigated with subsurface drip.)

Root masses and ears in corn hybrids that had no corn rootworm Bt (left) and compromised mCry3a Bt (right), Hart, Texas, 9/4/2018.

Clearly the failing Bt toxin provided some value. Had there been some Cry34/35 corn in this trial there would have been corn with relatively little damage and we could assess the yield loss experienced due to mCry3a failure. 

Diminished value is still value, but certainly not full value. Planting mCry3a corn next year in fields where mCry3a failed this year is going to result in significant value reduction, probably even greater than in 2018 because a higher percentage of the population will be resistant next year. In this case, if mCry3a must be planted again, a soil applied insecticide is necessary to provide some protection. The protection will not be as good as when the Bt was working well, but the benefits of the added insecticide will be significant.

Planting a corn hybrid with the Cry34/35 toxin either alone or in combination with mCry3a is a better option because the Cry34/35 is still working and the mCry3a is still kind of working. 

Reduced value in Bt corn with caterpillar control is a bit more complicated and will be addressed in a future edition of this newsletter. 

Wednesday, September 5, 2018

Terms We Will Soon Use in Discussing Resistance to Bt (GM) Crops

It has been quite a challenging summer with our Bt crops on the Texas High Plains. This year we have seen huge numbers of corn rootworms emerging from many fields planted to mCry3a, a corn rootworm Bt toxin that has been used year after year in continuous corn. These fields had significant root pruning caused by rootworm larvae and produced a large number of adults – survivors of the toxin. 

We have also seen corn earworm do significant damage to Bt corn that worked well a few years ago, and we have documented Bt cotton with unexpectedly high levels of bollworm damage and large larvae surviving the Bt toxins. In each case we collected insects from the field and sent them for laboratory assay to determine if the insects were resistant to the Bt toxins in the crops. These assays take time, but it is highly probable that in a few months we will be reporting that resistance has been confirmed in corn rootworm and corn earworm/cotton bollworm. 

To help explain what is going on in nature and in the discussion of resistance, here are some relevant terms that we will be using for the next few years. These are my explanations of what are sometimes complex things, and I in no way represent these as formal definitions.  

Resistance: A genetic shift in a population of insects that renders it significantly less susceptible to a toxin. Exposure to Bt toxins does not cause genetic mutation, but it does allow survival of those few insects that already have the genes to withstand the toxin. Resistance genes are fairly rare in the population when a Bt toxin is first used, but over time and repeated exposure to the toxin the genes become more and more common in the population. Ultimately, the population becomes predominately resistant with few insects being susceptible to the toxin.

Bt Toxins: Proteins toxic to insects that were originally isolated from Bacillus thuringiensis, a bacterium, that are now being produced by Bt (genetically modified) crops. The genes in the bacterium to create these proteins were transferred to the DNA of the crop plant, and the plant now produces the toxins. 

Allele/Gene: A gene is composed of two alleles, one contributed by the male parent and one contributed by the female parent. An analogy would be a zipper; one half (one allele) contributed by the female and one half (the other allele) by the male. An insect that has two alleles for susceptibility is called homozygous susceptible. An insect that has one allele for resistance and one allele for susceptibility is called heterozygous resistant, and an insect that has both alleles coding for resistance is called homozygous resistant. Genes code for various proteins and biological functions in the body. Insects have co-evolved with plants for thousands of years and already have a wide variation in genes to survive toxins present in plants and the environment.

Selection: Application of a mortality factor like a Bt toxin or insecticide. Individual insects without the gene(s) to survive the toxins are selected against (killed) and do not reproduce. Insects with the genes to survive the toxins do live to reproduce, and they pass their resistance genes to the next generation. 

Allele/Gene Frequency: The percentage or proportion of the alleles or genes in a population that are of a particular type. Repeated selection by Bt toxins generation after generation causes the resistance allele frequency to increase each generation, and a larger and larger percentage of the population is not killed by the toxin. The population becomes increasingly heterozygous resistant and homozygous resistant. 

Unexpected Injury (UXI) or Unexpected Damage (UXD): Significant insect injury to a crop that was not anticipated. Industry uses the term Unexpected Damage (UXD), but this is incorrect according to entomological definitions. Unexpected Injury (UXI) is the correct term and the one used by Entomologists. (By definition, injury is the level of harm or destruction to the plant from a pest; damage is the monetary loss to the crop due to insect injury.) Field investigations of UXI will look at a host of factors that might explain the high level of insect damage but, when these are ruled out and the damage is high enough, the field will become a Performance Inquiry.  

Performance Inquiry (PI): A term with regulatory implications that defines UXI that meets or exceeds thresholds of injury and warrants insects to be collected from the field for laboratory analysis and the field failure to be reported to EPA. For caterpillar pests, some seed companies have pre-determined levels of UXI that will trigger a Performance Inquiry and force a collection of insects from the field. Other seed companies do not have any formal definitions of damage that will trigger a PI. There are established PI levels for corn rootworms. Corn with a single Bt toxin must be considered a PI if the root damage exceeds 1.0 on the Iowa 0-3 rootworm damage scale and factors other than potential resistance have been ruled out. Corn with multiple rootworm toxins triggers a PI if the roots exceed 0.5 on the Iowa scale. 

Terms of Registration: An agreement between each seed company and EPA that covers, in part, how and where each Bt toxin will be planted, and what might happen if resistance is found. Refuge configurations (structured blocks, strips or seed blends (refuge in the bag)) are specified in the terms of registration. 

Regulatory Definition of Resistance: From a regulatory standpoint, the word resistance should only be used after a population suspected to be resistant has been collected from the field, reared in a laboratory, and the survival of its offspring shown in a laboratory bioassay to be significantly higher than survival of a colony that is known to be susceptible to the toxin(s). Some bioassays are conducted with artificial insect diet laced with known amounts of a Bt toxin. Other bioassays are conducted on plants expressing Bt toxins. 

Mitigation: An attempt to reduce the damage caused by resistant insects. Mitigation almost always occurs in the year following UXI and is an attempt to avoid yield and financial losses that result from resistance. 

Remediation: An attempt to lower the frequency of resistance alleles in a population of resistant insects. (This has never been done successfully in cases of Bt resistance in North America.)

The best publication to quickly determine the types of Bt in any commercialized corn in the US is the Handy Bt Trait Table by Dr. Chris DiFonzo at Michigan State University: https://www.texasinsects.org/bt-corn-trait-table.html.

Saturday, August 25, 2018

Texas Panhandle Corn Rootworm Probably Resistant to Some Bt Corn

There were a crazy lot of western corn rootworm beetles emerging from continuous corn fields this summer in the Texas Panhandle, and with seed purchase decisions on the horizon it might be a good idea to talk about Bt resistance. 

For two years now, Dr. Ed Bynum, Extension Entomologist in Amarillo, and I have seen extensive damage to Bt corn with the toxin mCry3a. These fields have been found in the zone from Dalhart in the north to Hart in the south. We have been seeing all the textbook symptoms of a failed Bt rootworm toxin: growers using soil applied insecticides in addition to their Bt seed, heavy root damage, plant lodging, lots of adult beetles on the wing because they were not killed as larvae feeding on roots, and aerial spraying of adult beetles to try and kill them before they clip silks or lay eggs that will become rootworm larvae the following year.

This year we received phone calls from consultants around Hart that were alarmed at the huge numbers of beetles and lodged plants in mCry3a corn. Dr. Bynum, Dr. Katelyn Kesheimer and John David Gonzales, IPM Agents in Lubbock/Crosby and Parmer, Bailey and Lamb counties, respectively, and I dug corn roots from several continuous corn fields near Hart. The roots from Cry34/35 corn were fine; only slight damage was observed. Roots from mCry3a corn rated 0.80 to 1.6 on the Iowa State rootworm injury scale. (Yes, we used gene-check strips to make sure the fields were mCry3a.) In practical terms this means the rootworms had removed 0.8 to 1.6 nodes of roots. EPA and the seed companies have negotiated levels of damage that might reasonably indicate there is a resistance problem, and for single toxin Bt corn like mCry3a the level is 1.0 on the Iowa Scale. Single toxin Bt fields with root ratings above 1.0 are supposed to trigger collection of beetles for resistance screening. 

Drs. Bynum, Kesheimer and Suhas Vyavhare (Extension Cotton Entomologist), and John David Gonzales and I collected 1,200 beetles from the mCry3a field mentioned above and sent them to a USDA-ARS lab for resistance screening. This process takes almost a year to complete and we will report the results when they come in. However, what we are seeing in mCry3a fields is highly consistent with what we would expect to see with resistance, so I am going to call it "probable resistance".

There are four Bt rootworm toxins in corn; mCry3a, eCry3.1Ab, Cry3Bb1, and Cry34/35. However, there is known cross resistance between the first three toxins listed above, so rootworm beetles experience these more like two toxins (mCry3a, eCry3.1Ab, Cry3Bb1) and Cry34/35. All four of these toxins have documented resistance, but resistance to the Cry3s is widespread, while resistance to Cry34/35 is only known in localized pockets – for now. 

Corn rootworm has been shown to have the ability to become resistant to single toxins in as few as four seasons when the same toxin is used in consecutive years. Most of the continuous corn fields in the Panhandle have been in Bt corn for far longer than four years, and most of them have been planted to the same toxin(s) in hybrids from the same seed company. 

The following table lists hybrids with only Cry3-type toxins for rootworm control. The toxins for caterpillar control, if any, are irrelevant to corn rootworm and are not listed.  



By far the best way to manage corn rootworms in Texas is to rotate to a non-corn crop; the rootworm larvae will hatch next year and starve because they don’t have an acceptable host. Beetles will not lay an appreciable number of eggs in a non-corn crop, so the field can be planted back to corn after just one year out of corn. 

However, crop rotation is not economically feasible in many circumstances, nor is planting a corn hybrid that does not have Bt for corn rootworm control. With pending seed purchase decisions in mind, in cases of rootworm resistance to a single toxin it is important to rotate to an entirely different toxin the following season. (Remember that mCry3a, eCry3a.1Ab and Cry3Bb1 are not very different as far as corn rootworm is concerned.) As far as we know, there is no resistance in Texas to Cry34/35. Every transgenic seed company has hybrids with Cry34/35, sometimes as the sole rootworm toxin but often in combination with one or more of the three Cry3-type toxins mentioned above. It might be a good time to contact your seed dealer and determine whether there are agronomically acceptable hybrids with Cry34/35 that can replace the hybrids that have only Cry3-type toxins. 

Determining the types of Bt toxins present in hybrids from every seed company is easy to do, just visit the Handy Bt Trait table

Wednesday, July 25, 2018

What's Up With All the Earworms in Bt Corn?

This is a jointly issued article from Extension Entomologists Drs. Pat Porter (Lubbock) and Ed Bynum (Amarillo).

There are large numbers of corn earworm larvae in Bt corn ears in the Texas Panhandle, and some people are spraying in an attempt to control them. 

The first question we are getting is what to spray, but the better question would be whether to spray. Corn earworm is usually an ear tip feeder, and on its own is seldom an economic pest of corn. However, this year we are seeing earworms doing more than just tip damage, and feeding lower in the ear. The reason(s) for this change in behavior are unknown. 

We have not forgotten last year though, when there was good evidence that just a little ear tip damage resulted in higher fumonisin levels. This could happen again this year, especially if the weather in August and September turns off wet and relatively cool. However, based on what we saw last year, the little bit of tip damage needed to promote fumonisin has already been done this year, so the contribution of ear damage to in-season fumonisin levels will depend largely on the weather between now and harvest.

Corn earworm eggs are laid on silks, and the small larvae feed on silks as they move toward the tip of the ear. Silks continue to grow for several days, so new growth will not have insecticide residue on it. That is in part why sweet corn growers often spray on a three-day schedule. Once inside an ear the larvae cannot be reached with insecticides. They only leave the ear when fully grown as they move toward the soil surface to tunnel in for pupation, but by this time all of the damage is done. And these fully mature larvae don’t feed much, so they won’t get an insecticide dose except by direct contact. 

There is very little benefit to trying to kill them while they are in the ear and after they finish doing damage. Corn earworm control for next year is not like western corn rootworm control. Western corn rootworm has one generation per year, and the beetles emerging in a field tend to stay in that field to lay eggs in the soil. The theory of spraying rootworm adults to control next year’s root feeding is that if most of the beetles can be killed before they lay eggs, the number of rootworm larvae the next year will be reduced. Corn earworm, however, is highly mobile has several generations, and multiple hosts to develop on. The immigrant moths come up from the south in spring and early summer, and immigration continues throughout the summer as locally reproducing populations increase as well. Corn earworm does not overwinter on the northern High Plains, so killing larvae as they leave the ear will have no effect on next year’s population. It might slightly reduce the number of moths in the next generation this year that will lay eggs in cotton (see below), but only slightly, and at significant expense. 

Western bean cutworm complicates things, but its management should follow established procedures and guidelines presented on page 17 here. Western bean cutworm has been shown to be resistant to Cry1F, the only older Bt toxin that provided any significant control or suppression. Cry1F Bt corn should be scouted and treated as if it was non-Bt corn. 

The second question we are getting is why all of the earworms? One answer would be that they have become resistant to most of the toxins in Bt corn. Dr. David Kerns, IPM Coordinator at College Station, has been running resistance assays on field collected populations of corn earworm (cotton bollworm) collected in central Texas and has reported significant levels of resistance to the once-partially-effective toxins Cry1Ab and Cry2Ab2. Cry1F never was very effective, and Cry1A.105 is a synthesized version of these toxins. 

The bollworm/earworm populations we have on the High Plains are comprised largely of moths that flew in from south and central Texas, so it would be no surprise to find they carry alleles (genes) for resistance to the older Bt toxins. Dr. Ed Bynum, Extension Entomologist in Amarillo, will collect larvae from a Bt corn field with high numbers of larvae in the ears tomorrow and send these to Dr. Kerns for laboratory screening for resistance. John David Gonzales, Extension Agent IPM in Parmer, Bailey and Lamb counties, sent a collection of 250 larvae from a Bt problem field to Dr. Kerns this week. The laboratory screening will take a couple of months to complete and the findings will be reported here. 

The third question we are being asked is what does this mean for Bt cotton which shares many of the same or similar toxins found in Bt corn.The answer is that we don’t know for sure, but we can pose a couple of scenarios.  

Corn earworm (cotton bollworm) greatly prefers to lay eggs in corn rather than cotton or sorghum. When given the choice between only cotton and sorghum, it tends to prefer sorghum more. But we have relatively little sorghum in the system this year. In areas where the corn planting is spread out over time, moths emerging from earlier planted corn will seek to lay their eggs in later planted corn that is silking or somewhat past green silk. This scenario played out last year on the High Plains, and the later corn intercepted a lot of the egg laying that would otherwise have gone to cotton if the later corn was not around. 

However, where much of the local corn crop is planted relatively early, there may not be later corn and sorghum available to draw the moths away from cotton. It is certainly possible in this situation that the moths emerging from corn will lay a high percentage of their eggs in cotton. If there is resistance to the Bt toxins in corn, the resulting larvae will be resistant to most of the Bt toxins in cotton. 

The one Bt toxin that is still highly effective on corn earworm (cotton bollworm) and western bean cutworm is Vip3a. Corn with Vip3a will not produce many moths at all, and cotton with Vip3a will not have much damage. The “Handy Bt Trait Table for US Corn Production” makes it simple to determine the Bt toxins in any commercialized corn. 




Thursday, July 19, 2018

Bollworm situation: West Texas cotton


I am glad folks are paying attention to what is happening in other areas. I had a few calls regarding bollworms this week--mostly in response to the hubbub in Central and South Texas. So far, worm pressure in cotton have remained light for the most part with some areas (e.g. parts of Swisher, Floyd, and Hale counties) just starting to show increase in numbers.

Our Bt sentinel research plots at Texas A&M AgriLife Research and Extension center in Lubbock are showing a few worms in non-Bt plots—no activity has been spotted in any of the Bt technology plots so far. As the season progresses, we may see increase in moth flights and the egg lay.

I wouldn’t spray for bollworm egg lay in West Texas cotton as very few eggs make it to worms (thanks to beneficials!). Treatment decisions should be made based on the amount of fruit injury and the presence of live worms in both Bt or non-Bt fields. Since newly hatched larvae must feed on the plant for the Bt toxin to be effective, delay decision making until you can determine the survivorship of larger worms. The threshold level before bloom is 30 percent or more damaged squares with worms present. After boll formation, the threshold is 6 percent or more damaged fruit with worms present.

Many of you are aware of the situation of worms breaking through some of the Bt traits across various cotton producing regions including the parts of Texas. We may experience similar events in the High Plains cotton if it turns out to be a heavy bollworm year. For now, let’s hope it remains a quiet one but don’t let your guard down. Irrespective of what Bt traits you have, keep scouting and be on the lookout.  Remember, once worms increase in size and dig into the lower plant canopy, it’s hard to get them even with the most effective diamide insecticides.

Monday, July 9, 2018

Sugarcane Aphid Arrives on the Texas Southern High Plains

Kerry Siders, EA-IPM in Hockley, Cochran and Lamb counties, just reported that Danny Quisenberry, a crop consultant, found sugarcane aphids on older grain sorghum four miles north of Earth in northern Lamb County.

Additionally, Greg Cronholm, a private crop consultant, just reported sugarcane aphids on sorghum in southern Castro County.

Neither of these populations is at a treatable level, just small colonies on isolated plants at present. If this year's infestation pattern follows those of 2014-2017, then it is likely the aphids are already present in Hale, Floyd and Lubbock counties, although we have had no reports of this as yet.

Wednesday, June 20, 2018

Sustained High Numbers of Fall Armyworm Moths

This week's fall armyworm pheromone trap graph appears below. The 7-year average for this week of June is 110 moths per week. This week's capture was 707.

This does not appear to be a phenomenon confined to the Lubbock Research Center, as Katelyn Kesheimer, EA-IPM in Lubbock and Crosby counties, reported several hundred moths in the Crosby County traps she operates. (Rain filled the traps so it was not possible to count all the moths, but there  were hundreds.) Additionally, Blayne Reed, EA-IPM in Hale, Swisher and Floyd counties, drove to Plainview from College Station late last week and reported that fall armyworm moths were very abundant at rest stops along the way.

A prior post from April of this year discussed some genetic work being done by Ashley Tessnow, a graduate student in the Dept. of Entomology at College Station. She determined that the Lubbock area fall armyworm moths we sent her last year were 30% rice strain and 70% corn strain as a season average. She also noted that the relatively big flight toward the end of June last year had 93% corn strain.

We don't know where these moths are coming from. Some are doubtless progeny of the moths that arrived here earlier in the season, but it is likely a majority of the moths caught this week have come up from south Texas. Ashley's work will ultimately help us to know the geographic origins of our moths.

Most Bt corn will not get appreciable damage, but non-Bt corn should be scouted. Yesterday I noted that there were a lot of newly hatched larvae in my non-Bt corn at the Experiment Station just as the oldest larvae were cycling out. Actually there were a lot of plants infested by larvae from egg masses that had hatched in the last 24 - 48 hours. Some of these newly infested plants had four or more larvae per whorl and may get "dead heart" due to the very high numbers of caterpillars feeding on the youngest leaves in the whorls. Sorghum should be scouted as well.


Information on scouting and thresholds can be found for corn here and sorghum here. Essentially, both of these crops can withstand 30% whorl damage before an insecticide is justified. This does not, however, apply when deadheart is likely to happen. There are a few fields of corn in the area that are silking now, and these should be scouted regardless of whether they are Bt or non-Bt. 

While scouting it would be a good idea to look for corn earworm eggs and larvae; the moths are very abundant in my whorl stage corn although I do not have any pheromone trap data to report.