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Monday, November 19, 2018

Bt Corn Seed Selection in Light of Resistance in Corn Rootworm

By Patrick Porter and Ed Bynum, Extension Entomologists in Lubbock and Amarillo, respectively. 

The August 25th edition of this newsletter discussed how our mCry3a Bt corn was no longer able to control western corn rootworm in an area from Hart, Texas, north to the top of the Panhandle. In that newsletter, even though we were seeing all of the classic signs of resistance, we used the term “probable resistance”, only because our laboratory assays on field collected beetles will not be completed until next year. 

In addition to mCry3a, that newsletter suggested that since there is cross resistance between all of the Cry3-type toxins (mCry3a, eCry3.1Ab and Cry3Bb1), none of these toxins could be expected to provide good control of western corn rootworm. Dr. Aaron Gassmann at Iowa State University, a leading authority on corn rootworm resistance, said, “Cry3Bb1, mCry3A, and eCry3.1Ab all appear fairly similar to the rootworm. Resistance to one is likely to confer resistance to the other two.” 

As seed purchase decisions are made for next year’s growing season, it is time to put the cards on the table and discuss options for corn rootworm management.


By far the best option is rotation to a non-corn crop. 

Rotation will result in death of the entire rootworm population in the field because the larvae will not have a suitable host on which to feed and they will die. Since our rootworm beetles don’t lay many eggs in non-corn crops, the field can be planted the following year with no risk of a damaging rootworm population. 

When rotating to a non-corn crop, the volunteer corn that germinates must be killed when small to prevent rootworms from surviving and developing to beetles. The surviving beetles might lay eggs and re-infest the field, and the presence of corn in the field could attract other beetles from a considerable distance.

Of course crop rotation is often not an option, so here are the answers to some commonly asked questions. These answers are based on a field being in the resistance zone for Cry3-type toxins. If fields were planted to these toxins for the last several years and had lodging and high numbers of beetles, then resistance is likely. 


Is there any difference between a Cry34/35 (only) hybrid and one that has both Cry34/35 and a Cry3-type toxin?

It is better to plant corn with a pyramid of toxins rather than Cry34/35 alone. Resistance to the Cry3-type toxins is not complete so, in pyramids of the two types, the Cry3 will still provide some measure of additional root protection over Cry34/35 alone. This “partial protection” will also help preserve rootworm susceptibility to Cry34/35 because some of the insects with resistance alleles for Cry34/35 will be killed by Cry3-type toxins and won’t pass genes on to the next generation. See the table below for a full list of Bt corn hybrids active against corn rootworm, and the type of toxin(s) they contain. 

Several studies by academics and the seed industry have shown that, in areas where there is resistance to Cry3-type toxins, pyramids of Cry3s and Cry34/35 do not benefit from the addition of soil applied insecticides. Similarly, in our area there is probably no economic benefit from using soil applied insecticides on pyramids of Cry34/35 plus a Cry3-type toxin.


If I can use a Bt corn that has Cry34/35 without a Cry3-type toxin, will my roots be protected? 

Probably. There is no known resistance to Cry34/35 in our area and root protection should be very good. However, there are some caveats. One caveat is that, due to resistance to Cry3-type toxins, some fields have enormous numbers of eggs in them and the Cry34/35 will be challenged. We have seen instances of significant root damage in Cry34/35 corn under heavy rootworm pressure. If a continuous corn field had extremely high numbers of beetles last year and adult control was not used, then it might pay to use a soil applied insecticide when planting Cry34/35 seed. 

The other caveat is that toxin expression is lower in plants grown under stress, so proper agronomic conditions need to be met if the Cry34/35 is going to do the best job possible.


What if I have resistance but have to plant a Cry3-type (only) toxin?

In this case expect damage equal to or worse than last year, as a higher percentage of the population is now resistant. (Winter rootworm mortality is usually not a factor in our area, and adult sprays last year provided only suppression of egg laying.) The use of a high rate of insecticidal seed treatment and an at-plant soil applied insecticide is strongly recommended. 


How much protection is provided by insecticidal seed treatments, soil applied insecticides, and beetle sprays?

The answer varies by the amount of insecticide on the seed, but even at the highest amounts available on commercial corn seed, protection will be insufficient at moderate and higher infestations. If soil applied insecticides have been used continuously when planting Bt rootworm corn hybrids and spraying for beetles, the rootworms could also have developed resistance to the insecticides. 

Until this past year Bifenthrin insecticide has been used almost exclusively for both soil applications and beetle sprays. This has put a lot of pressure for selecting rootworm resistance. Researchers from Kansas State University and the University of Nebraska has shown different levels of resistance to Bifenthrin in rootworm populations across Kansas and Nebraska. Steward EC insecticide, which is not a pyrethroid, has recently received a supplemental label as a foliar application for beetle control. 


Does corn without a rootworm Bt toxin have a place?

Corn without a Bt toxin is a good choice for ground coming out of rotation to a non-corn crop because the rootworm pressure will be essentially zero. The seed will be treated with an insecticide and fungicide similar to the Bt hybrids. Planting non-Bt corn in a field that had high rootworm populations the previous year is not a good idea, even with high rates of insecticide in the seed treatment and with soil applied insecticides used at planting. 

Unfortunately, seed companies have not put as much breeding effort toward their non-Bt hybrids, so in some cases the agronomics and yield potential are inferior to the Bt hybrids. This is not true across the board, so consult your seed dealer(s) to examine the yield potential of hybrids that do not have rootworm protection. 


The Bt toxins in every type of hybrid from every seed company are listed in the Handy Bt Trait Table

Wednesday, November 14, 2018

Good News: Some Single Toxin Bt Corn Being Withdrawn from the Market

Dr. Chris DiFonzo, Entomologist at Michigan State University and author of the Handy Bt Trait Table, and I got a note from a corn seed dealer a couple of weeks ago concerning the removal of some single toxin Bt corn hybrids from the market after 2019. Apparently there was a letter circulating from one of the seed companies to this effect, and he wanted to know whether it was true and, if so, why it was being done - some of his customers really like their single toxin hybrids. 

It is true, and it is a good and necessary thing. Back when Bt corn was first introduced, most hybrids had only a single toxin for caterpillar pests, and, a few years later, if there was a corn rootworm toxin it was single as well. A few more years down the road, seed companies began selling “pyramids” of toxins; a combination of two or more toxins targeted at a pest. Not only did this improve efficacy, but it also slowed the rate of resistance development as explained in the following scenario. 

If insects with resistance to toxin A were allowed to develop on plants that contained only single toxin A, then most of them would live and pass their genes on to the next generation and resistance to toxin A would evolve rather quickly. Pyramids were meant to slow the process down because in a pyramid of toxins A and B, insects with resistance to toxin A would still be killed by toxin B (unless they also had resistance to toxin B). Similarly, insects that had resistance to only toxin B would be killed by toxin A. In each case, the resistance genes would be removed from the population. The chance of an insect having resistance to both toxins A and B was initially quite small (but is not so today now that we have grown Bt corn and cotton for more than 20 years).

Seed companies, realizing the risk of their single toxins being selected generation after generation, soon began cross-licensing their toxins to each other in an attempt to build pyramids as fast as possible. 

What has changed is that we now have insect species where resistance to one or two of the toxins in a pyramid is fairly common. If a pyramid is built from toxins A and B, and an insect is now completely resistant to toxin A, then it is only toxin B that can kill it. So in reality, the insect is being selected for resistance to toxin B alone now that toxin A has no effect. But if resistance to toxin A is not complete, and toxin A still has some effect (not full effect), then toxin A still provides some partial protection to toxin B. 

This “partial protection” scenario is where we are today with all of our pyramids of toxins for corn rootworms; every toxin is compromised, but some more than others. In caterpillar control we are trying to protect Vip3a, the newest toxin in our most modern pyramids.

We are trying to buy time and prevent resistance to all of the toxins in our pyramids. And, since putting a single toxin out there by itself is the fastest way to get resistance to that toxin (and destroy the “partial protection” it still might offer in pyramid hybrids), it makes perfect sense to get the single toxin hybrids off the market as fast as possible.

All of the seed companies have committed to removing all of their single toxin corn hybrids, “as soon as possible”. (Single toxin cotton was removed years ago). Some companies have been more successful than others, but all are trying. The loss of the few single toxin corn hybrids currently on the market is a small price to pay if their removal will delay resistance a few more years. We are in a tight spot with our Bt toxins in corn and cotton; resistant insects are closing in and we need to extend the life of our current pyramids long enough that the next generation solutions can come online.

If you want to know which toxins are in the corn seed you are buying, the 2019 version of the Handy Bt Trait Table was published earlier this month.

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More background on how we got here can be found in previous editions of this newsletter:


Wednesday, November 7, 2018

Virus-Based Insecticides for Control of Headworms in Sorghum and Caterpillars in Other Crops

A "new" insecticide option is being tried and promoted on the High Plains for control of some caterpillar pests, especially sorghum headworms (corn earworm and fall armyworm). There are some appealing aspects to this, but there is a lot we don't know as well.

Dr. Ed Bynum just posted a nice summary of the history of these insecticides based on nucleopolyhedrovirus (NPV), how they work, and some aspects of application and labeling. Insecticidal Virus Products for Pest Control: What are the Latest Facts, Panhandle Pest News, 7 November 2018.

Because these are live viruses and spread in the field, the minimum plot size needed for research is very large. We would be interested in trying one or both of these headworm products on large fields next year. By large I mean way bigger than we have at the Experiment Station. There would be large blocks of treated and untreated sorghum, and ground application equipment would be essential.

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.