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Sunday, December 10, 2017

Bt Corn Update, and New Bt Trait Table Published

As final seed purchases are being made in the Texas Panhandle, here is a brief update on the status of Bt corn. But before your eyes glaze over from the discussion below of new Bts and resistance to older Bts, I want to highlight a publication that makes it easy to tell which toxin packages and herbicide traits are in which type of corn. Dr. Chris DiFonzo at Michigan State University publishes the annual "Handy Bt Trait Table for U.S. Corn Production", and the 2018 version was posted online today. (I am a contributor to this publication, but Dr. DiFonzo does the heavy lifting.)  In just two pages she lists the types of Bt present in all commercialized corn in the U.S.A., and the table presents the trade names for traits, Bt event, protein(s) expressed, targeted insects and herbicide traits.

The 2018 Trait Table also lists the insect x Bt combinations with documented field-failures, confirmed resistance, or cross-resistance. These statements are based on published lab assays and/or field research. The resistance column is intended to alert growers and consultants to potential management problems, influence seed selection, and encourage field scouting. It is important to note that the Trait Table is a national publication, so check with your local seed company or extension personnel for the types of Bt resistance present in your area.

For those looking at a printed version of this newsletter, the Handy Bt Trait Table can be found here: https://www.texasinsects.org/bt-corn-trait-table.html .

And now to the Bt corn update.

Monsanto is marketing Trecepta, its version of Vip3a pyramided with other toxins. I worked with Trecepta in 2017 and was impressed with its insect resistance and yield. For Vip3a corn we have Monsanto's Trecepta, DuPont Pioneer's Leptra, and Syngenta's Viptera. This type of corn is virtually immune to caterpillar damage whether the pest is western bean cutworm, fall armyworm or corn earworm. I have worked with Leptra for six years and Trecepta for one and, in that time, have seen only two live caterpillars in many thousands of ears I have examined.

On the resistance front, laboratory studies conducted primarily in Canada by Dr. Jocelyn Smith (with coauthors from academia and seed companies) have shown that western bean cutworm (WBC) is now resistant to Cry1F (often sold as Herculex). Control with Cry1F, even in the Texas Panhandle, has been slipping for several years, and it is now conclusive that resistance is a big part of the reason. All of the Cry1F registrants and licensees have removed WBC from the list of insects their non-Vip3a Bt corn will control. All of the companies correctly point out that the only Bt corn that will control WBC are the hybrids with Vip3a (combined with other ineffective toxins which vary by company). Bt hybrids that do not contain Vip3a should be scouted and treated according to extension recommendations.

Southwestern corn borer is known to be resistant to Cry1F in southeastern Arizona and southwestern New Mexico. The good news is that we have no indication that this resistance has come to Texas. DuPont Pioneer made significant efforts in 2017 to determine where the resistance does and does not occur. As part of this they funded John David Gonzales, IPM Agent in Bailey, Parmer and Castro counties, to make SWCB collections, and that project will be ongoing in 2018. The other good news is that in places where the resistance does exist, planting Bt corn with two toxins rather than just Cry1F has eliminated economic yield losses.

In the less than good news category, Dr. Ed Bynum, Extension Entomologist in Amarillo, and I have been in continuous corn fields in the Dalhart area that had unusually high amounts of corn rootworm damage in mCry3a hybrids. There have been many instances of western corn rootworm resistance to Bt toxins in other parts of the country, and is known that there is cross resistance between Cry3Bb1, mCry3a and eCry3.1Ab. It looks to me like we have resistance to these toxins in the Panhandle. However, resistance should be determined through laboratory analysis of the offspring of adults collected from fields, and that is why it is important to contact extension personnel or your seed dealer if you are seeing indicators of resistance. These indicators include fields that have been planted to the same Bt toxins for three or more years, the need to use soil applied insecticides on top of the Bt, high levels of root pruning, goosenecking or lodging of plants, or consistently high numbers of adult beetles in July or August. It is important to report suspected resistance while there are still plenty of adult beetles in the field to collect. Dr. Bynum and I would be glad to discuss resistance with anyone who thinks they might have a problem.


Thursday, October 26, 2017

Southern Bt Crops: Getting Boxed In

Insects have developed resistance to the older Bt toxins in cotton and corn on a local or regional scale. A quick look at the situation in the southern U.S.A. finds that in the last two years, resistance has been documented over a large geographic area in cotton bollworm/corn earworm (Helicoverpa zea) to the cotton Cry1Ac and Cry2Ab2 toxins. In corn, the limited efficacy of Cry1Ab, Cry2Ab2 and Cry1F has slipped from where it was years ago. This year in the mid-south, university personnel are reporting as much earworm damage in Bt corn as in non-Bt corn.

The "new" toxin, Vip3a, is highly effective on bollworm/earworm, and seed companies are putting it into new corn hybrids and cotton varieties alongside suites of the older toxins for which resistance has developed. Yes, readers can already see what is wrong with this picture; once Vip3a hybrids and varieties are widely planted, bollworm/earworm will be selected for successive generations on this toxin that now has only partial protection from the other toxins because they are already compromised. There will be two generations of Vip3a resistance selection on corn, and then another 1-2 generations of selection on cotton.

When bollworm/earworm becomes resistant to Vip3a, cotton will suffer economic damage because bollworm is a major pest. Resistance will be noticed in cotton first, in part because Vip3a toxin expression is lower in cotton than in corn. When Vip3a fails in cotton, growers will then begin to spray their crops 1-3 times per season with the diamide class of insecticides, the most effective class on bollworms. This in turn will select bollworm for resistance to diamides. Cotton production costs will rise and profits will decline. Any new toxins are seven to ten years away, so profitable cotton production depends on keeping bollworm susceptible to Vip3a for as long as possible.

On the other hand, on the vast majority of acres in the southern U.S., corn does not need the protection afforded by Vip3a. (The exceptions would be the few areas where fall armyworm is a pest, and in very small areas where fumonisin levels can increase due to insect damage to ears.) Corn earworm is not a significant pest in field corn; it damages the tips of ears and is not a major contributor to yield loss. This fact, however, has not stopped the seed companies from marketing Vip3a corn as a breakthrough solution to the corn earworm "problem". It is easy to sell technology that results, at least for a few years, in a completely undamaged ear, whether that technology is needed or not.

Unnecessary insect control aside, seed companies have put their most advanced genetics into Vip3a corn, so even without insects in the system these hybrids will probably yield more than older hybrids with compromised Bts or no Bt. Because they can't get advanced genetics in older, non Vip3a hybrids, growers will end up planting Vip3a corn for the yield potential rather than a need for Vip3a.

As a consequence, it is easy to predict that Vip3a corn acres will expand in the south and therefore hasten the demise of bollworm susceptibility in cotton. If this happens, the cotton industry will suffer major losses and corn growers will barely notice. Earworm/bollworm moths resistant to Vip3a (and perhaps the diamide insecticides) will fly north and threaten the sweet corn industry.

The older Bt toxins are failing, and Vip3a stands as the primary means of caterpillar control for some pests. There is already a field collected colony of fall armyworm that is resistant to Vip3a, but as yet bollworm/earworm is susceptible, although cotton varieties with Vip3a did suffer bollworm damage this season (which does not mean resistance).

I have long been a proponent of GM crops and still am. However, the current situation highlights the fact that we are just on a different version of the old Pesticide Treadmill. We are being boxed in; trying to come up with the next great thing before the last great thing loses effectiveness. I hope the next great thing comes along in time.

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For a bit of background, older issues of this newsletter have discussed the convergence of Bt toxins in our crops and what it means for resistance evolution.

Is There Still Value in GM Crops? (9/16/16)
Bt Corn and Resistance Clouds (2/6/16)
Shuffling the Deck Chairs in Bt Crops (9/10/16)







Monday, October 9, 2017

Fumonisin Levels and Insect Damage in Corn

I am not smart enough to be a Plant Pathologist, and in fact had two courses in it in college and still don't understand it. The classic "disease triangle" taught in pathology says that disease occurs when there is a pathogen, susceptible host and conducive environment. This year we seem to have had a happy triangle for Fusarium species, the causative agents of fumonisins.

Not much is known locally about how these fungi interact with our corn, but it is thought that drought stress followed by warm, wet weather, especially at flowering, favor the fungi. Being just an entomologist, I tend to think there is a baseline risk for significant fungal infection based on the susceptibility of the host (hybrid genetics) and environmental conditions. Without insects in the system there will be a given level of fungal growth and fumonisin creation. In my simplistic entomologist's picture, the baseline level is what it is and can vary from year to year, but insect damage can add to this level by opening wounds on the ear and/or by insects carrying fungal spores into the ear.

Dr. Ed Bynum and I did some work at Lubbock in 2012 that looked at the amount of fumonisin in ears with three different levels of insect damage, and more fumonisin was found with higher levels of insect damage. This was one hybrid of non-Bt corn that we sprayed with different timings of insecticide so as to get the three damage levels.

Figure 1. Type of ear damage and fumonisin levels associated with that damage, 2012.
This year there is a need to try and determine to what extent insect damage might be contributing to fumonisin levels, but this is not easy to do unless the hybrids have the same genetic background (inherent susceptibility) and are grown in the same field under the same conditions. One seed company has a small plot field trial near Ralls, and they were kind enough to allow me to sample ears from their new hybrid that contains Vip3a and other toxins, and an older Bt type that has fewer toxins but still the same genetic background as the new type of corn. This is a fair comparison for determining the role of insects. The older type of Bt corn averaged 3.6 damaged kernels per ear, while the new corn with Vip3a was essentially undamaged. Even the silks on the new type of corn were intact. The photos below represent what I saw in the field today.

Figure 2. New hybrid with Vip3a and other toxins (top), and older Bt with two toxins (bottom). The new hybrid was essentially without insect damage. In the older hybrid the insect damage was only at the tip, but fungal growth could proceed through much of the ear. 

The same photo as above, but rendered in an infrared simulation that highlights the kernels damaged by fungi. The grain from these ears was sent for laboratory analysis of fumonisin content, and the relatively undamaged ears on the top of the photo had an average of 6.0 ppm fumonisin. The ears on the bottom row had an average of 208 ppm. 

It is common on the High Plains for nearly every ear of corn to have corn earworm damage, and this year was no different and not significantly worse. In the opinion of this entomologist, the problems we are having this year are primarily due to environmental conditions that favored Fusarium. Having said that, I have worked with Vip3a corn for six years, and in all that time have seen only two live caterpillars in thousands of ears examined. Vip3a corn is essentially bulletproof for now, and if the goal is to reduce caterpillar damage then this type of corn is the way to go. Of course it is more expensive than older Bt technologies. All of the seed companies put other Bt toxins in with Vip3a. Pioneer sells their Vip3a corn as Optimum Leptra or AcreMax Leptra, Monsanto is now beginning commercial sales for 2018 as Trecepta, and Syngenta calls it Agrisure Viptera or Agrisure Duracade 5222. This is not to say that these hybrids won't have fumonisin problems; the inherent susceptibility might be more or less. It is to say that they will have less insect damage, which our data suggest ultimately plays a role in fumonisin levels.

Update on 10/11/17: Erin Louise Bowers did her Ph.D. dissertation on the benefits of transgenic corn in reducing fumonisin levels. She found that Cry1Ab +Vip3a corn had lower fumonisin levels that other types of Bt corn and non-Bt corn. The work is here.






Friday, September 29, 2017

SCA After the Rains: Now What?

We are now concluding five straight days of rain on the southern High Plains, but sugarcane aphids are still with us. I spent some time today collecting infested leaves and examining the aphids under a microscope, and I have to report that I can't find any evidence of the fungi that hammered populations on the Gulf Coast. (Although I will keep monitoring the situation.) Most of the aphid colonies I observed looked just fine, and there were some beneficial insects like syrphid fly and lady beetle larvae feeding on them. Dr. Katelyn Kesheimer, IPM Agent in Lubbock and Crosby counties, took 7 Day After Treatment data in a sugarcane aphid efficacy trial yesterday between rain events, and she reported that there was a slight decrease in aphid numbers on the untreated plots, but nothing to write home about.

So the rains did not really reduce the number of aphids, but, significantly, the cooler temperatures slowed them down. Aphid development and reproduction is slower in cooler temperatures, so the explosive population growth potential is not going to be here until we get significantly warmer. The practical effect of this is that fields that still require treatment, or will require treatment, do not have to be sprayed as quickly as they would be in hotter conditions. This is good for a few reasons, one of which is that it will pay to wait a few days.

We know that our insecticides do not work as well when it is cold, or, put another way, they work better when it is warm. Current predictions put the warmest days next week as Sunday - Tuesday, and then Friday - Sunday. If an application needs to be made, make it during the window of warmest days. Given that we don't really have hot weather in the forecast, it would not be a good idea to cut insecticide rates in the face of these moderate temperatures.

Dr. Kesheimer included a generic formulation of imidacloprid in her efficacy trial because growers are using it due mostly to its relatively low cost and a marketing push. We already have older data that this off-label insecticide does not provide good sugarcane aphid control, and her 7DAT data are reinforcing what we already know. Transform and Sivanto remain the effective sugarcane aphid insecticides.

Friday, September 8, 2017

Sugarcane Aphid Increasing on Late Sorghum

It Is Not Over for the High Plains

Even though it is getting late in the season, sorghum is still at risk from sugarcane aphid, especially later planted sorghum. In Lubbock we are seeing leaves with thousands of aphids, and for the last two weeks many of these have been winged. These aphids have and will continue to ride the winds as they do each year. If this year is like the past three years, the aphids will spread westward and northward. Dr. Ed Bynum in Amarillo is reporting treatable populations in his area. The rains did not stop the aphids, and there is no reason to think they will stop before the first or second hard freeze. Last year we harvested sorghum at the Halfway Experiment Station after first freeze and still had plenty of aphids on the plants and in the heads.

What I am trying to say is that if you have grain or forage sorghum in the field, this is no time to get complacent. The photos below were taken at the Lubbock Research Center this morning before sunrise.

Leaves being killed by aphids, and honeydew darkening the soil where it dripped. 

Leaves on late planted sorghum completely covered by honeydew from the thousands of aphids feeding on the undersides of leaves above. All of the sorghum in this field looked this way. 

Mid-June planted sorghum. The untreated row is on the left, obviously. The row on the right was sprayed with 5 oz. of Sivanto. 

Friday, September 1, 2017

Grain Sorghum: Nearly Perfect Storm in Lubbock and Lynn Counties

After writing in this newsletter last week that fall armyworm was not a significant threat this year, Katelyn Kesheimer, Lubbock and Crosby county IPM Agent, and I visited some fields in southern Lubbock county and south to the middle of Lynn County in the last four days. I take it back; fall armyworm is very numerous in sorghum in these places south of central Lubbock County where my traps are located. We encountered fields at panicle exertion or already booted that had as many as six worms per head, with an average of 2-3 mid-sized worms being the norm. For the most part these were fall armyworms in southern Lubbock County, but corn earworms seemed to increase in frequency as we went south. In a field 6 miles west of Tahoka we were seeing something like the 70% fall armyworm and 30% corn earworm. The age structure of the populations was approximately 45% small larvae, 45% medium larvae and 10% large larvae, but of course this will change quickly. Large larvae are by far the most destructive, and the goal is to treat the field before many of them are present. Kerry Siders, IPM Agent in Hockey, Cochran and Lamb counties, reported in his newsletter tonight that headworms were increasing in his counties, and the majority of these were corn earworms.

Fall armyworm larva feeding on a sorghum panicle in southern Lubbock County yesterday.


Prior to the arrival of sugarcane aphids, control options for caterpillars would have been a pyrethroid, Lannate or Carbaryl. Pyrethroids are not very effective on fall armyworms over 1/2 inch in size, so some area crop consultants are now adding a pint of Lorsban to act as a synergist with pyrethroids. HOWEVER, WE FOUND SUGARCANE APHIDS IN ALL OF THESE FIELDS. The use of a pyrethroid and/or Lorsban would eliminate the biological control agents in the field that are suppressing the sugarcane aphid population.

What we have now in these areas is a real problem. The best control practice would be to go after the worms with a soft insecticide that does not kill the biological control agents that keep sugarcane aphid in check. These insecticides would be Blackhawk or Prevathon. Blackhawk is approximately half the price of Prevathon, but DowAgroSciences has told us that there are no supplies of Blackhawk left in the warehouses because of brisk sales this year in the mid-South. So that leaves Prevathon, which is excellent on both caterpillar species. However, an application of 14 oz/acre of Prevathon, the lowest labeled rate, would cost on the order of $18 per acre + application costs. We cannot recommend less than labeled rates, but area Independent Crop Consultants tell us that 10 oz of Prevathon with 5 GPA by air provides good control of both caterpillar pests. This rate would cost approximately $12.80 per acre + application costs.

If one chooses to follow the pyrethroid + Lorsban path in a field with sugarcane aphids, then it is likely that a follow-up application will be needed for the aphids; at least 5 oz/acre of Sivanto or 1.25 oz/acre of Transform. But this is not a given; the aphids south of Lubbock County do not seem to be increasing as fast as they did in years past. Scouting will be essential.

At this point we do not know what to recommend with so many headworms in the system and aphids in the field; it comes down to economics. There are no inexpensive options here that do not elevate risk from sugarcane aphid, and we can't predict the future with respect to whether sugarcane aphids will require treatment later. (But note that some fields in southern Lubbock County are well over treatment thresholds for both pests.)

We have a sorghum headroom treatment threshold calculator here.  Our written thresholds and scouting procedures are here on page 23.

Another unknown is sorghum midge. The late planted crop is at risk, and with Blackhawk (which is effective on sorghum midge) out of the picture, we will have to resort to pyrethroids, which in turn will increase the risk of sugarcane aphid while not being much use on fall armyworm. Yes, there are no inexpensive answers to this emerging multi-pest situation.

Saturday, August 26, 2017

Sorghum: Sugarcane Aphids, Headworms and Midge

We have been closely monitoring sugarcane aphid numbers at the Lubbock Experiment Station and hoping the rain would knock them down. Unfortunately this has not happened, and our untreated plants in bloom now have 500 - 2,000 aphids per leaf and the lower half of the canopy has severe leaf damage. There are plenty of winged adults, too, and these will be flying off to infest other fields. With all of the late sorghum planting after failed cotton, there is now a very wide range of sorghum maturities out there and the younger plants are still subject to the full force of the aphid. High Plains insecticide action thresholds for each growth stage are presented on page 5 of our SCA Management Publication. There is also a statement about re-treatment thresholds.

However, we now have a 2-3-axis threat because cotton bollworm/corn earworm egg laying has really picked up and is now at a level of concern in both cotton and sorghum, and sorghum midge can still injure crops yet to complete bloom. In sorghum, cotton bollworm and fall armyworm comprise the headroom complex, and these insects feed directly on the developing kernels and can cause significant yield loss. Our treatment thresholds are based on the size of the worms, number of plants per acre, cost of control and market value of the grain, and these thresholds are presented on page 22 of Managing Insect and Mite Pests of Texas Sorghum. While cotton bollworm numbers are high, thankfully fall armyworm numbers are fairly low.


We think our High Plains bollworms are still susceptible to pyrethroid insecticides even though there has been some weakness in susceptibility in south Texas. A headworm population that is predominately cotton bollworm (but not fall armyworm) can be taken out with pyrethroids - EXCEPT that using them will eliminate most of the biological control in the field and stimulate a sugarcane aphid and/or yellow sugarcane aphid population increase. 

If a field reaches treatment threshold for either headworms or sorghum midge then insecticides should be applied to protect yield. However, if sugarcane aphids are present in the field then the choice of insecticide is important. We have some "soft" insecticides for headworms that will not remove the beneficial insects that are important for aphid control. Unfortunately, except for Blackhawk insecticide, this is not the case for sorghum midge, and any "hard" insecticide application (for either pest) should be followed up by careful monitoring of aphid populations. Insecticide options in these multi-pest situations are presented in "Insecticide Selection for Sorghum at Risk to Sugarcane Aphid Infestations".