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 Cry1Ab, 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)

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.

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.

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. 

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.

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".

West Texas Cotton: Scout for Aphids and Bollworms

Suhas Vyavhare, Extension Cotton Entomologist, Texas A&M AgriLife Extension Service

Cotton will benefit from some of the timely rainfall we have been receiving recently. However, it is going to make conditions better for survival and multiplication of several pest species as well. In general, the hot and dry weather of West Texas helps to keep bollworm numbers in check (through desiccation of eggs). Humid and cloudy conditions over the last couple of weeks, however, may increase egg hatch rate and worm survival in cotton. In addition, added new growth on plant terminals will help both bollworms and aphids thrive better.

Over the last 10 days, we have spotted many fields with cotton aphid infestations. Aphid colonies are mainly concentrated to plant terminals but as the numbers build-up, they may move on to the leaves. Overall, beneficial numbers seem to be lower compared to the previous year, but they are present. Isolated showers will also help wash out honeydew and some of the aphids from the plants. If aphid colonies are spotty and mostly restricted to plant terminals, I would wait and monitor the situation over the next few days. Often aphid populations crash out in response to beneficials and rain. Click the link below to access more detailed information on cotton aphids: http://lubbock.tamu.edu/files/2017/07/Cotton-aphid_ENTO074.pdf

I have also come across a few reports of bollworm damage to non-Bt cotton. In our research trials, bollworm damage ranges from 5-6% boll injury in non-Bt cotton and <1% in Bt cotton. The threshold is 6% fruit injury with the presence of live worms in both Bt and non-Bt crop. Among various insecticide options for bollworm control, Diamide insecticides (Prevathon and Besiege) are the most reliable choices. Remember, Besiege contains both a diamide and a pyrethroid so it would be a better choice if stink bugs are present too. However, if aphid colonies are present in the field, the pyrethroid component may flare-up aphids. If a field needs to be treated for both aphids and bollworms, Prevathon can be tank-mixed with any of the commonly used aphidicides such as acetamiprid (Intruder).

Along with the proper insecticide selection, coverage is also important getting the desired level of worm control. In fields with dense plant canopy, it is important to get material down in the lower canopy where worms are in protected places.  Air induction nozzles recommended for newer herbicide technologies produce coarser spray which may not penetrate through the dense plant canopy and provide thorough coverage. Penetration through plant canopy can be improved with flat fans or hollow cone tips and by increasing final volume (no less than 10 GPA with a preference of 15 GPA for ground rig). If using an airplane, use at least 5 GPA.