Some Bt Sweet Corn No Longer Effective

I received a few calls this summer from sweet corn growers who wondered why they had corn earworm larvae in their Bt sweet corn. The short answer is that corn earworm has now developed resistance to the older set of sweet corn toxins and only the newest toxin will provide good control.

I ran a sweet corn sentinel plot trial at Lubbock this summer to determine the ability of Bt sweet corn to control corn earworm (as compared to non-Bt sweet corn).

The bottom line was that I found roughly twice as many healthy corn earworm larvae in the older types of Bt corn than in the non-Bt corn. There are a few reasons for this; a) corn earworm is largely resistant to Cry1Ab and Cry1A.105+Cry2Ab2, toxins which worked well several years ago, and b) it has been shown in the scientific literature that resistant larvae on Bt sweet corn largely lose their cannibalistic drive. On non-Bt corn there is usually one big surviving larva per ear by the time the larvae are older. On Bt sweet corn where the insects are resistant or partially resistant to the toxin(s) they are not as prone to cannibalism and there are often multiple surviving older larvae.

Here are some data from this summer's trial at the AgriLife Research Center at Lubbock.

The two columns on the right give us an idea about the health of the larvae. Corn earworm has six instars, and those reaching third instar will probably not be killed by Bt and have a good chance of completing development. I did notice a slight developmental delay in resistant caterpillars on the older Bt toxins as compared to non-Bt corn, but it was minor, perhaps a setback of a couple of days in development.

Attribute II has the older Cry toxin from Attribute and the newer toxin, Vip3a. At least for the High Plains of Texas, if you are a commercial sweet corn grower and want to use a Bt hybrid, it would be worth sourcing seed with Vip3a as one of the toxins.

Lab Results Indicate Last Year's Rootworms Not Resistant to Bt Corn

Last year we had huge numbers of corn rootworm beetles in corn around Hart in early July, and I wrote at the time that they were "probably resistant" to the mCry3a rootworm toxin. I was seeing classic resistance signs: lodged corn in Bt fields, root damage, and many, many beetles. In that article I said that we had collected insects to send to a USDA lab where they would be assayed for resistance, and that I would report the results of the assay here in FOCUS on Entomology.

The results are in, but before I provide them I will tell a bit more of the story on how we arrived at these results. The 1,200 beetles we sent to the USDA-ARS Lab in Columbia, Missouri were healthy and laid many eggs. They were tested by USDA-ARS personnel, and personnel at the University of Missouri. Eggs were held over the winter at the USDA Lab, and when they were ready to hatch we asked the seed company that sold the corn, and the seed company that was the registrant of mCry3a for permission to assess the resistance level by using standard toxin overlay procedures on artificial diet. Both of these companies refused to allow the bioassay. Their justification was that the field where we collected the insects was not an official "Performance Inquiry", so they were not obligated under the terms of their EPA registration to test this population. (This is true, but why the resistance to finding out whether there was resistance?) However, Texas A&M has an agreement in place with the registrant of the toxin that we can use commercialized hybrids to assess resistance, so we went that direction.

Our Missouri colleagues sent the results last week, and they showed only a slight elevation in tolerance to mCry3a in the Texas population as compared to a known susceptible population provided by the USDA-ARS lab in Brookings, South Dakota. (The Brookings lab is key to maintaining several corn rootworm strains used in the investigation of resistance, as it is impossible to find non-selected corn rootworms in nature since Bt corn has been planted for so long over such a wide area.)

So, as promised, I have provided the results of the resistance screen for the insects we collected near Hart last year. I am indebted to our friends at the USDA-ARS lab in Missouri for doing this work. The official determination is that they are only slightly less susceptible to mCry3a than a population that is known to be susceptible. They are not, according to formal screening, resistant to mCry3a. I am still scratching my head as to why our mCry3a fields (and fields all the way to Colorado) had root damage, lodging and clouds of rootworm beetles last year, but I have to go with the science. And I completely trust my colleagues in Missouri.

In July of this year I visited the Hart area again and saw few beetles and little damage. In my opinion, the very heavy spring rains after planting drowned many of the small rootworm beetle larvae, and this in turn reduced potential damage, even to non-Bt corn. (This phenomenon is well documented in the Midwest, but not so documented here since we don't often have abundant spring rains.) Additionally, another contributor to the lack of high beetle numbers this year is the fact that growers abandoned, for the most part, hybrids that had only the mCry3a toxin and switched to hybrids that had the Cry34/35 toxin with or without mCry3a. This is good rootworm control and resistance management! The best option is to plant a hybrid that has Cry34/35 and any of the Cry3-type toxins. It is easy to determine which Bt toxin combinations are present in any particular hybrid by looking at the Handy Bt Trait Table.

Managing cotton aphids

Cotton aphids have been more common than normal this season in the Southern High Plains region. Aphids are present in more or less numbers in almost every cotton field. They are often found on the underside of leaves or feeding on the plant terminals. In several fields, aphid infestation is evident with the accumulation of honeydew causing the appearance of sticky and shiny leaf surfaces. I know of a few spots where aphid treatments were necessary. For the most part, however, aphid numbers remain well below economically damaging levels. Beneficial insects (e.g. lady beetles, green lacewing, etc.) are ubiquitous and are working their magic to keep aphid population growth in check. Fields that are treated with broad-spectrum insecticides earlier in the season may not have enough beneficials and will have to be monitored closely for potential aphid outbreaks.  

I would not suggest aphid treatment unless aphids are very numerous, and honeydew is accumulating. Give the beneficials and natural control (rain, aphid fungus, etc.) a chance before the treatment decision is made. Having said that, given the current hot and dry conditions, I wouldn’t let too many aphids suck the juice out of plants that are already under stress either. The action threshold is 40-70 aphids per leaf prior to first cracked boll. Late in the growing season and once open bolls are in the field, honeydew can accumulate on the lint of the open bolls. Even under low infestation levels, cotton aphids can excrete enough honeydew to contaminate the lint, causing “sticky cotton”. The threshold drops down to 10 aphids per leaf after the first cracked boll.

Suggested insecticides are listed in the “2019 Insect and Mite Pests Control Suggestions for Cotton" https://lubbock.tamu.edu/files/2019/08/2019-Cotton-Insect-Control-Suggestions_ENTO090.pdf

For detailed information, checkout our Cotton Aphid fact sheet: https://agrilifecdn.tamu.edu/lubbock/files/2017/07/Cotton-aphid_ENTO074.pdf

Cotton aphids

Cotton aphid colony on the plant terminal

Lady beetle larva

Leaf curling resulted from aphid feeding

Texas High Plains Cotton: Keep Watch on the Fleahoppers

This is just a heads up that there have been a few reports of cotton fleahoppers in cotton. Cotton fleahopper adults are pale green to gray-green; nymphs are lighter-colored with reddish eyes. Fleahoppers prefer to feed on small squares (pinhead size) and can cause substantial square loss if present in enough numbers. When scouting for fleahoppers, pay attention to both number of insects in field and the percent square retention especially during the first three weeks of squaring. They typically don’t target large (> matchhead size) squares.  Thus, they are normally concentrated in the top few nodes of the plant. Scout by visually inspecting plant terminals, the top three nodes. Adults are active flyers, but nymphs can be spotted when observed carefully.  

Cotton fleahopper adult (Photo: Salvador Vitanza)

Cotton fleahopper nymph (Photo: Xandra Morris)

Here is a link to a video on how to scout for fleahoppers in cotton.

https://www.youtube.com/watch?v=epVctkRkTHs

Use an economic threshold of 25-30 cotton fleahoppers per 100 terminals to determine when treatment is needed. After, first bloom, fleahopper control is rarely justified. Avoid using broad-spectrum insecticides as they can negatively impact beneficial insect populations and cause outbreaks of aphids and bollworms.

Additional information on cotton fleahopper management can be found at: https://agrilifecdn.tamu.edu/lubbock/files/2017/06/Cotton-fleahopper_ENTO073.pdf

https://lubbock.tamu.edu/files/2019/04/ENTO-075-2019.pdf

FYI – I am also seeing some black fleahoppers in cotton. There isn’t much information available about their impact on cotton—include them along with cotton fleahopper counts and base your treatment decisions taking into account the number of insects present and percent square loss.

Lygus bug (pictures below) numbers have remained sparse so far. But there have been scattered reports of treatment level infestations of lygus bug in few fields. Be alert for lygus but do not confuse other harmless true bugs for it.  

Adult lygus bug (Photo: Pat Porter)

Adult lygus bug

Sugarcane aphid has arrived, fall armyworm flight very large

Small colonies of sugarcane aphids have been found on grain sorghum in Lubbock and Hale counties in the last two days. Greg Cronholm, Independent Crop Consultant in Hale County, texted last night and reported finding a few small colonies. I found some today on blooming sorghum on the Experiment Station in Lubbock. Scouting and management guidelines are here.

Fall armyworm trap captures continue to be very high; more than twice the eight-year average.

 To put the current flight in perspective, the graph below shows what we faced at this time in July over the past few years. The magnitude of the current flight strongly suggests that fields be scouted for larvae and damage.

Big Fall Armyworm Flight Underway

The weekly fall armyworm pheromone trap graph shows a capture 2.35 times the eight year average.

Of interest, an average of 100 of these 310 moths were caught last night, so we might be on the front end of the second big flight this year. This would be expected, as many of the moths are probably progeny of the big flight the last week of May.

Yellowstriped armyworm captures in the western bean cutworm traps continue to be high, and I got a call last week about YSAW quickly defoliating greenhouse tomatoes.

Greg Cronholm, Independent Crop Consultant in Hale County, texted this week to say he was finding the first western corn rootworm adults of the season.

Moths and bugs: A strange start to the season

The 2019 season is beginning on an odd note. First we had an unusually heavy flight of fall armyworms, and the larvae are easy to find in early corn and sorghum. Then this week my western bean cutworm traps began picking up large numbers of yellowstriped armyworm (YSAW) moths (70 in one night). It has been several years since I ran pheromone traps for western bean cutworm, and in the past they did not catch yellowstriped armyworm. Either the pheromone lure has changed over time, or the lures have always been partially attractive to YSAW and we just have a lot of them in the system this year.

Male yellowstriped armyworms from the pheromone trap.

Next up we have unusually heavy populations of stink bugs. There have been several calls about Conchuela stink bugs in wheat and rye, and Dr. Ed Bynum addressed this in his newsletter earlier this week. Monti Vandiver, Syngenta, sent us a photo of some species of green stink bug at his home. We have yet to detect large numbers of these in the field.

In summary, 2019 is starting off as a buggy, wormy year.

Last summer I described large numbers of western corn rootworm beetles and damage associated with their larvae in corn planted near Hart, Texas all the way up to the northern Panhandle. At that time I said we would know their resistance status to mCry3a Bt corn in June of 2019 because we had made a collection and sent it to a USDA lab for resistance bioassay. As it turns out, the field where these insects were collected did not quite reach the formal threshold to be declared "Unexpected Injury", so the seed companies involved (the company that sold the seed and the company that owns the rights to the toxin) would not authorize the resistance bioassay to be conducted. They are not obligated to do so when a field is not an official UXI, although of course it would be most useful to know whether we have resistance to mCry3a or not. 

Non-rotated corn fields in the areas affected last year should be ok if planted to hybrids that contain Cry34/35. Fields planted to mCry3a (only) corn, or perhaps Cry3Bb1 (only) corn, should be closely monitored for root pruning and lodging. We expect beetle emergence from corn to begin in a couple of weeks.

Texas High Plains cotton: Time to scout for thrips and wireworms

With the cotton planting in full swing and the early-planted crop making its way up, it is time to begin scouting for early-season insect pests. Wireworms are a growing problem in the region especially in fields with reduced tillage and those following grain crops. Wireworm larvae damage cotton by feeding on the root, hypocotyl (stem of the germinating seedling), and cotyledon of plants before emerging from the soil. Wireworm injury usually results in stunting; however, heavy pressure can kill plants and reduce plant stand substantially. There are no rescue treatments for wireworms--early field scouting will help making timely replant decisions if necessary.

Wireworm injury (Photo: Pat Porter)

 

Thrips are the number one insect-pest of seedling cotton in the region. Their feeding causes foliar deformity (leaves crinkle and cup upward), plant stunting and delays in maturity. Thrips species composition in the Texas High Plains region is mainly formed by onion thrips and the western flower thrips. Preventive insecticide seed treatments provide good control against these species up to 3 weeks after planting. However, this can vary with growing conditions and the weather. When scouting for thrips, there is truly no substitute for whole plant inspections from a representative sample from across the whole field. It is important to remember that there will always be adult thrips on cotton. If plants are growing well, presence of adult thrips alone will not warrant foliar insecticide application. The presence of immature thrips (wingless) is a good indicator of whether the seed treatments are running their course and reproduction is taking place. Consider applying a foliar insecticide at the first or second true leaf stage when the emerging leaf shows signs of thrips injury and especially if immature thrips are present. Also, beware of “look-a-like” thrips symptoms from sandblasting, residual herbicides and high temperatures. As the cotton emerges, it is very important to keep a close watch at the early season pests to make timely management decisions and give a good head start to the crop.

Thrips injury

Additional information on thrips management can be found at: http://lubbock.tamu.edu/files/2017/05/ENTO-069_fn.pdf

Check out a video to learn how to scout for thrips:

https://www.youtube.com/watch?v=uD2dIDQmRb0

Wireworm fact sheet: https://agrilifecdn.tamu.edu/lubbock/files/2017/05/Wireworms_ENTO-068.pdf