Monday, February 24, 2020

Army cutworms damaging wheat

A researcher at the Lubbock Center asked us to tell her what was eating up her ryegrass plots. Suhas Vyavhare and I checked the plots and some wheat fields near Lubbock and found abundant army cutworm larvae. The growth stages are varied from small to about one-inch, so there is plenty of damage yet to come.

Nebraska has a nice 2017 army cutworm alert that states the treatment threshold is four larvae per square foot (for grain production).  Our publication Managing Insect and Mite Pests of Texas Small Grains (page 8) says, "In outbreak years, fields can have 10 - 20 cutworms per square foot." What we saw today was not to that point, but it is still early and many larvae were small and hard to find. Typical damage includes chewing on leaves, cut plants and severed stems.

The first thing you will see when scouting is the leaf damage. During the day, the cutworms will be beneath the soil surface near the plants.

Army cutworm larva.

Clipped stem on 6-inch wheat plant.

Thursday, February 6, 2020

Know what you are buying in your Bt corn hybrids (including sweet corn)

The 2020 version of the Handy Bt Trait Table has just been published. This two-page quick reference guide lists the Bt and herbicide packages in all US field corn in an easy to follow manner. It also lists the pests the corn claims to control and those that have developed resistance to the trait packages. This year's changes include a discussion of finding European corn borer resistance to Cry1F corn in Canada. The table also now lists Bt toxins to which corn earworm (cotton bollworm) is resistant.

But wait, there's more! The first ever version of the Handy Bt Trait Table for Sweet Corn is now available as well. There are only a few Bt combinations available in sweet corn and the new table lists them all, including those for which resistance has been determined. Basically, only Vip3a sweet corn now provides good control of corn earworm in the south. The table lists all hybrids available in the US by company, hybrid and Bt type.

The Handy Bt Trait Tables for both field corn and sweet corn can be found here: https://www.texasinsects.org/bt-corn-trait-table.html

The following graph shows the number of corn earworm larvae per 25 ears of Bt and non-Bt sweet corn grown on the Experiment Station in Lubbock in 2019. It is easy to see that the older toxins are no longer working and in fact have many more larvae than the non-Bt types. (This is because the caterpillars are resistant to the older Bt toxins and also lose the behavioral tendency to cannibalize when on the Bts.) The graph also shows that Vip3a is doing a very good job of controlling corn earworm.

Tuesday, October 8, 2019

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.

Sunday, September 1, 2019

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.

Friday, August 23, 2019

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

Thursday, July 11, 2019

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.
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
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

Wednesday, July 10, 2019

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.