Japanese Beetle Management Guidelines

Japanese beetles (Fig. 1) have been arriving throughout Illinois over the last couple of weeks, and are becoming pretty conspicuous in some areas. Our crops are well behind their usual progress when Japanese beetle emergence occurs, which could impact scouting and management decision making. Several of my colleagues recently wrote an in-depth article on the history, distribution and management of this pest1; you can read the full open-access article here. Some notes on management follow by crop:

Japanese beetle adult

Fig. 1. A Japanese beetle adult hanging out on a corn leaf

Corn: Silk clipping is the primary concern with Japanese beetle infestations in corn. While the beetles will nibble on the leaves also, this does not amount to much. Many fields this season are likely to begin silking when Japanese beetles are at their peak, so scouting will be especially important. Silk clipping by Japanese beetles (as well as corn rootworms) can interfere with pollination. The effect of this feeding can be compounded by heat and drought stress2, which could be an issue in many fields this year given the late timing of pollination. Feeding tends to be concentrated on field edges, so thorough scouting within the field is necessary to determine if a treatment is justified. A rescue treatment with an insecticide is recommended if the following additions are observed:

  • Silks are being clipped to within ½ inch throughout the field
  • There are 3 or more beetles per ear (consider reducing this number if silk clipping is occuring under drought and heat-stress conditions)
  • Pollination is ongoing/less than 50% complete (especially during the first 5 days of silking).

Soybean: Control of Japanese beetles in soybean is rarely justified in Illinois, even though the damage is often conspicuous. Soybeans are fairly tolerant of defoliation in general. The only “wild card” this year is that, like corn, our soybeans are well behind their normal level of development when Japanese beetles (and other defoliators) become active. Making a rescue treatment decision for defoliators is a three-step process:

  • Determine the overall level of defoliation in the field. The recommended economic threshold is 30% defoliation prior to bloom, and 20% defoliation after bloom. Train your eye to accurately measure defoliation, and be careful not to over-estimate the extent of the damage (Fig. 2)
  • If a field is above the economic threshold, sample using a sweep net, shake sheet, or other sampling method to identify the insect responsible and verify that it is still present in the field. (Avoid “revenge” applications, which will not provide an economic return).
  • Choose an insecticide and rate that will provide effective control of the target insect. (Efficacy results from 2018 can be found in the 2018 Applied Research Results on Field Crop Pest and Disease Control report here. Results from trials conducted previously at the University of Illinois can be found in the “On Target” summaries of field crop insect management trials here.
Soybean defoliation levels

Fig. 2. Soybean leaves with differing levels of defoliation. Most observers tend to over-estimate the actual level of defoliation in the field

Most insecticides that control Japanese beetles have a relatively short period of residual control. This is no big deal in corn, as the critical period to protect silks is short anyway. In soybean, the short period of residual activity is another great reason to abide by the economic thresholds for defoliating insects; yield-reducing numbers of Japanese beetles in soybean are rare, and multiple applications for this insect are usually a wasted expense.

1 Shanovich et al. 2019. Journal of Integrated Pest Management 10: 9

2 Steckel et al. 2013. Journal of Economic Entomology 106: 2048-2054

Author contact: Nick Seiter | nseiter@illinois.edu | 217.300.7199


RSVP for the Champaign Pest and Pathogen Field Day!

Come to Champaign, Illinois on July 22nd for the first annual field crop Pest and Pathogen Field Day from 9am-noon.  Registration, doughnuts, and coffee will start at 8:30 am. Parking for the event will be available at the Agricultural and Biological Engineering farm on the UIUC South Farm Facility, located at 3603 South Race Street, Urbana, IL, 61802.  Click HERE to register.

Join us to walk research plots and learn about insect and disease identification in field crops, current research on field crop entomology, nematode, and plant disease research, and discuss local and regional production issues with entomology and plant pathology experts from the University of Illinois Department of Crop Science.

Examples of some of topics that will be discussed:

Seed treatments for suppressing soil borne diseases of soybean and corn

Lesion nematodes in corn and soybean

Understanding HG types and resistance to soybean cyst nematode

Current research projects of tar spot on corn

Bacterial leaf streak of corn

Red crown rot in soybeans

Fungicides in crop production

Mycorrhizae in crop production

Corn root worm research

Defoliators in field crops

Thrips and Soybean Vein Necrosis Virus

Understanding residual control of insect pests

Cover crops and insects

and much more!

RSVP today- this is a free field day, bring sunscreen, a hat, and plenty of questions!


Scouting for Early Season Pests in Corn and Soybean After a Late Start

It goes without saying that this spring has been a challenge. With extreme planting delays throughout the state, crop development is well behind normal expectations, while insect pest populations have continued to progress. In addition, the tight schedule we have faced has forced planting into less than ideal conditions in terms of both soil moisture and weed control, which can have consequences for insect pest management. There are a few pests in particular to target during early season scouting this season:

 

True armyworm, black cutworm, variegated cutworm

 

These insect pests are more likely to be a problem in later planted fields, especially where late burndown herbicide applications allowed weed cover to build up (unfortunately, an all too common occurrence this season). All three usually develop on weedy plant species, then move to corn or soybean when their weedy hosts mature or are killed with a herbicide; armyworms are more of a concern where there are dense populations of grasses, while black and variegated cutworms have a wider host range that includes legumes and other broadleaf plants in addition to grasses. While all of these can cause defoliation, the cutworm species can reduce stand directly when their feeding “cuts” the plant close to the ground. (Note: be sure to follow Kelly Estes on Twitter [@ILPestSurvey] for periodical updates on moth flights for true armyworm, black cutworm, and other pests).

 

Image of two variegated cutworm larvae

Fig. 1. Variegated cutworm larvae from a heavily damaged soybean field. The overall color varies quite a bit from brown to blue or gray, but look for yellow or white markings along the back (Photo: Victoria Kleczewski, Growmark)

 

image of a true armyworm larva

Fig. 2. A true armyworm larvae. Note the light colored bands on the side, the net-like pattern on the head, and the dark bands on the prolegs. Photo: Frank Peairs, Colorado State University, Bugwood.org

True armyworm larvae in wheat

Fig. 3. Several different developmental stages of true armyworm larvae in wheat. (Photo: Robert Bellm)

Slugs

 

Slugs are primarily an issue in no-till or conservation tillage fields which have a lot of residue and moisture. The wet conditions that favor slug damage can also lead to problems with seed slot closure, which exacerbates slug damage by allowing them to feed on the developing plant as the seed germinates. Unfortunately we do not have a good rescue treatment for slugs in soybean in Illinois. The best management strategy is to plant into a warm, dry seedbed (not always an option this season), and tillage is the best control we have available.

Slug in an open seed slot

Fig. 4. A slug in a seed slot left open due to wet planting conditions. (Photo: Nick Seiter)

Slug damage to a soybean cotyledon

Fig. 5. Slug damage to a soybean cotyledon (Photo: Jennifer Woodyard, University of Illinois Extension)

 

Bean leaf beetle

 

Bean leaf beetle feeding is often noticed on soybean fields that are among the earliest planted in the state; when there are relatively few acres that have emerged, the highly mobile beetles are concentrated in those few fields. Usually this damage is mostly cosmetic, as soybeans are excellent at overcoming early defoliation. The economic threshold for defoliation of soybeans prior to bloom in Illinois is an average of 30% of leaf tissue removed with the defoliator still present in the field.

Bean leaf beetle on seedling soybean

Fig. 6. A bean leaf beetle and its feeding damage on a young soybean plant (Photo: Nick Seiter)

 

Scouting is necessary to determine both the necessity and timing of an insecticide application for these insect pests. We want to avoid “revenge sprays” that occur after the insect has either progressed through its life cycle (in the case of the caterpillar pests) or moved along to another field (bean leaf beetles) and is no longer damaging the crop. As always, feel free to contact me if you are seeing anything unusual in the field related to insect management. Here is hoping for improved conditions as the season moves forward.

 

Contact: Nick Seiter nseiter@illinois.edu Twitter: @nick_seiter

Research Assistant Professor, Field Crop Entomology

 


Insect Trapping Update: May 15

Cooperators around the state are monitoring black cutworm and true armyworm traps this spring.

 

Black Cutworm

Continued flights have occurred this last week, but at low levels overall. Reports of corn emergence have been trickling in and along with those reports are also those of black cutworm feeding. As a reminder, some management suggestions from Nick Seiter’s 2018 article (http://bulletin.ipm.illinois.edu/?p=4151):

  • Infestations are more likely in later planted corn, as delayed planting means larger cutworm larvae are present at earlier stages of corn development.
  • Black cutworm moths prefer to lay their eggs on grasses, not bare ground. Therefore, fields with grassy weeds present at or shortly before planting are more likely to experience damaging populations. Similarly, monitor fields closely if a grass cover crop (e.g., cereal rye) is terminated while corn is susceptible to cutworm damage (emergence to ~V5).
  • The economic threshold for black cutworm is 3% of plants cut with black cutworms still present in the field. Look for plants that look like they have been cut roughly with scissors close to the base (Fig. 1); plants with intact roots (Fig. 2) were most likely dug up by birds and do not represent cutworm damage. Remember, larvae (Fig. 1) do their feeding at night and hide in residue or just below the soil surface during the day, so you will have to do a little bit of digging near the base of the plant to find them.
  • Several Bt corn trait packages offer suppression of black cutworm, but these might be less effective under heavy infestations or against later stage larvae. Most pyrethroid insecticides labeled for use in corn will do an excellent job of controlling larvae as a rescue treatment; just remember that they only pay off when an economic threshold has been reached.

Projected potential cutting dates in Illinois based on black cutworm trap catches. Pinhole feeding in southwestern Illinois (photo courtesy of Kelli Bassett, Pioneer) and cut plant found in Hancock County (Stephanie Porter, Golden Harvest).

 

True Armyworm

Sporadic flights seen again this week. Numbers were overall low, with the exception of Bureau county. This is just a selection of sites to give you an idea regionally what is going on around Illinois. I have had reports of armyworm in wheat in southern Illinois – varying larval instars and densities.

True armyworm trap totals from selected sites in Illinois. Reports of armyworm in wheat of varying sizes and densities (Robert Bellm, Brase Farms).

 

Despite the cool temps and rain, insects are still out there and scouting will be key. Feel free to share field observations with us – kcook8@illinois.ed or nseiter@illinois.edu


Wheat Insects – What to Watch for This Spring

Consider adding some insect scouting to your wheat management routine this spring if you are not doing so already. While damaging insect pest infestations are pretty sporadic in Illinois, missing one can be costly. The first step in managing these infestations is knowing what to look for.

 

Aphids

 

There are several species of aphids that infest wheat in Illinois, and they can be difficult to tell apart without careful examination (the figure captions below provide some tips on how to identify the different species; note that aphids are tiny insects, and you will probably need a hand lens to distinguish them). The primary concern with aphids is their ability to transmit barley yellow dwarf virus (BYDV); however, high populations of aphids feeding on plants can reduce vigor, lead to wilting, and coat the plants in sticky “honeydew” and sooty mold. In addition, greenbug feeding introduces a toxin that can reduce yield through stunting of the plants.

 

Image of two bird cherry-oat aphids

Fig. 1. Bird cherry-oat aphid. Note the brown-ish coloration at the rear of the body and the overall round or “globular” shape compared with the other aphid species. Photo: Frank Peairs, Colorado State University, Bugwood.org

 

image of an English grain aphid

Fig. 2. English grain aphid. This species has darkened cornicles (the “tail pipes” at the rear end of the body) and a narrower overall body shape. Photo: Kansas Department of Agriculture, Bugwood.org

 

image of a greenbug aphid

Fig. 3. Greenbug (or greenbug aphid). Greenbugs can be recognized by the dark green stripe down the back of the body. Greenbugs are less common in Illinois wheat than the other two species, but they are more damaging at lower numbers due to a toxin contained in their saliva. Photo: Frank Peairs, Colorado State University, Bugwood.org.

 

Armyworm

 

Armyworms are a sporadic issue in wheat in Illinois, but under high pressure they can cause substantial damage. The species we see in the spring is often called the “true” armyworm to distinguish it from fall armyworm (which, as the name suggests, arrives later in the season) and several other species. The true armyworm caterpillar has a broad, lighter-colored stripe on either side of the body, a net-like pattern on its head, and dark bands on each proleg. Armyworms feed on leaves, resulting in a raggedy appearance. Leaf feeding itself generally does not have much of an impact unless it is severe. However, occasionally armyworm larvae clip seed heads when leaf material becomes scarce, and this can result in serious yield losses.

 

image of a true armyworm larva

Fig. 4. A true armyworm larvae. Note the light colored bands on the side, the net-like pattern on the head, and the dark bands on the prolegs. Photo: Frank Peairs, Colorado State University, Bugwood.org.

 

(Just a note: the images in this article are used under the terms of a Creative Commons Attribution License, and were obtained through www.ipmimages.org, which is an excellent insect identification resource).

Keep watching the Bulletin for seasonal updates if and when we start to see issues pop up. Until then, happy scouting!

 

Author: Nick Seiter, Research Assistant Professor, Field Crop Entomology

nseiter@illinois.edu | 217.300.7199

 


Grape Colaspis in Corn and Soybean: a Pre-Season Primer

Grape colaspis is a common insect, but it only occasionally affects corn and soybean production in Illinois. However, 2018 was that rare year where “outbreak” levels of infestation occurred in parts of the state, resulting in stand reductions and, in some cases, replanting of damaged areas. Information can be tough to come by for an occasional pest like this one, but I will summarize what we know and what we should expect going forward.

Grape colaspis adult in a soybean trifoliate

Fig. 1. A grape colaspis adult peeking out of a soybean trifoliate. The adults resemble bean leaf beetles in size and shape, but have stripes along the back (on the “elytra” which are shield-like front wings that protect the beetle’s soft body). Larvae, which feed below ground and resemble tiny white grubs, are the damaging stage. (Photo: Nick Seiter)

 

Biology. Matt Montgomery provided some detail on the life cycle, identification, and habits of grape colaspis in a Bulletin article from 2003, which can be found here: http://bulletin.ipm.illinois.edu/pastpest/articles/200311b.html. (The date of the article gives you a rough idea of the last time we had widespread issues with this insect in Illinois). A couple of key points:

  • Adult females lay clutches of eggs in the soil in fields of soybean, alfalfa, and other legumes in June and July.
  • The larvae that hatch from these eggs feed on root hairs, gradually moving to larger sections of the roots.
  • When temperatures begin to cool in the fall, the larvae burrow down 8-10 inches into the soil profile, where they spend the winter.
  • When temperatures warm in the spring, the larvae move back up the soil profile and resume feeding on roots. High populations of larvae at this time can prune roots and lead to stand reduction.

 

Management. Plants with grape colaspis damage may appear wilted, stunted, or as if they have a nutrient deficiency from above the soil surface. The damage is often patchy and most pronounced on high/well drained portions of the field. Uprooting damaged plants will reveal the larvae, which resemble white grubs but are much smaller. While plants will usually overcome the initial damage, severe feeding can kill plants and reduce stands. Because egg laying occurs in soybean, alfalfa, and other legumes, the damage can occur in rotated corn or continuous soybean.

 

Control. Any chemical control measures for grape colaspis must be applied at planting. Insecticide seed treatments with many of the same active ingredients as those used in corn and soybean have been used successfully for grape colaspis control in rice in the southern U.S. However, because of the sporadic nature of this pest in corn and soybean, there are limited data available for at-plant control options such as insecticide seed treatments and soil insecticides in the Midwestern U.S. Consider these control options on fields that have a history of grape colaspis damage, referring to the label for correct use. Like many soil-dwelling insects, there are no viable rescue treatments for grape colaspis; once damage has occurred the only management decision is whether or not replanting is necessary on part or all of the field. (There are several resources available to examine the economics of this decision, including the IL Corn Replant Calculator which can be used to guide decisions during the season: https://go.aces.illinois.edu/CornReplantCalcWeb).

 

Remember, this insect is sporadic in Illinois; history suggests that issues in 2018 do not necessarily translate to another big year in 2019. Hopefully this one will pass us by this season, but learn to recognize this insect and its damage just in case.

 

Author: Nick Seiter, Research Assistant Professor, Field Crop Entomology | nseiter@illinois.edu | 217.300.7199 | Twitter: @nick_seiter


Feedback sought on Pest Degree Day Calculator Upgrade

The Illinois Degree Day calculator has been available to Illinois producers since 2004. Hosted by the Illinois Climate Network (ICN), this pest management tool was developed to help aid producers in monitoring insect development throughout the growing season and aid in pest management decisions.

The calculator uses weather data from 19 network stations across Illinois to provide degree day accumulations and forecasts for 30 agricultural and invasive pests based on long-term averages. While this has been a great resource for many years, technology has changed, along with how information is disseminated.

Our main goal is to provide a calculator for priority pests for Illinois growers and deliver that information in way that is most useful and effective for them. In order to do that, we need your help. During February and March, we are collecting feedback from a short survey at https://go.illinois.edu/PDDSurvey. The information we collect will be used to design new tools to better communicate with growers.

The current plan is to have the new tools available by the end of 2020. During this time, the pest degree day calculator will remain available at the WARM website (https://www.isws.illinois.edu/warm/).


What effect will cold temperatures have on pests and pathogens?

Nathan Kleczewski Research assistant Professor and Extension Field Crop Pathologist

Nick Seiter- Research Assistant Professor and Extension Field Crop Entomologist

 

Many in the Illinois agricultural community are wondering what effects the recent extreme cold might have on pests and pathogens. While it would be nice if the cold temperatures we are experiencing could help to reduce our potential for pest damage, past experience tells us that the most serious pests we deal with are unlikely to be impacted much by these conditions.

Many of the pathogens and insect pests that commonly affect field crops in Illinois are well adapted to survive our winter conditions.  In many cases, pathogens produce recalcitrant survival structures (e.g. cysts in soybean cyst nematode, oospores in Phytophthora, sclerotia in white mold).  These structures allow the pathogen to survive extreme conditions including cold, drought, and flooding. Different species of insects overwinter in different life stages, including eggs (for example, western corn rootworm), larvae (Japanese beetles), pupae (corn earworm, though they do not survive the winter in most of Illinois), or adults (stink bugs). The overwintering stage has characteristics that help these insects to survive the winter, either by adjusting its physiology to better survive the cold, seeking out an overwintering site that protects it (such as soil, tree bark, or leaf litter), or both. The overwintering sites that insects find mean that they are not experiencing the same temperatures that we are when we venture outside. Wind chill has little effect for this reason (even though it has a major, unpleasant effect on us).

Extreme cold temperatures can impact some insects and plant pathogens, particularly those that may not overwinter as well (e.g. powdery mildew).  When cold weather pushes into the Southern regions of the country it can push certain diseases, such as rusts, further south, delaying disease onset in Illinois and other regions further north. The same is true of migratory insects, such as black cutworm and fall armyworm, which do not usually overwinter in Illinois; colder temperatures during winter often delay the arrival of these insects, and may ultimately lead to lower numbers. The opposite is also true – warmer than normal temperatures during the winter can allow these migratory insects to become a problem earlier in the season.

Although cold temperatures may not impact most of the diseases we encounter in Illinois field crops, fluctuation between conditions of cold and warm may have a negative impact on some diseases.  Dormancy by fungi can be broken by environmental conditions such as higher temperatures.  This is similar to what occurs in plants, where warm weather may result in trees flushing out buds and flowers.  Consequently, the wide swings in temperature that we have experienced during the 2018/19 winter may negatively impact some diseases. While some insects (such as stink bugs) can also break dormancy during brief warm periods, many of our most serious pests will stay “hunkered down” until the spring and avoid these fluctuations. Unfortunately, insects and plant diseases are unlikely to suffer as much from the recent cold as we have. The best way to reduce the impact of insects and pathogens on those cold days is to stay inside, grab a hot cup of coffee, and curl up to the latest UI Extension recommendations or UI applied research results guide.


Field Performance of Seed Treatments and Soil Insecticides for Corn Rootworm Control

Authors: Nick Seiter and Joe Spencer

Producers across east-central Illinois have enjoyed low western corn rootworm pressure for several years, due to a combination of saturating rains during rootworm egg hatch and widespread use of Bt corn hybrids. Following a low point in the rootworm population in 2015, statewide monitoring of corn and soybean fields has documented a slow western corn rootworm population rebound in some areas.  Recent low corn pest abundance (combined with lower commodity prices) provides an opportunity to become reacquainted with rootworm monitoring and non-Bt options for their management. While relying on soil insecticide or a seed treatment to protect corn roots may not fit into every growers’ operation every year, rotating among different rootworm management tactics should be considered a part of the best management practices for corn rootworms in the transgenic era. Rotating between different rootworm management tactics and Bt modes-of-action is necessary because western corn rootworm populations are evolving resistance to the Bt proteins expressed in Bt corn hybrids. In addition, monitoring adult populations in fields that will be planted to corn the following year will help to assess the need for control (whether a Bt trait or an insecticide).

In 2018, we conducted a series of field trials to evaluate control options for corn rootworm. These trials were planted following a 2017 “trap crop” of late planted corn and pumpkins to artificially increase rootworm populations in the field. Root masses (5 per plot) were removed during the early reproductive stages (R1-R3), cleaned using pressure washers, and rated for corn rootworm damage using the 0-3 Node-Injury Scale developed by researchers at Iowa State (Oleson et al. 2005). The rootworm population at this location consisted almost entirely of western corn rootworm, and previous bioassay data indicated a high level of resistance to the “Cry3” Bt traits within the population. Note that additional information and data for these trials (as well as additional insect and disease management trials) are available in the recently published “Applied Research Results on Field Crop Pest and Disease Control,” available at the following link: http://cropdisease.cropsciences.illinois.edu/wp-content/uploads/2018/12/Pestpathogenappliedresearchbook2018.pdf. In addition, readers are encouraged to consult “on Target” for summaries of applied research trials conducted by University of Illinois personnel from 2004-2014: http://ipm.illinois.edu/ontarget/.

Seed Treatments. Seed treatments are nearly ubiquitous on seed corn planted across the Corn Belt.  In our trials, the seed treatments Poncho Votivo and Poncho Votivo 2.0 offered significant root protection from corn rootworm larvae compared to an untreated control (Table 1).  For many years, some corn hybrids have been marketed with seed treatments at what has been described as the ‘rootworm rate’.  These data indicate that at modest larval pressure (ca. 1.9 on the 0-3 Node Injury Score scale), these seed treatments provide some root protection; however, based on previous studies these treatments should not be relied upon alone for control under heavy rootworm pressure. Note that all hybrids used in this trial expressed Cry3Bb1 for root protection. The relatively high root pruning observed in the untreated plots illustrates that resistance to the “Cry3” proteins is an issue at this site.

Soil-Applied Insecticides.  We tested soil-applied insecticides with a non-Bt hybrid for rootworm control, and all insecticide materials tested in 2018 reduced injury from corn rootworm larval feeding compared with the untreated control. This trial was conducted under relatively low larval pressure (1.07 on the 0-3 node-injury scale in the untreated plots), and no distinctions among the different insecticides could be made.

Before commercialization of Bt corn hybrids, a soil-applied insecticide was one of the only options available to growers anticipating economic rootworm injury in continuous or rotated corn.  Over the years, soil-applied insecticides were regularly evaluated in University of Illinois Insect Management Trials (see previously linked “on Target” reports). They typically provided significant reductions in corn rootworm larval damage to corn roots compared to untreated controls.  Oftentimes, soil-applied insecticides provided root protection equivalent to, or approaching that provided by single trait Bt corn hybrids with similar yield results (see 2013 “on Target” report). Ultimately, Bt corn’s season-long root protection that was as good as or better than a soil-applied insecticide, reduced pesticide exposure, and simplified planting operations were powerful motivations that drove adoption of Bt corn. However, use of a granular or liquid soil-applied insecticide on a non-rootworm Bt corn hybrid remains a viable tactic to protect corn roots without the use of a Bt corn hybrid. If you are interested in using one of these products and have not done so in a while, now is a good time to verify that your application equipment is in good shape. Rotating corn hybrids that incorporate Bt traits with non-Bt corn treated with a soil-applied insecticide should be considered as a strategy to slow resistance evolution, especially in areas that are currently experiencing only moderate corn rootworm pressure.

Oleson, J. D., Y. Park, T. M. Nowatzki, and J. J. Tollefson. 2005. Node-injury scale to evaluate root injury by corn rootworms (Coleoptera: Chrysomelidae). Journal of Economic Entomology 98: 1-8.

 

Table 1. Mean (± standard error) node-injury ratings of corn rootworm larval feeding injury on corn hybrids expressing the Bt trait Cry3Bb1 treated with Poncho Votivo, Poncho Votivo 2.0, or Untreated at Urbana, IL in 2018.

Treatment

Node-injury ratings

10 July (R1)

Untreated 1.83 ± 0.18 aa
Poncho Votivo 0.57 ± 0.06 b
Poncho Votivo 2.0 0.55 ± 0.10 b

a Means followed by the same letter within a column are not different based on the Fisher method of least significant difference (α = 0.05)

 

Table 2. Mean (± standard error) node-injury ratings of corn rootworm larval feeding injury on non-Bt corn treated with granular and liquid insecticides at planting at Urbana, IL in 2018.

Treatment

Node-injury ratings

10 July (R1)

Untreated 1.07 ± 0.12 aa
Capture 3RIVE 3D (16 oz/a) 0.37 ± 0.07 b
Force CS (9.9 oz/a) 0.22 ± 0.05 b
Aztec 4.67G (52.3 oz/a) 0.25 ± 0.06 b
Ampex EZb (12 oz/a) 0.15 ± 0.02 b
Poncho 1.25 mg ai/seed 0.19 ± 0.03 b
Poncho 0.5 mg ai/seed 0.31 ± 0.07 b
Ampex EZb (8 oz/a) 0.13 ± 0.01 b

a Means followed by the same letter within a column are not different based on the Fisher method of least significant difference (α = 0.05)  b Note that Ampex EZ is not labeled for use in corn at the time of this publication

Contact:

Nick Seiter nseiter@illinois.edu, University of Illinois Department of Crop Sciences

Joe Spencer spencer1@illinois.edu, University of Illinois Natural History Survey


Despite local Bt resistance, growers still have options

The development of resistance to Bt Cry toxins by the western corn rootworm is a growing concern, highlighted by the recent confirmation of field-evolved resistance to Cry34/35Ab1 in Iowa (https://www.agriculture.com/crops/corn/why-managing-corn-rootworm-just-became-more-complicated). Across the Corn Belt there are western corn rootworm populations with resistance to multiple Bt Cry toxins expressed in Bt corn. In Illinois, Bt resistance can be found in western corn rootworms from both continuous and first-year cornfields; rotation-resistant populations are vulnerable to Bt resistance. Since the first report of western corn rootworm Bt resistance in 2011, resistance to each of the Bt toxins expressed in Bt corn hybrids (i.e. Cry3Bb1, mCry3A, eCry3.1Ab and Cry34/35Ab1) has been documented at multiple Corn Belt locations (Gassmann et al. 2016: https://doi.org/10.1093/jee/tow110, Zukoff et al. 2016: https://doi.org/10.1093/jee/tow073). Repeated use of the same Bt trait(s) in the same continuous (or rotated) fields imposed strong selection for rootworm resistance to those traits. This problem is not limited to rootworms; Tabashnik and Carrière (2017) have recently documented examples of increasing Bt resistance around the globe (https://doi.org/10.1038/nbt.3974).

The introduction of corn hybrids that expressed two different Bt toxins was an important innovation in Bt technology that offered a way to slow the development of resistance and manage rootworms already resistant to a Bt trait. Because these “pyramided” Bt corn hybrids expressed Bt proteins with two different modes of action, a larva feeding on a pyramid’s roots must have resistance to both Bt proteins to survive.  Thus, pyramided hybrids provided a “second line of defense” against western corn rootworms in areas where some Bt resistance may already be present.

Cross-resistance among the structurally similar “Cry3” family of Bt Cry proteins (Cry3Bb1, mCry3A, and eCry3.1Ab) has further complicated rootworm management (Gassmann et al. 2014: https://doi.org/10.1073/pnas.1317179111). With cross-resistance, rootworms with resistance to one Cry3 toxin were also resistant to the other two. This significantly reduces grower options, as despite the availability of many different commercial Bt corn hybrids, there are functionally only two different Bt modes of action with activity against western corn rootworm beetles. Choosing an effective hybrid requires information about which traits work well against the rootworm populations in a particular field or area and information about which Bt Cry toxins are expressed in different Bt corn hybrids. Information about the Bt traits expressed in commercial Bt corn hybrids can be found in the “Handy Bt Trait Table” (https://www.texasinsects.org/bt-corn-trait-table.html).

Though western corn rootworm Bt resistance is increasing across the Corn Belt, field efficacy trials and bioassays reveal that some susceptibility to one or more proteins remains in most populations.  Furthermore, a recent publication from University of Nebraska (Reinders et al. 2018: https://doi.org/10.1371/journal.pone.0208266), showed that the level of resistance to a particular Bt protein(s) could vary from field-to-field even within a portion of a county. In east central Illinois, we have conducted a series of experiments to examine the susceptibility of local western corn rootworm populations to Bt traits.

Measuring local patterns of Bt susceptibility.

Local western corn rootworm populations have been periodically assessed for their susceptibility to the Cry toxins expressed in Bt corn hybrids.  Last spring, a variety of single trait-, pyramided- and non-Bt corn hybrids were planted for a Bt efficacy trial at the UI Agricultural and Biological Engineering (ABE) Farm in Urbana, Illinois.  Laboratory Bt resistance bioassays were also conducted using the offspring (larvae) of rootworm populations collected from the ABE farm during 2017.

Local field efficacy of Bt traits.  Bt corn hybrids expressing three of the four Bt toxins commercially available for corn rootworm management (Cry3Bb1, mCry3A, and Cry34/35Ab1; eCry3.1Ab Duracade®, a Cry3 toxin, was not tested) were evaluated as single trait hybrids and in pyramided hybrids with one other Bt protein (i.e. Cry34/35Ab1 + Cry3Bb1 proteins and Cry34/35Ab1 + mCry3A) (Table 1).

 

Table 1. Hybrid number and corn rootworm Bt trait(s) expressed in single- and pyramided Bt corn hybrids evaluated for node injury in 2018 ABE Farm Bt field efficacy trials.

aStandard commercial seed treatments were present on all hybrids. CRW: corn rootworm.

 

Local rootworm pressure has been low for several years. To assure significant western corn rootworm pressure for the efficacy trial, in 2017, the site was late-planted with corn and pumpkins to attract egg-laying adults. The 2018 trial was planted on May 2nd in a 6 x 6 grid of 8r x 9m plots. On July 18th, five roots were dug from each plot and evaluated for rootworm larval feeding injury using the Iowa State University 0-3 Node-Injury Scale (NIS)(Oleson et al. 2005: https://doi.org/10.1093/jee/98.1.1).

Western corn rootworm larval pressure in the trial was only modest leading to a pattern of variable results. The roots of non-CRW Bt hybrids sustained nearly two full nodes of injury (NIS = 1.987 ± 0.335; mean ± SE) (Table 2) which was greater than injury to all of the Bt corn hybrids, except the Cry3Bb1 single trait hybrid. The injury to roots of the mCry3A-expressing single-trait hybrid was not significantly different from injury to the Cry3Bb1- or the Cry34/35Ab1-expressing single trait hybrids.  The lowest NIS (=best root protection) occurred on the two pyramided hybrids, though this was not significantly different from the NIS for the single trait Cry34/35Ab1-expressing hybrid.

While a significant contribution of Cry34/35Ab1 to better root protection is evident from these data, the level of root protection provided by Cry3 traits (i.e. Cry3Bb1- and mCry3A-expressing single trait hybrids) compared to unprotected roots (i.e. non-CRW Bt) would be unacceptable in a commercial setting. Unsatisfactory Cry3 toxin performance is hardly surprising, given Cry3 cross-resistance, prior local evidence of reduced Cry3 Bt toxin efficacy, and current bioassay data (below). In single trait hybirds, Cry3 traits are unlikely to meet root protection expectations.

 

Table 2. Node Injury Scores (NIS) for Bt corn hybrids from the field efficacy trial. There were n=5 roots evaluated per replicate and n=6 replicates per each of the six corn hybrids; n=180 total roots evaluated.

aAll hybrids also expressed Lepidopteran-specific Bt traits. b Data were Log10 (NIS + 0.5) transformed before ANOVA and comparison of Least Squares Means with Tukey HSD using JMP Pro 13; untransformed means are shown. NIS scores sharing the same letter are not significantly different at alpha <0.05.

 

The importance of the Cry34/35Ab1 Bt toxin can be highlighted by pooling the NIS data according to whether the Cry34/35Ab1 Bt toxin was or was not expressed in a Bt hybrid (regardless of Cry3Bb1 or mCry3A expression) (Table 3).  After a re-analysis, the pooled Cry34/35Ab1 (+) hybrids are found to experience significantly less root injury than the two Cry34/35Ab1 (-) hybrids or the non-Bt control.

 

Table 3. Node Injury Scores (NIS) for the field efficacy trial from Table 2, data were pooled according to Bt-corn hybrid Cry34/35Ab1 protein expression category. Five roots were evaluated per replicate and n=6 replicates per each of the six corn hybrids; n=180 total roots.

aData were Log10 (NIS + 0.5) transformed before ANOVA and comparison of Least Squares Means with Tukey HSD using JMP Pro 13; untransformed means are shown.

 

Western corn rootworm Bt resistance bioassays. Single-plant Bt resistance bioassays compared the survival of larvae from a suspected-Bt resistant population and a Bt-susceptible population from a USDA laboratory colony (Brookings, SD). Larval survival was measured using corn hybrids expressing single Bt toxins (Cry3Bb1, mCry3A, or Cry34/35Ab1) and their respective non-Bt near-isoline hybrids. The suspected-Bt resistant rootworm larvae were the offspring of adults collected from the ABE field during 2017. Because, these larvae were the siblings of those attacking the plants in the field efficacy trial, we expected similar results.

Corn hybrids were obtained from the respective trait licensees and greenhouse-grown singly in 1-liter cups. When plants reached the V5-V6 stage, they were each inoculated with n=10 newly-emerged larvae. Twelve cups were inoculated for each of the six treatments, for a total of n=72 cups per rootworm population. Inoculated cups incubated for 17d at 25°C before transfer to Berlese funnels to recover surviving larvae. The bioassay was replicated twice.

 

 Table 4. Mean proportion larval survival for western corn rootworm on the roots of Bt- and non-Bt isoline hybrids in single-plant Bt-resistance bioassays.

aANOVA was performed on Log10(proportion survival + 0.5) transformed data. Untransformed data are depicted; JMP Pro 13 (2013 SAS Institute) was used to perform analyses. Means sharing the same letter within a trait family do not differ significantly (P<0.05) based on least-squares means (Tukey HSD).

 

Survival of larvae from the suspected-resistant, Urbana population on Cry3Bb1- and mCry3A-expressing corn was significantly higher than the Bt-susceptible population. The Urbana population survived as well on the Cry3 hybrids as they did on the non-Bt isoline hybrids indicating that they were resistant and had little if any susceptibility to either of the Cry3 proteins. In contrast, survival of larvae from the same population on Cry34/35Ab1-expressing corn was low and did not differ from that of Bt-susceptible WCR populations indicating that the Urbana field population was susceptible to the Cry34/35Ab1 toxin.

Results from field efficacy trials and Bt-resistance bioassays suggest that the Urbana, Illinois western corn rootworm population is susceptible to the Cry34/35Ab1 toxin, but resistant, or nearly so, to the Cry3 toxins (i.e. Cry3Bb1 and mCry3A).  Based on these data, growers anticipating economic populations of western corn rootworm should select Bt corn hybrids (pyramided hybrids) that express the Cry34/35Ab1 toxin. Whenever possible, rotating to soybean (a recommended Best Management Practice or BMP) should also be a first choice when an economic population is anticipated. The Urbana population’s minimal susceptibility (only in the field efficacy trial) to Cry3 toxins was too low to prevent significant injury to single trait hybrids and did not add to the efficacy of the Cry34/35Ab1 toxin expressed in pyramided corn hybrids.

Conclusions. Local populations of western corn rootworm in east central Illinois are still mostly susceptible to Cry34/35Ab1, despite the recent confirmation of resistance to this toxin in Iowa. The efficacy of pyramided hybrids in our area is largely dependent on this toxin, as local populations are mostly resistant to the “Cry3” toxins.  Western corn rootworm abundance monitoring is the first step when initiating an integrated pest management approach to corn rootworm. Rotating with a non-Bt alternative (e.g. non-host soybeans, a seed treatment or liquid/granular soil-applied insecticide) rather than using a Bt hybrid in every field every year helps to preserve the efficacy of Bt traits. When a Bt hybrid is used, planting pyramided Bt corn hybrids that express Cry34/35Ab1 is also an appropriate response. Finally, when field-specific monitoring indicates that rootworm populations will be below economically damaging levels in continuous or rotated cornfields, forgoing rootworm control helps to prevent needless selection of Bt-resistant individuals.  Using pest monitoring to guide selection of the appropriate management techniques fosters long-term strategies that are justified by data and responsive to field-specific pest threats.

Given evidence for field-evolved resistance to Cry3 and/or Cry34/35Ab1 traits elsewhere in the Corn Belt, it is important to protect local Bt trait efficacy by adopting IPM-based approaches. Remember that local adoption of BMPs can help mitigate resistance in problem fields. When the next generation of rootworm protection products reach the market, the new RNAi-based mode of action is slated to be pyramided with existing Cry3Bb1 and Cry34/35Ab1 Bt traits in a product called “SmartStax®Pro”. Careful stewardship of current Bt traits is in the best interest of growers. Not only does it preserve the remaining management options, but it will help to prolong and reinforce the efficacy of new products.

Joe Spencer and Nick Seiter

 

Links to helpful scientific literature:

Gassmann, A.J., J.L. Petzold-Maxwell, R.S. Keweshan, and M.W. Dunbar. 2011. Field-evolved resistance to Bt maize by western corn rootworm. PLOS ONE 6:e22629.  https://doi.org/10.1371/journal.pone.0022629

Gassmann, A.J., J.L. Petzold-Maxwell, E.H. Clifton, M.W. Dunbar, A.M. Hoffmann, D.A. Ingber, and R.S. Keweshan. 2014. Field-evolved resistance by western corn rootworm to multiple Bacillus thuringiensis toxins in transgenic maize. PNAS, 111 (14):5141-5146. https://doi.org/10.1073/pnas.1317179111

Gassmann, A.J., R.B. Shrestha, S.R.K. Jakka, M.W. Dunbar, E.H. Clifton, A.R. Paolino, D.A. Ingber, B.W. French, K.E. Masloski, J.W. Dounda, and C.R. St. Clair. 2016. Evidence of Resistance to Cry34/35Ab1 corn by western corn rootworm (Coleoptera: Chrysomelidae): root injury in the field and larval survival in plant-based bioassays. J. Economic Entomology, 109(4): 1872–1880. https://doi.org/10.1093/jee/tow110

Oleson, J.D., Y.L. Park, T.M. Nowatzki, and J.J. Tollefson. 2005. Node-injury scale to evaluate root injury by corn rootworms (Coleoptera: Chrysomelidae). J. Economic Entomology, 98:1–8. https://doi.org/10.1093/jee/98.1.1

Reinders J.D., B.D. Hitt, W.W. Stroup, B.W. French, and L.J. Meinke. 2018. Spatial variation in western corn rootworm (Coleoptera: Chrysomelidae) susceptibility to Cry3 toxins in Nebraska. PLoS ONE 13(11): e0208266. https://doi.org/10.1371/journal.pone.0208266

Tabashnik, B. and Y. Carriere. 2017. Surge in insect resistance to transgenic crops and prospects for sustainability. Nature Biotechnology. 35(10):926-935. https://doi.org/10.1038/nbt.3974

Zukoff, S.N., K.R. Ostlie, B. Potter, L.N. Meihls, A.L. Zukoff, L.French, M.R. Ellersieck, B.W. Wade French, and B.E. Hibbard. 2016. Multiple assays indicate varying levels of cross resistance in Cry3Bb1-selected field populations of the western corn rootworm to mCry3A, eCry3.1Ab, and Cry34/35Ab1. J. Economic Entomology, 109(3):1387-1398. https://doi.org/10.1093/jee/tow073