Insect monitoring in soybean: what to look for during pod fill

At this point in the season, most of our insect monitoring efforts are focused on soybean. There are several pests that can damage soybean during pod fill, and proper scouting is necessary to identify and, occasionally, control these insects. While not an exhaustive list, these are some of the insects and insect relatives to be on the lookout for as the growing season winds down.

Stink bugs. Stink bug (Fig. 1) feeding during pod fill (particularly R5- R6) can reduce soybean yield and quality. These insects feed directly on the developing seeds, resulting in wrinkling, shriveling, and discoloration in addition to reductions in yield. This damage can be compounded by pathogens and weather; note also that pathogen and weather-related quality issues can sometimes be confused for stink bug damage. Unfortunately, many fields in Illinois are not thoroughly scouted for insects during pod fill, and infestations sometimes go unnoticed until the crop is graded at the elevator.

Image of brown marmorated stink bug adult in soybean

Fig. 1. While not the most common stink bug in soybean, the invasive brown marmorated stink bug has been found more frequently in Illinois over the last several years.

Image of sweep net sampling in soybean

Fig. 2. Using a sweep net to sample insects in soybean

The most effective way to scout for these insects is with a sweep net (my personal favorite) or a drop cloth. (Both of these methods are also effective for many other soybean pests). A sweep net (Fig. 2) is swung through the canopy perpendicular to the rows a set number of times (usually 25 “sweeps” per sample). With a drop cloth, a small section of row (usually 3 feet) is shaken vigorously over a cloth, and the insects that are dislodged from the soybean canopy are counted. An insecticide application is warranted if you meet or exceed the economic threshold, which is 9 per 25 sweeps with a sweep net or 1 per row foot using a drop cloth. Note that the window of residual activity provided by insecticides for stink bug control is short (generally < 1 week); therefore, preventative applications targeted to a certain growth stage are unlikely to be effective. The most effective applications are those that are made only when (and if) a damaging population occurs –a rare event in Illinois.

Spider mites. Unfortunately, we have struggled with drought stress in parts of Illinois in recent weeks. While not a problem every year (and not an insect), spider mites often become an issue when soybeans are drought stressed. Spider mite feeding causes yellow to brown discoloration of soybean foliage (Fig. 3), and can result in severe stress to the plant. The infestations often (but not always) begin at field edges. Closely examining infested foliage will reveal the mites and the webbing that they produce. Shaking the mites onto a white piece of paper and/or using a hand lens might be necessary, as they are quite small.

Image of spider mite damage in a field with close-up

Fig. 3. Soybean foliage discolored from spider mite damage; inset shows a close-up of the underside of a damaged leaf, which has a “sandblasted” appearance.

Dectes stem borer. This insect caused some unexpected damage in southern Illinois in 2018. The stem borer larva feeds on the pith inside the soybean stem. As the plant matures, the larvae can girdle stems which leads to lodging, especially in situations where harvest is delayed. While we do not have an economic threshold for this insect (or a reliable way to control the larvae with insecticides), infested fields can be identified and, where possible, prioritized for earlier harvest to reduce their potential for lodging. Adult dectes stem borers are gray, long horned beetles that can be found using a sweep net or drop cloth. The first sign of infestation by the larvae is usually “flagging” of petioles in which a dectes larva has been feeding (Fig. 4). The larvae themselves can be observed by slicing the stem in half (Fig. 5).

Image of damage to soybean petiole from dectes stem borer

Fig. 4. “Flagging” of a dying petiole that has been fed on by a dectes stem borer larva (image: Scott Stewart, University of Tennessee).

Image of dectes stem borer larva inside of a soybean stem

Fig. 5. A dectes stem borer larva inside of a soybean stem.

Bean leaf beetle. Bean leaf beetle adults (Fig. 6) tend to be both the first and the last defoliating pest to enter soybean fields. As with other defoliating insects, the decision of whether or not to treat should be made based on (1) the level of defoliation in the field (the economic threshold post-bloom in Illinois is 20% defoliation) and (2) the continued presence of the pest within the field. Estimate the overall percent defoliation by collecting individual leaflets throughout the field. There are now several smarphone apps available that can help you to “calibrate” your defoliation estimation skills (e.g., BioLeaf Foliar Analysis for Android) (Fig. 7).

Image of a bean leaf beetle adult

Fig. 6. A bean leaf beetle feeding on seedling soybean foliage.

Image of defoliated soybean leaf

Fig. 7. A partially defoliated soybean leaflet measured by a smartphone app; approximately 25% of the leaf area has been removed by insect feeding.

As always, if you have any questions or are seeing anything unusual in the field, don’t hesitate to contact me. Happy scouting!

Author contact:

Nick Seiter | nseiter@illinois.edu | 217.300.7199


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