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


And the Survey Says…

Figure 1. What pests were most prevalent in Illinois corn and soybeans in 2018? The survey says…

 

For those that attended Agronomy Day this past August, the title and above graphic may look familiar. As field and research season winds down, we’re able to finish collecting and summarizing data. One of our biggest summer projects is the annual corn and soybean survey. While some of that information was shared at Agronomy Day, the complete results are summarized below.

As a recap, this survey has been carried out across the state for several years (2011, 2013–2018). In 2018, 40 counties representing all nine crop reporting districts were surveyed, with five corn and five soybean fields surveyed in each county. These surveys have been conducted with the goal of estimating densities of common insect pests. The estimates provided in this article should not be considered a substitute for scouting individual fields and making informed pest management decisions—even areas of the state that appear to be at low risk could have contained fields with high densities of a given insect pest.

Figure 2. Average number of Japanese beetles per 100 sweeps.

As I’ve talked with growers throughout the summer, in their opinion, the top insect pest of 2018 is the Japanese beetle. And both the survey results and I agree.

Within the soybean fields surveyed, 100 sweeps were performed on both the exterior of the field (outer 2 rows) and interior (at least 12 rows beyond the field edge) using a 38-cm diameter sweep net. The insects collected in sweep samples were identified and counted to provide an estimate of the number of insects per 100 sweeps (Tables 1 and 2).

Japanese beetle populations were higher statewide compared to 2017. Western Illinois saw record numbers last year and populations stayed high in 2018. The highest Japanese beetle populations remained in western Illinois, but numbers increased dramatically in the northwest as well (from 54 beetles per 100 sweeps to 175).

Table 1. Average number of insects per 100 sweeps on the edge of the field.

 

Table 2. Average number of insects per 100 sweeps on the interior of the field.

Western corn rootworms are always a concern, but populations have been very low in recent years. In addition to sweep samples in soybeans, cornfields were sampled for western corn rootworm by counting the number of beetles on 20 consecutive plants beyond the end rows of a given field—a beetle per plant average was calculated for each field. A mild winter followed by favorable conditions at egg hatch and adult emergence helped the small populations from 2016 gain some traction in 2017 (Table 3). However, per plant averages were lower in all districts again in 2018. Populations were variable. Many fields had low to nonexistent populations, but there were fields with higher numbers. It is important to remember, fields are randomly selected. We have no knowledge of insect management strategies that are used – soil insecticides, transgenics, or foliar applications.

Table 3 Mean number of western corn rootworm beetles per plant in corn by crop reporting district and year.

As we’ve seen repeatedly, grape colaspis populations are highly variable. Despite having reports of sporadic larval injury in the spring, adult populations were lower in 2018 compared to last year. We did see more stinkbugs as well as green cloverworms and soybean loopers statewide. While the majority of the stink bugs are green and brown, we did not pick up any of the southern species like red banded and redshouldered stink bugs in the survey. Brown marmorated stink bug was found for the first time in soybean field sweeps in several counties, though.

 

Funding for survey activities was provided by the USDA National Institute of Food and Agriculture. This survey would not be possible without the hard work and contributions of many people. I would like to thank Illinois Cooperative Agriculture Pest Survey Program interns Evan Cropek, Hannah Hires, Calli Robinson, and Cale Sementi as well as Department of Crop Science intern Matt Mote.


Western Corn Rootworm: Adult Sampling and Economic Thresholds

Authors: Nick Seiter, Joe Spencer, and Kelly Estes

Based on degree day accumulations, western corn rootworm egg hatch should be underway in much of Illinois (roughly south of Peoria as of May 29; you can view your specific location using the degree day calculator here: https://www.isws.illinois.edu/warm/pestdata/sqlchoose1.asp). We are probably just over a month away from seeing the emergence of the first adult beetles. With low rootworm populations for the last several years, there has been a renewed interest in adult sampling. The only way to determine if larval densities will be high enough to justify a control action in a specific cornfield next spring is to monitor adults in the field this summer. Doing this correctly will require some preparation to obtain the correct materials. Now is a good time to review your monitoring procedures for western corn rootworm adults.

The most common monitoring tool for western corn rootworm adults is a 5.5 × 9-in yellow card trap coated in sticky material (e.g., Pherocon® AM No-Bait trap, Trécé, Inc., Adair, OK). The yellow color attracts the beetles, and when they land on the sticky substance they become trapped (Fig. 1). We recommend placing 12 of these traps uniformly throughout each field that you are monitoring beginning in late July. If the field you are monitoring is planted to corn this season (i.e., continuous corn), simply place each trap on a corn plant just above an ear. If you are monitoring a soybean field this season that will be rotated to corn next year, you will need to place each trap on a stake so that it will sit just above the soybean canopy. PVC pipes (½” diameter) are a relatively cheap and easy material that you can use to make these stakes, but wooden, metal, or plastic stakes also work.  Use poles that are long enough to allow trap height to be raised as the soybean crop grows taller.  Replace each trap once a week for 3-4 weeks, count all western corn rootworm adults stuck to the trap upon collection, and determine the average number of adults collected per trap per day.

 

Yellow sticky card trap

Figure 1. Yellow sticky card trap used to monitor western corn rootworm adults.

 

We recommend using the economic thresholds recently updated by our colleagues at Iowa State University [1] to determine if a control action is needed in corn the following spring (Table 1). If the beetle numbers you see on your traps are above these thresholds, a corn hybrid with Bt traits targeting corn rootworm or a soil insecticide is justified in that field when corn is planted the following spring. While monitoring for western corn rootworm takes some effort, it is the only way to get field-specific information on the economic need for a control tactic the following year.

Table 1. Economic thresholds for western corn rootworm in continuous or rotated corn.

Sticky Trap Location Economic Threshold
Corn (continuous corn) 2 beetles per trap per day
Soybean (rotated corn) 1.5 beetles per trap per day

 

1.             Dunbar MW, Gassmann AJ. Abundance and Distribution of Western and Northern Corn Rootworm (Diabrotica spp.) and Prevalence of Rotation Resistance in Eastern Iowa. Journal of Economic Entomology. 2013;106(1):168-80.


Increased Insect Densities Reflected in Annual Corn and Soybean Survey

 

Thirty-six counties representing the nine crop reporting districts were surveyed at the end of July/beginning of August as part of our annual statewide corn and soybean survey. The surveys were performed by sampling five corn and five soybean fields per county. For the past several years (2011, 2013–2017), surveys in corn and soybean fields have been conducted with the goal of estimating densities of common insect pests. The estimates provided in this article should not be considered a substitute for scouting individual fields and making informed pest management decisions—even areas of the state that appear to be at low risk could have contained fields with high densities of a given insect pest.

Crop Reporting Districts

Figure 1. Illinois crop reporting districts surveyed during 2017 annual corn and soybean insect survey.

 

Western corn rootworm beetles were sampled in cornfields by counting the number of beetles on 20 consecutive plants beyond the end rows of a given field—a beetle per plant average was calculated for each field. A mild winter followed by favorable conditions at egg hatch and adult emergence helped the small populations from 2016 gain some traction in 2017 (Table 1). Per plant averages are up compared to recent years, though looking at the big picture, these numbers are still considered low. The district average from the northeast (1.95 beetles per plant), was affected by a single field in LaSalle county that average 7 beetles per plant which leads to a very important point to consider with this survey. Fields are randomly selected. We have no knowledge of insect management strategies that are used – soil insecticides, transgenics, or foliar applications.

Table1

Within an adjacent soybean field, 50 or 100 sweeps were performed on both the exterior of the field (outer 2 rows) and interior (at least 12 rows beyond the field edge) using a 38-cm diameter sweep net. The insects collected in sweep samples were identified and counted to provide an estimate of the number of insects per 100 sweeps (Tables 2 and 3).

Table2

 

Table3

The number of western corn rootworm adults in soybean fields throughout the state was low as well. The greatest number of beetles in soybeans occurred in McLean County, 8.00 beetles per 100 sweeps. All other counties sampled had fewer than 5 beetles per 100 sweeps (range of 0 to 2.4 per 100 sweeps).

Japanese beetles continued to increase in number from 2016 in the western part of Illinois. Both Fulton and McDonough counties recorded over 200 beetles per 100 sweeps in several fields, with their county averages of 525 and 340 beetles per 100 interior sweeps, respectively. Undoubtedly some of the highest numbers I’ve seen in this survey.

Overall, grape colaspis numbers were higher in several districts. This follows earlier reports during the growing season of grape colaspis feeding in soybeans. Unfortunately, there is no direct correlation between grape colaspis presence in soybeans and potential for larval injury in corn the following year. Numbers continue to be variable for this insect, but were high in the east southeast counties and should bear watching in 2018.

Stink bug injury in soybeans continues to make news in the southern states. We saw little damage caused by stink bugs in this survey, though numbers were slightly higher than past years. We continue to monitor for potential spread of not only the southern species like red banded and redshouldered stink bugs, but also the spread of brown marmorated stink bug as it gets its foothold here in Illinois.

Funding for survey activities was provided by the USDA National Institute of Food and Agriculture. This survey would not be possible without the hard work and contributions of many people. I would like to thank Illinois Cooperative Agriculture Pest Survey Program interns Evan Cropek, Ryan Pavolka, Emma Sementi, Jacob Styan and Hannah Hires as well as Department of Crop Science interns Lacie Butler, Kaela Miller, and Matt Mote.


A Mixed Bag of Insect Densities in 2016 Corn and Soybean Surveys

Once again, statewide surveys of insects in corn and soybean fields were conducted during the summer of 2016.  A total of 33 counties were surveyed this year. The surveys were performed during the first week of August by sampling five corn and five soybean fields per county. For the past several years (2011, 2013–2016), surveys in corn and soybean fields have been conducted with the goal of estimating densities of common insect pests. Densities are reported for the various USDA crop reporting districts of Illinois to highlight portions of the state where the risk of economic insect damage is greatest. The estimates provided in this article should not be considered a substitute for scouting individual fields and making informed pest management decisions—even areas of the state that appear to be at low risk could have contained fields with high densities of a given insect pest.

Western corn rootworm beetles were sampled in cornfields by counting the number of beetles on 20 consecutive plants beyond the end rows of a given field—a beetle per plant average was calculated for each field. Much like 2015, the number of western corn rootworm adults in corn was very low throughout the state (Table 1).

Table 1  ∙  Mean number of western corn rootworm beetles per plant in corn by crop reporting district and year
District 2011 2013 2014 2015 2016
Northwest 0.26 0.33 0.05 0.02 0.02
Northeast 0.15 0.20 0.02 0.00 0.02
West 0.01 0.10 0.01 0.01 0.00
Central 0.35 0.37 0.74 0.02 0.05
East 0.31 0.81 0.51 0.01 0.01
West-southwest 0.01 0.20 0.06 0.00 0.01
East-southeast 0.02 0.01 0.00 0.00 0.00
Southwest 0.00 0.00 0.00 0.01 0.01
Southeast 0.00 0.03 0.01 0.00 0.02
Means were determined by counting the number of beetles on 20 consecutive plants for between 15 and 50 fields per district.

 

Within an adjacent soybean field, 50 or 100 sweeps were performed at least 12 rows beyond the field edge using a 38-cm diameter sweep net. The insects collected in sweep samples were identified and counted to provide an estimate of the number of insects per 100 sweeps. Depending on the year, five or ten pairs of corn and soybean fields were sampled at random in each county visited. The number of western corn rootworm adults in soybean fields throughout the state was very low as well. The greatest number of beetles in soybeans occurred in McLean County, 8.00 beetles per 100 sweeps. All other counties sampled had fewer than 5 beetles per 100 sweeps (range of 0 to 2.4 per 100 sweeps.)

Table 2  ∙  Mean number of various insect pests per 100 sweeps in soybean by crop reporting district and year

District

Year Bean leafbeetles Japanesebeetles Western cornrootworm beetles Grasshoppers Greencloverworms Soybeanloopers

Stink bugs

Northwest 2011 0.0 31.7 0.3 0.2 0.2 0.0 0.2
2013 0.3 28.3 1.0 0.4 0.0 0.0 0.1
2014 0.3 14.5 1.0 0.7 0.9 0.2 0.5
2015 1.1 13.4 0.0 1.6 1.9 0.1 0.5
2016 1.1 21.8 0 3.2 2.0 0.0 0.8
Northeast 2011 1.4 13.0 0.3 0.1 0.1 0.0 0.1
2013 0.5 13.8 10.0 0.5 0.2 0.1 0.0
2014 0.2 18.3 3.0 0.3 0.6 0.1 0.6
2015 0.7 12.9 0.1 1.7 2.3 0.0 0.6
2016 8.3 1.3 0.0 5.9 2.9 0.0 0.0
West 2011 0.7 9.5 0.1 0.6 0.7 0.0 0.2
2013 1.0 5.0 0.4 0.3 0.3 0.0 0.2
2014 11.7 2.1 0.2 1.2 0.4 0.2 1.5
2015 1.6 17.5 0.0 2.8 1.3 0.1 0.5
2016 0.9 89.4 0.7 1.5 6.6 1.4 0.2
Central 2011 3.3 24.1 0.9 0.5 0.1 0.0 0.1
2013 0.5 0.9 6.4 0.3 0.4 0.1 0.2
2014 2.4 0.7 18.9 0.6 2.6 0.3 0.7
2015 5.8 2.7 0.2 4.0 0.5 0.0 0.7
2016 16.8 2.0 5.2 4.0 10.0 0.0 0.0
East 2011 17.0 5.3 7.0 1.1 5.4 0.0 0.3
2013 1.4 2.2 9.8 1.0 1.4 0.0 0.1
2014 1.9 0.4 10.2 0.7 3.0 0.0 0.7
2015 5.5 2.0 0.1 3.8 2.3 0.0 0.8
2016 13.4 0.8 0.13 2.3 11.3 0 0.0
West-southwest 2011 1.4 7.0 0.0 1.3 6.1 0.0 0.5
2013 1.3 2.4 1.5 0.5 1.4 0.0 0.1
2014 1.8 7.3 0.4 0.4 0.9 0.3 1.9
2015 5.4 22.2 0.0 5.8 1.5 0.1 1.7
2016 4.0 10.5 0.3 5.2 12.8 0 0.6
East-southeast 2011 4.1 2.0 0.4 1.3 23.8 0.0 0.1
2013 1.1 0.5 0.1 0.4 1.6 0.0 0.0
2014 1.7 0.4 0.0 0.5 2.7 0.0 0.7
2015 0.9 2.7 0.0 1.7 3.4 0.5 2.1
2016 0.8 2.0 0.0 2.2 7.0 0.0 0.1
Southwest 2011 2.6 2.7 0.0 1.0 4.4 0.0 0.4
2013 1.2 0.4 0.1 0.3 3.4 0.0 0.2
2014 8.4 0.2 0.0 0.6 6.1 0.1 1.3
2015 0.8 2.1 0.0 1.1 2.7 0.0 0.3
2016 1.2 12.0 0.0 4.0 13.2 0.0 0.1
Southeast 2011 1.9 2.5 0.0 0.9 9.7 0.0 0.3
2013 0.5 0.5 1.5 0.1 2.4 0.2 0.3
2014 2.4 0.8 0.1 0.4 2.2 0.2 1.2
2015 0.2 2.5 0.0 1.1 3.3 0.1 0.3
2016 1.9 7.7 0.53 1.1 6.1 0 0.6
Means were determined by counting the number of insects in a 50- or 100-sweep sample for between 15 and 50 fields per district. The stink bug species reported here are the green stink bug and the brown stink bug.

 

Increased densities of some of the defoliating insect species were observed in several districts. Samples were screened for bean leaf beetles, Japanese beetles, grasshoppers, green cloverworm, soybean loopers, and stink bugs. As with many years, Japanese beetles “hot spots” were observed around the state. The western part of the state yielded the most impressive numbers (89.4 per 100 sweep average), with 240 per 100 sweeps and 108 per 100 sweeps recorded in Pike and Warren counties, respectively.  Interestingly, we had higher numbers across the board for green cloverworm in 2016. A few counties had noticeable bean leaf beetles in the samples (Central – 16.8 bean leaf beetles per 100 sweeps and East 13.4 bean leaf beetles per 100 sweeps). No brown marmorated stink bugs were detected in any of the soybean or cornfields that were sampled, though this species has been confirmed in many Illinois counties (Figure 1).

BMSB August 2016

Funding for survey activities was provided by the USDA National Institute of Food and Agriculture and the Illinois Soybean Association. This survey would not be possible without the hard work and contributions of many people. I would like to thank Illinois Cooperative Agriculture Pest Survey Program interns Evan Cropek, Colleen Musson, Ryan Pavolka, Emma Sementi, and Jacob Styan as well as Department of Crop Science interns Lacie Butler and Sarah Luce.