Agronomy • 2019-04-29

Integrated Management of Corn Rootworm

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Crop Insights written by Mark Jeschke, DuPont Pioneer Agronomy Research Manager

Summary

Western and northern corn rootworm have a history of adapting to and overcoming control practices, which has increased the complexity and difficulty of successfully managing these pests.
DuPont Pioneer trials conducted in 2012-2014 found that performance of Pioneer® brand Optimum® AcreMax® 1 (AM1) and Optimum® AcreMax® Xtra (AMX) products, which include the Cry34/35Ab1 Bt protein, was very consistent, even under intense corn rootworm pressure.
The addition of a soil insecticide to AMX and AM1 products tended to result in a slight increase in corn yield, although it did not always reduce rootworm feeding.
Although DuPont Pioneer research has shown that Cry34/35Ab1 remains an effective tool for corn rootworm management, maintaining the efficacy of this trait will require responsible management.
Rotation to a crop other than corn is the single most effective way to counter corn rootworms developing resistance.
Timely treatment for control of corn rootworm adults (beetles) can be very effective in reducing egg-laying and subsequent problems the following year.

Introduction

Corn rootworm is the primary pest of corn in the major corn-growing areas of North America. The impact of corn rootworm larvae on yield varies greatly depending on the timing of rootworm feeding, available soil moisture, and the hybrid’s ability to regenerate damaged roots. Plants with damaged root systems are more susceptible to drought stress and lodging. In addition to root feeding by larvae, beetle feeding on corn silks during pollination can result in poor seed set and subsequent yield loss.

Of all corn rootworm species, the western (Diabrotica virgifera virgifera) and northern (Diabrotica barberi) are the 2 most damaging species in the midwestern U.S. and in Canada. Both species can be found throughout much of the Corn Belt, often coexisting in the same fields. Other species include Mexican corn rootworm (Diabrotica virgifera zeae), which is locally important in Oklahoma and Texas, and southern corn rootworm (Diabrotica undecimpunctata howardi), which is found throughout the U.S. but rarely causes economic damage.

*Northern (left) and western (right) corn rootworm adults.*

Management Challenges

Western and northern corn rootworms have a history of adapting to and overcoming control practices, which has increased the complexity and difficulty of successfully managing these pests. Insecticide resistance has been documented in populations of western corn rootworm, both to soil applications for larva control and foliar applications for adult control. Crop rotation was long an effective and widely used management strategy; however, both species have developed adaptations that have challenged the effectiveness of soybean crop rotation in many areas.

A population of western corn rootworm, dubbed the “eastern variant,” developed the ability to defeat 2-year corn-soybean rotations by laying its eggs in soybean fields rather than corn fields. Larvae hatch the following spring into the corn year of the rotation, allowing their survival. First discovered in eastern Illinois in 1987, this population quickly spread to Indiana and eventually moved across the entire state to Ohio and Michigan counties. It also moved north and west in Illinois (Cook et al., 2005) and can now be found in southern Wisconsin and eastern Iowa as well (Dunbar and Gassmann, 2013; Prasifka et al., 2006).

Northern corn rootworm populations defeated rotation by a different adaptive mechanism – extended diapause. Diapause is a winter dormancy stage of rootworm eggs. Eggs exhibiting extended diapause remain viable in the soil for 2 or more years before hatching, allowing the insect population to survive until corn returns to the rotation. First documented in the 1920s, rotation-resistant northern corn rootworms can now be found throughout much of the northern Corn Belt. Extended diapause can last up to 4 years and has shown adaptability to rotation patterns over time; i.e., fields with corn every other year have a relatively high percentage of eggs that hatch in the second year, and fields with corn every third year tend to have more eggs that hatch the third year, etc. (Levine et al., 1992).

Bt Traits

Bt corn hybrids have been engineered to express genes isolated from the common soil bacterium Bacillus thuringiensis. The insecticidal properties of Bt were first recognized in 1901, and the use of Bt formulations as insecticides, which began in 1920 has continued to the present. Work started in the 1980s to transfer the genes that code for the insecticidal proteins into corn (Dow 2006). Bt corn hybrids were first commercially grown in the U.S. in 1996 and Bt hybrids designed to protect against corn rootworms were introduced in 2003. Bt corn has been widely adopted by corn growers in the U.S., accounting for 80% of total corn acres in 2014 (Figure 1).

Bt corn (all types) as a percent of total corn acreage in the U.S. 2001 - 2014 (USDA-ERS). — **Figure 1.** Bt corn (all types) as a percent of total corn acreage in the U.S. 2001-2014 (USDA-ERS.)

Corn rootworm Bt hybrids have been rapidly adopted in areas where corn rootworms are a concern. An Iowa State University survey found that 77% of growers who were planting corn in 2013 planned to plant hybrids with Bt corn rootworm protection (Arbuckle and Lasley, 2013). Four different Bt proteins are currently used in transgenic corn for control of corn rootworms (Table 1).

Table 1. Bt technologies currently on the market for protection against corn rootworm.

Bt Protein	Commercial Name	Year Introduced
Cry3Bb1*	YieldGard VT Rootworm/RR2™	2007
Cry34/35Ab1	Herculex® RW	2005
mCry3A	Agrisure® RW	2007
eCry3.1Ab	Agrisure Duracade™	2014

*Originally commercialized as YieldGard® Rootworm in 2003

Two Bt traits, Herculex RW and Agrisure RW, are currently used in Pioneer® brand corn products for corn rootworm protection. These include several products with Herculex RW, Agrisure RW in Pioneer® brand Optimum® TRIsect® and Optimum® AcreMax® TRIsect® products, and both traits in Pioneer brand Optimum® AcreMax® XTreme and Optimum® Intrasect® XTreme products (Table 2).

Table 2. Bt proteins in Pioneer brand corn rootworm products.

Bt Protein(s)	Product Name
Cry34/35Ab1	Herculex® RW Herculex® XTRA Optimum® AcreMax® 1* Optimum® AcreMax® RW* Optimum® AcreMax® Xtra* Optimum® Intrasect® Xtra
mCry3A	Optimum® TRIsect® Optimum® AcreMax® TRIsect®*
Cry34/35Ab1, mCry3A	Optimum® AcreMax® XTreme* Optimum® Intrasect® XTreme*

*Integrated refuge products; Bt traits present in the major components.

Insect Resistance to Bt

The potential for insect pest populations to become resistant to Bt was recognized before Bt corn entered the market, with the first instance reported in 1985 (McGaughey, 1985). To reduce the probability of insects developing resistance to Bt corn, the Environmental Protection Agency (EPA) mandated certain provisions on the use of Bt corn products. One of the most important EPA requirements is that growers implement an IRM (insect resistance management) program, which includes planting an insect refuge.

The goal of a refuge is to ensure that susceptible insects are available in sufficient numbers to mate with any resistant survivors from Bt fields. Susceptible × resistant matings dilute resistance in the population and reduce the probability of building up resistant insect populations. To be effective, the refuge must be large enough and close enough to the Bt field and be planted with a similar hybrid under similar management practices.

There are 2 types of refuge products for Pioneer brand hybrids with Bt traits: integrated and structured. Some Bt products have an integrated refuge with refuge seed blended in the bag, while other Bt products require a structured refuge. A structured refuge requires a grower to plant a portion of a field with another product that does not contain the insect-control traits of the Bt product.

DuPont Pioneer Research

Because of the history of management challenges with corn rootworm species and their propensity for overcoming control tactics, DuPont Pioneer conducts ongoing studies of corn rootworm management practices. The 2 primary objectives of these studies are to evaluate performance and efficacy of Pioneer® brand corn rootworm products and to determine the value of additional control methods, such as soil-applied insecticides, in protecting corn yield.

On-Farm Trials

On-farm strip trials were conducted at 128 midwestern locations in 2013 and 2014 to evaluate the efficacy and grain yield performance of Pioneer® brand Optimum® AcreMax® 1 (AM1), Optimum® AcreMax® Xtra (AMX) and Optimum® AcreMax® XTreme (AMXT) products with and without the use of soil-applied insecticide. Both AM1 and AMX products include Cry34/35Ab1 for rootworm protection, while AMXT products include 2 modes of action, Cry34/35Ab1 + mCry3A. Trials were placed primarily in corn-on-corn fields in areas with a history of moderate to severe corn rootworm feeding.

Results reported here are combined for AM1 and AMX products, which have the same CRW protection trait and percent integrated refuge component. Entries were not identical across all on-farm trials; consequently, summary charts and tables shown here reflect data from the subset of locations at which the products and treatments were included.

Small-Plot Trials

Small-plot trials in 2012 and 2013 evaluated the efficacy and grain yield performance of AMX and AMXT products under varying levels of corn rootworm feeding pressure. Trials conducted in 2014 also included Pioneer® brand Optimum® AcreMax® TRIsect (AMT) products in addition to AMX and AMXT products. Replicated trials were conducted at 12 locations in 2012, 9 locations in 2013, and 11 locations in 2014. Eight or 9 hybrid platforms were evaluated in each year of testing, and trial locations extended from central Nebraska to central Indiana each year.

Results

Among 128 on-farm trial locations in 2013 and 2014, high CRW pressure (defined as >1.5 on the Iowa State 0-3 NIS (Oleson et al., 2005)) was observed in 15 locations (Figure 2). Corn rootworm feeding was very intense at several of these locations. Four trials had CRW injury ratings between 2.0 and 2.5 (very high) in the unprotected check, and 5 had CRW injury greater than 2.5 (severe).

Corn rootworm insect pressure at on-farm strip trial locations in 2013 and 2014. — **Figure 2.** Corn rootworm pressure at on-farm strip trial locations in 2013 and 2014.

Review a larger map.

Corn rootworm feeding pressure was generally higher in 2012 and 2014 small-plot testing locations than in 2013. In 2012, 3 of 12 locations experienced high corn rootworm pressure (defined for these experiments as > 1.75 on the Iowa State 0-3 NIS), and 4 of 11 locations had high pressure in 2014. Conversely in 2013, 3 of 9 testing locations had moderate feeding pressure (between 0.75 and 1.75 on the Iowa 0-3 node injury scale), and none experienced high feeding pressure.

Performance of AM1 and AMX products was very consistent in 2013 on-farm trials, even under intense corn rootworm pressure (Figure 3). Average corn rootworm injury was significantly reduced relative to the unprotected check at all levels of corn rootworm pressure. Average corn rootworm injury in AM1 and AMX products was only 0.52 among the 3 locations with pressure greater than 2.5 in the check, indicating that these products continue to provide a high degree of protection under severe corn rootworm pressure.

The addition of a soil insecticide to AMX and AM1 products tended to result in a slight reduction in corn rootworm feeding, although the effect was not statistically significant even under severe corn rootworm pressure (Figure 3).

Corn rootworm injury (CRW) scores with AM1/AMX products under different degrees of CRW pressure. — **Figure 3.** Corn rootworm injury (NIS scores) observed with AM1/AMX products,
AM1/AMX products + soil-applied insecticide and no CRW protection (check)
in on-farm trials with low to moderate (n=10), high (n=4), very high (n=4)
or severe (n=3) CRW pressure.

Averages designated with the same letter within a CRW pressure grouping were not
significantly different at á = 0.05.

Small-plot trials in 2012 and 2013 evaluated performance of both AMX and AMXT products. Across 3 locations with high corn rootworm pressure in 2012, average corn rootworm feeding was 0.22 on AMX products and 0.13 on AMXT products compared to 2.02 in the unprotected check (Figure 4). Nearly identical results were observed in 2013 and 2014 on-farm trials that included both AMX and AMXT products. Average corn rootworm feeding was 0.27 on AMX products and 0.09 on AMXT products compared to 2.09 in the unprotected check.

Average corn rootworm injury (NIS scores) observed with AMX products, AMXT products and no corn rootworm protection. — **Figure 4.** Average corn rootworm injury (NIS scores) observed with AMX products,
AMXT products and no corn rootworm protection in 3 small-plot trials in 2012
and 9 on-farm trials in 2013 and 2014 with high corn rootworm pressure.

Averages designated with the same letter within a trial type grouping were not
significantly different at á = 0.05.

Small-plot trials in 2014 evaluated performance of AMT, AMX, and AMXT products with and without a soil-applied insecticide. All CRW-protected products had greatly reduced CRW damage relative to the unprotected check (Figure 5). All CRW-protected products also had small but significant reductions in CRW feeding with the addition of a soil-applied insecticide. These results contrast with those from the 2013 on-farm trials where no such reduction was observed (Figure 3).

Average corn rootworm injury (NIS scores) observed with AMT, AMX, and AMXT products with and without a soil-applied insecticide in 4 small-plot trials with high corn rootworm pressure in 2014. — **Figure 5.** Average corn rootworm injury (NIS scores) observed with AMT, AMX and
AMXT products with and without a soil-applied insecticide in 4 small-plot trials
with high corn rootworm pressure in 2014.

In all cases, corn rootworm feeding was slightly lower on AMXT products than AMX or AMT products although not always to a statistically significant degree. Corn rootworm protection would be expected to be slightly better with AMXT products due to the combined effects of trait efficacy (dual mode vs. single mode) and a lower percentage of refuge plants (5% vs. 10%).

Corn yield in on-farm trials tended to be slightly greater with the addition of a soil insecticide to a single-Bt trait product, with increases ranging from 2.9 to 3.8 bu/acre at locations with high, very high or severe corn rootworm pressure (Figure 6). A slight increase in average yield was also observed with a soil insecticide application among sites with low to moderate corn rootworm pressure, suggesting that yield improvements may be partially due to the control of insect pests other than corn rootworm.

Average yield of AM1/AMX products without and with a soil-applied insecticide in trials with low-moderate, high, very high and severe corn rootworm pressure. — **Figure 6.** Average yield of AM1/AMX products without and with a soil-applied insecticide
in trials with low-moderate, high, very high and severe corn rootworm pressure.

* significantly different at á = 0.10. **significantly different at á = 0.05.

Comparison of corn rootworm feeding and yield performance between AM1/AMX products and non-corn rootworm protected corn with an insecticide demonstrated the superior corn rootworm protection provided by the Pioneer® brand Optimum® AcreMax® family of products (Table 3). Soil-applied insecticides significantly reduced corn rootworm feeding relative to an unprotected check; however, AM1/AMX products performed significantly better than the soil-applied insecticide, both in terms of corn rootworm feeding and corn yield.

Table 3. Corn rootworm injury and corn yield of AM1/AMX products, non-corn rootworm corn with a soil-applied insecticide, and an unprotected check across 7 on-farm trials with high corn rootworm pressure in 2013.

	CRWNIS	Yield
AM1/AMX	0.45 A	210 A
Non-CRW Corn with Insecticide	1.01 B	191 B
Check	2.14 C	181 B

Averages designated with the same letter were not significantly different at á = 0.05.

Results of DuPont Pioneer on-farm and small-plot research trials showed that the Optimum AcreMax family of products with Herculex® RW provided excellent protection against corn rootworm, even when pressure was severe. AM1 and AMX products provided significantly better corn rootworm protection and greater yield than a soil-applied insecticide under high corn rootworm pressure. Addition of a soil-applied insecticide to AM1 and AMX products did not always significantly improve corn rootworm protection but did result in slightly higher corn yield in on-farm trials, likely due in part to control of insect pests other than corn rootworm.

Integrated Management Practices

Corn rootworm species have repeatedly proven their ability to overcome the continual use of a single management tactic. Rootworm populations have evolved resistance to insecticides, crop rotation, and now 2 of the 3 Bt traits that have been used for rootworm protection up to this point. Although DuPont Pioneer research has shown that Cry34/35Ab1 remains an effective tool for corn rootworm management, maintaining the efficacy of this trait, as well as successfully managing corn rootworm in the long term, will require an integrated management strategy that incorporates multiple tactics.

Break the Cycle

Rotation to a crop other than corn is the single most effective way to counter corn rootworm resistance. Rotating out of corn at least 1 in 5 years or on 20% of total acreage annually can significantly reduce selection pressure and have a positive impact on resistance management. Volunteer corn can serve as a host for corn rootworm beetles in soybeans. Controlling volunteer corn early will reduce this risk as well as competition with the soybean crop.

Even in areas where rotation-resistant populations are present, crop rotation will still reduce selection pressure and may provide some benefit in reducing corn rootworm population levels. Surveys in Illinois have found substantially lower activity of western corn rootworms in soybean fields than those observed 10 years ago, likely due to extensive use of Bt hybrids and/or soil insecticides in first-year corn (Gray, 2013). Research in Iowa found northern corn rootworm populations with extended diapauses were commonly present in first-year corn but often at levels below what would be expected to result in economic damage, indicating that rotation is still a viable corn rootworm management tactic (Dunbar and Gassmann, 2013).

Continuous use of a Bt trait places a significant amount of selection pressure on the insect population and increases the likelihood of resistance development. To help prevent this occurrence, growers should consider:

Avoiding 3 or more years of continuous use of the same Bt trait in a field.

»Where rotation to a different crop is not practical, plant a non-corn rootworm Bt product every 3 years.

-- If corn rootworm adult populations are moderate or low, soil-applied insecticides alone may provide satisfactory control the following season.

Planting corn with 2 modes of action for corn rootworm protection, such as AMXT products, to help delay resistance.

» Individuals resistant to 1 mode of action will still likely be controlled by the other.

» This is not an effective IRM strategy however, in fields where populations resistant to Cry3Bb1 and mCry3a already exist, because Cry34/35Ab1 would be the only effective mode of action.

Manage Corn Rootworm Populations

High corn rootworm populations can make management very challenging. In these situations, depending on 1 management tactic can be extremely risky. Scouting fields to determine beetle population levels allows growers to make more informed decisions on tactics necessary to successfully manage corn rootworm the following season. Start scouting when silks appear in the field and continue weekly through grain fill. The fields with the highest likelihood of heavy corn rootworm pressure are continuous corn fields.

Be aware that beetles can migrate to late-planted fields, late CRM hybrids, and delayed maturity areas, creating very high localized populations. Rootworm beetles are very attracted to fresh silks and will move and concentrate in fields where fresh silks are present. Fields that silk late due to delayed planting or maturity can act as trap crops for beetles. Timely treating of gravid or pregnant females can be very effective in reducing egg-laying and subsequent problems the following year.

Protect Yield Potential

In situations where corn rootworm pressure is expected to be extremely high, consider using a soil-applied insecticide in combination with a hybrid containing a single Bt trait for corn rootworm protection. Situations that may justify the added level of yield protection include fields that matured later than surrounding fields the previous season and likely attracted a high concentration of beetles, as well as fields where a Cry3Bb1 or mCry3a hybrid failed to provide adequate corn rootworm control the previous season. Soil-applied insecticides may also be useful to protect against secondary pest species such as seedcorn maggots, white grubs, and wireworms.

DuPont Pioneer trials conducted in 2013 found relatively little yield benefit to adding a soil-applied insecticide to AM1 and AMX products, even with extremely high corn rootworm pressure. However, some university trials have found a greater benefit on average, particularly in corn that is experiencing drought stress. A survey of several University of Illinois and Iowa State University research trials conducted between 2009 and 2013 found no yield advantage on average to adding a soil-applied insecticide when corn rootworm pressure was low, but an average yield advantage of 4.3 bu/acre under moderate pressure and 8.2 bu/acre with high corn rootworm pressure (Table 4).

Table 4. Yield advantage of soil-applied insecticides used with Herculex® XTRA and AM1 products under low (NIS < 0.5), moderate (NIS 0.5-1.5), and high CRW pressure (NIS > 1.5) in Univ. of Illinois and Iowa State Univ. trials, 2009-2013.

CRW Pressure	Locations	Average Yield Advantage
		(bu/acre)
Low (< 0.5)	9	-1.0
Moderate (0.5-1.5)	7	4.3
High (>1.5)	13	8.2

(Data summarized from Estes et al. 2012 and 2013, Tinsley et al. 2009, 2010, and 2011, and Gassmann and Weber 2010, 2011, 2012, and 2013. Some data were excluded from the summaries shown here due to abnormally low yield levels resulting from extreme drought in 2012.)

Although soil-applied insecticides are a good tool to protect roots and yield potential, they are not effective in reducing corn rootworm populations or helping in resistance management (Petzold-Maxwell et al., 2013). This is primarily due to the small zone of root coverage protected by these insecticides (typically only a 6- to 8-inch zone of protection in the application area). Corn roots can spread from row to row, leaving a large portion of the root system unprotected by insecticide. One of the advantages of Bt corn rootworm products is their protection of the entire root system. Soil-applied insecticides vary in efficacy depending on the product and environmental conditions.

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