Pioneer® brand hybrids with the Herculex® RW^ and Herculex XTRA^ Traits

• Corn growers must manage corn rootworm (CRW) to prevent losses, especially in corn-after-corn production and where rotation is no longer effective. Management options now include hybrids with Herculex® RW^ (HXRW) or Herculex XTRA^ (HXX) traits.
• Researchers evaluated HXRW and HXX performance. Pioneer and university researchers conducted studies in 2006 to evaluate the performance of hybrids with HXRW and HXX traits vs. other CRW management options.
• Hybrids with the HXRW trait had less feeding damage. Averaged across 14 university test sites, hybrids with the HXRW trait had avg. root node injury scores of 0.15. Hybrids with the YieldGard® Rootworm (YGRW) trait had over twice as much root feeding with injury scores of 0.32.
• The HXRW advantage increased under high CRW pressure. In locations with very high CRW feeding, the advantage for HXRW vs.YGRW hybrids increased (0.31 for HXRW hybrids; 0.71 for YGRW hybrids.)
• Hybrids with the HXRW trait were more consistent. Hybrids with HXRW and HXX traits had consistency ratings of 96 and 98, compared to YGRW or YieldGard Plus (YG+) hybrid ratings of 77 and 84, respectively.
• Hybrids with the HXRW trait gave season-long protection. Under very high CRW pressure in Urbana, IL, the HXRW hybrid score was 0.47 in July, and did not increase in August. The YGRW hybrid root injury score increased from 0.96 in July to 1.46 in August.
• Hybrids with the HXX trait yielded more. Across all locations, HXX hybrids outyielded Aztec-treated plots by 7 bu/acre and untreated plots by 21 bu/acre. At locations with moderate to high CRW pressure, the advantage for HXX hybrids was 11 bu/acre and 37 bu/acre, respectively.
• Hybrids with the HXRW and HXX traits yielded similarly to hybrids with the YGRW and YG+ traits, when averaged across 16 2006 Pioneer research locations or grouped by low, moderate or high CRW pressure.

Corn rootworm (CRW) is the most damaging corn pest in North America, reducing farm income by over one billion dollars annually due to management costs and lost yield. CRW larvae damage corn by tunneling within maize roots, feeding at tips of nodal roots and destroying the root’s growing point. If densities are high, one or more nodes of roots may be destroyed. Damaged plants are more susceptible to drought stress and lodging, which can result in lower yields and harvest losses. Yield losses of 10% to more than 30% can occur with moderate to high rootworm populations.

Crop rotation historically controlled this pest, but changes in rootworm biology have neutralized its effectiveness in some areas. One such area is centered in Illinois and now includes several surrounding states. This problem was first discovered in Ford County, Illinois, in 1987 (Levine, et al. 2002). A new type of western corn rootworm, dubbed the “eastern variant”, developed the ability to defeat rotation by laying its eggs in soybean fields, which then hatched into first-year corn fields. 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) (Figure 1), and can now be found in southern Wisconsin and eastern Iowa (Prasifka, et al. 2006).

Figure 1. Spread of the eastern variant population of western corn rootworm in Illinois (Cook, et al., 2005).

Another type of corn rootworm also damages first-year corn. Populations of northern CRW in some areas have developed extended diapause, a phenomenon that keeps eggs dormant in the soil for an additional season, so that larvae hatch into corn fields in two-year rotations. Extended diapause has been most prevalent in parts of South Dakota, Minnesota, northwest Iowa and northeast Nebraska. Studies and observations indicate that the problem is expanding.

In areas where crop rotation is not effective, and in corn-after-corn fields, growers must manage CRW to prevent losses. With the registration of the Herculex® RW (HXRW) and Herculex XTRA (HXX) Insect Protection traits, growers now have new options to help control this pest. HXRW is a transgenic Bt trait, providing protection against western, northern, and Mexican CRW. HXX is a stack of the Herculex I and Herculex RW traits. Pioneer hybrids with the HXX trait protect against a broad array of destructive plant and ear-feeding insects in addition to corn rootworm, including corn borer, black cutworm, western bean cutworm, and several other pests. This Crop Insights will report on research studies conducted in 2006 to evaluate the effectiveness of the HXRW and HXX traits for managing corn rootworm populations.

HXRW and HXX traits were evaluated for efficacy against CRW larval feeding in small plot replicated trials at 16 locations in 2006. These traits were tested against other CRW management options, including Aztec® granular soil applied insecticide, Poncho® 1250-treated seed, and an untreated check. In addition, competitive hybrids were chosen to represent YGRW and YG+ technologies (Table 1).

Table 1. Corn rootworm management systems and hybrids tested for efficacy against larval CRW at 16 locations, 2006.

Each treatment was replicated four times per location and one to two hybrid platforms were evaluated in each treatment at a given location. Hybrids with the appropriate CRM were selected for each growing environment. Individual plots were four rows wide by 17.4 feet in length.

In mid-July five plants were randomly selected from row one or four of the four-row plot; plants were dug and washed, and roots were evaluated for corn rootworm larval injury using the 0.0-3.0 node injury scale developed at Iowa State University (Oleson et. al). Consistency ratings were also calculated for each treatment. Consistency ratings are a measure of the percent of roots evaluated for feeding injury that had an injury level below 0.25, the economic injury level suggested by several university extension entomologists. Corn grain yield was recorded from the center two rows of the four-row plots.

Western (L) and northern (R) corn rootworm beetles.

Root Injury Ratings
A listing of all 2006 testing locations and the CRW injury score from the untreated check plots are shown in Table 2. Six locations had high infestation levels with an average node injury score that was greater than 1.0 in the untreated check. Two of the testing locations had moderate infestation levels with an average node injury score of 0.9 and eight locations had low infestation levels with an average node injury score that was less than 0.5 in the untreated check.

Table 2. Testing locations, average node injury score in the untreated check plots, and relative CRM of hybrids evaluated in 2006.

* Node injury score of the untreated check at that location.

Hybrids with HXRW and HXX traits provided excellent protection from larval corn rootworm feeding in this study. When data were averaged across six locations with an average node injury score greater than 1.0 in the untreated check, these hybrids provided protection that was better or equivalent to other available corn rootworm management systems (Figure 2).

Figure 2. Node injury scores for several CRW management systems. Data from six high-pressure locations in 2006: IA (2), IL, MN, IN and WI. Two hybrids per treatment.

Hybrids with HXRW and HXX traits had lower root node injury scores than hybrids with the YGRW and YG+ traits. This difference was statistically significant when comparing hybrids with the HXX trait to hybrids with the YGRW trait.

Consistency Ratings
Consistency ratings are another way entomologists evaluate available rootworm management tools. Consistency ratings are derived by calculating the percentage of dug roots that have a node injury score below a defined economic injury level. Several, but not all, university entomologists use 0.25 as the cutoff for calculating consistency ratings. For example, if 96 out of 100 roots that were dug and evaluated for a given treatment in this study had a node injury score equal to or less than 0.25, the consistency rating for that treatment would be 96%. Consistency ratings were calculated for each of the treatments tested in this study and results are presented in Figure 3.

HXRW hybrid root Conventional root

Figure 3. Consistency ratings for several CRW management systems. Data from six high-pressure locations in 2006: IA (2), IL, MN, IN and WI. Two hybrids per treatment.

Averaged across the six locations with an average node injury score greater than 1.0 in the untreated check, consistency ratings of HXRW and HXX technologies hybrids were 96% and 98%, respectively. Both exhibited significantly higher consistency ratings than YGRW and YG+ technologies in this study.

Grain Yield
Grain yield for various corn rootworm management systems under moderate, high, and low corn rootworm pressure are presented in Figures 4 to 6, respectively.

Averaged across all 16 locations, grain yield of hybrids with the HXX trait was significantly higher than that of Aztec®-treated, Poncho®-treated, and untreated check plots (Fig. 4).

Figure 4. Grain yield for CRW management systems across all testing locations (average untreated check node injury score = 0.8, range = 0.06-1.86). 16 locs in 2006: IA (3), IL(3), IN(2), NE(2), MN(4), CO and WI. 2-3 hybrids per treatment.

Averaged across eight locations with moderate to high corn rootworm pressure, grain yield of hybrids with the HXRW trait was 7 and 33 bushels higher than Aztec-treated and untreated check plots, respectively. Similarly, grain yield of hybrids with the HXX trait was 11 and 37 bushels higher than that of Aztec-treated and untreated check plots, respectively. Hybrids with Herculex® technologies yielded similarly to hybrids with YieldGard® technologies (Figure 5).

Figure 5. Grain yield for CRW management systems under moderate to high CRW pressure (average untreated check node injury score = 1.4) 8 2006 locations: IA (3), IL(2), IN, MN and WI. Two to three hybrids per treatment.

Even under very low corn rootworm pressure, yield of hybrids with HXRW and HXX traits was not significantly different compared to other management systems (Figure 6). Hybrids with Herculex technologies yielded similarly to hybrids with YieldGard technologies under low infestations.

Figure 6. Grain yield for CRW management systems under low CRW pressure (average untreated check node injury score = 0.2, range = 0.06-0.46). 8 locations in 2006: IL, IN, MN(3), NE(2), and CO. Two to three hybrids per treatment.

University Trials
Several university entomology researchers from the Corn Belt evaluated Herculex RW technology for efficacy against corn rootworm larval feeding in 2006. Node injury score data comparing hybrids with the HXRW trait to hybrids with the YGRW trait were combined and summarized across 14 testing environments from seven universities (Figure 7).

Figure 7. Node injury scores for in-plant CRW technologies summarized across 14 testing environments from seven universities, 2006.

Averaged across these 14 university testing locations, plots with HXRW technology had lower node injury scores in July compared to plots containing YGRW technology.

Based on node injury scores in the untreated check, the 14 testing locations were divided into moderate (0.45 to 1.00), high (1.01 to 2.00), and very high (2.01 to 3.00) infestation categories. Data are summarized in Figure 8. Under high and very high infestation locations, hybrids with the HXRW trait performed better than hybrids with the YGRW trait.

Figure 8. Node injury scores for in-plant CRW technologies summarized by level of CRW infestation. 14 testing environments from seven universities, 2006.

At various university CRW efficacy trials, hybrids with the HXRW trait were compared against conventional hybrids treated with Force® granular insecticide, and untreated. Figure 9 summarizes the results for 15 locations where CRW damage in the untreated check exceeded one node of roots destroyed.

Figure 9. Node injury scores for HXRW, Force and untreated check hybrids, 2006. Data from university CRW efficacy trials at 15 locations with moderate to high (> 1.0 feeding damage).

As Figure 9 demonstrates, hybrids with the Herculex® RW trait were more effective, on average, in preventing rootworm damage than conventional hybrids with Force.

Season Long Protection
Pioneer Agronomy Sciences researchers evaluated roots for node injury score from four high-pressure testing locations in 2005 and 5 locations in 2006. Roots were evaluated in July and then again in August to determine if hybrids with HXRW and HXX technologies provided season long protection (Figure 10). Results were similar in both years. Although node injury scores increased from July to August in the untreated check hybrid, node injury scores did not increase significantly in hybrids with Herculex technologies.

Figure 10. July and August node injury ratings of several CRW technologies, averaged across four locations, 2005.

University of Illinois researchers also evaluated plots in August to determine if corn rootworm technologies provided season long protection (Steffey, et al., 2006). At the Urbana, IL testing location, under very intense rootworm feeding pressure, the root injury score for the YieldGard® Rootworm technology increased from 0.96 in July to 1.46 in August, whereas the injury score for the Herculex XTRA technology did not increase from July to August (Figure 11).

Figure 11. July and August root damage ratings, 2006 Univ. of Illinois efficacy trials, Urbana, IL (Steffey, et al., 2006). Herculex XTRA = Pioneer® brand 34A18; YieldGard Rootworm = DK61-68. Check = DK61-72, untreated.

Cook, K.A., S.T. Ratcliffe, M.E. Gray , K.L. Steffey, 2005. Western corn rootworm variant scouting information sheet. University of Illinois, Urbana-Champaign. (ipm.uiuc.edu/fieldcrops/insects/western_corn_rootworm/wcr.pdf)

Levine, E., J.L. Spencer, S.A. Isard, D.W. Onstad, and M.E. Gray. 2002. Adaptation of the western corn rootworm to crop rotation: evolution of a new strain in response to a management practice. Amer. Ent. 48. no. 2. (www.entsoc.org/pubs/periodicals/ae/AE-2002/summer/Feature-Levine.pdf)

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) J. Econ Entomol. 98(1): 1-8).

Prasifka, P.L., J.J. Tollefson, and M.E. Rice, 2006. Rotation-resistant corn rootworms in Iowa. In Integrated Crop Management Newsletter, IC-496(21)-July 24, 2006. Iowa State University, Ames, IA. (www.ipm.iastate.edu/ipm/icm/2006/7-24/resistantcrw.html)

Steffey, K., M.Gray, R. Estes, and J. Schroeder, 2006. Preliminary root ratings from corn rootworm control trials - DeKalb, Monmouth, and Urbana. In The Bulletin, no. 21, August 18, 2006. University of Illinois, Urbana-Champaign. (www.ipm.uiuc.edu/bulletin/article.php?id=613)