CROP INSIGHTS 
Corn Rootworm: Research Results and Management Options
by Steve Butzen, Murt McLeod and Tom Doerge
Summary
CRW larva, highly magnified 7. |
Corn rootworm was the most damaging pest of corn in the U.S. in 2004. Damage was amplified by a prolong-ed CRW development and feeding period due to cool summer temperatures. In addition, CRW populations that damage corn after soybeans expanded their geographical range. To help determine optimum management strategies against this pest, Pioneer has conducted extensive research studies across the Corn Belt over the last four years. This Crop Insights will discuss corn rootworm damage in 2004, Pioneer long-term research results on CRW management systems, and options available to growers in 2005 to help manage this pest.
Young corn rootworm larvae damage corn by tunneling within maize roots. Older larvae also feed at tips of nodal roots and may destroy 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.
In 2004, cooler than normal temperatures prevailed in June and July throughout most of the primary corn-growing areas of North America (Figure 1).
Average Temperature Departure from Mean in ° F.
|
Figure 1. Temperature departure from normal, June 15 to July 15, 2004. Source: Midwest Climate System (MICIS). |
These cooler temperatures slowed the development of many insects, including corn rootworms. As a consequence, corn rootworm feeding was extended longer than normal into August in some areas. Even where late feeding was not observed, damage to corn by corn rootworm feeding appeared to be greater than normal in 2004. In some areas, this feeding damage was followed by strong storms that caused root-damaged corn fields to lodge.
Pioneer Agronomy Sciences researchers conducted experiments from 2001 to 2004 to evaluate the efficacy of several management systems against CRW larval feeding and its impact on root damage, lodging and grain yield. Both replicated small plots and on-farm strip tests were used in these studies.
Small plot replicated trials were conducted at 10 to 12 locations per year from 2001 to 2003, and in 20 locations in 2004. Table 1 shows products and rates evaluated:
Table 1. Insecticide treatments evaluated in Pioneer Agronomy Sciences studies, 2001-04.
Treatment |
Application Rate |
Placement |
Poncho1 1250 |
1.25 mg per kernel |
On Seed |
Force2 3.0 G (2001-03) |
4.0 oz /1000 ft. row |
T-Band |
Aztec1 2.1 G (2004) |
6.7 oz /1000 ft. row |
T-Band |
Check |
– – |
– – |
Two Pioneer® brand hybrids with different root lodging scores were evaluated in each year of the study. All treatments were replicated four times per location in a split-plot design with insecticide treatment as the whole plot and hybrid as the sub-plot. Plots were 17.5 feet long by 10 feet (4 rows) wide. Plots were maintained consistent with local agronomic practices.
The previous crop in these studies was corn, except for two locations each year (four locations in 2004) where plots were planted on soybean stubble to evaluate the treatments against first-year western corn rootworm variants. All infestations represent natural population densities of CRW -- plots were not manually infested with rootworm eggs.
At approximately V2, the total number of plants in the center two rows of each plot was recorded as early stand count, to determine if any of the treatments either enhanced or reduced stand establishment. Five consecutive plants per plot were dug from row one in mid to late July and rated for corn rootworm larval feeding damage using a 0.00 to 3.00 node injury scale developed at Iowa State University (Table 2). At harvest, the number of root-lodged plants per plot was recorded and the middle two rows of each four-row plot were machine-harvested for grain yield.
Table 2. Corn rootworm damage rating system* used by Pioneer researchers in this study.
Rating |
Description |
0.00 |
No feeding damage (lowest possible rating) |
1.00 |
One node (circle of roots) or the equivalent of an entire node eaten back to within approximately two inches of the stalk (soil line on the 7th node) |
2.00 |
Two nodes eaten |
3.00 |
Three or more nodes eaten (highest possible rating ) |
Damage in-between complete nodes eaten is noted as the percentage of the node missing, e.g., 1.50 = 1 1/2 nodes eaten, 0.25 = 1/4 of one node eaten, etc. |
|
*Rating system developed at Iowa State University.
Results and Discussion
Corn rootworm feeding damage was at high levels at most but not all of the testing locations over the four-year study. Only those locations with significant rootworm larval feeding pressure (node injury scale ratings for the untreated check equal to or greater than 1.0) were combined for analysis each year.
|
Corn rootworm larval feeding on non-treated hybrid 8. |
Early stand count was not affected by treatment at any of the testing locations over the four-year study. This indicates that the seed treatment does not reduce germination or plant stand establishment.
Node injury scale ratings for Poncho 1250 and soil insecticides from 2001 to 2004 are presented in Figure 2. In all four years of the study, Poncho 1250-treated plots scored a 1.0 for root damage, demonstrating the consistency of this product from year to year. This score was significantly better than that of the untreated plots in all four years. Soil insecticides scores were statistically equal to those of Poncho 1250 in two years of the study, and lower (indicating less damage) in the other two years.
|
Figure 2. Control of corn rootworm with a seed treatment and a granular insecticide, 2001-04. *Reps = locations x 4 reps/location x 2 hybrids. |
Yield data for this study are presented in Figure 3. In all years, yields for plots treated with either Poncho 1250 or soils insecticide were higher than those of the untreated checks, by an average of 13.6 bu/acre. Averaged over all four years of the study, yields of Poncho 1250-treated plots and soil insecticide-treated plots were almost identical.
|
Figure 3. Grain yield (bu/acre) for Poncho 1250 and soil insecticide, 2001-2004. |
In 2004, Pioneer hybrids with the YieldGard3 Rootworm trait were added to the study as another experimental treatment. However, due to a limited number of hybrids with the YGRW trait, the genetics of these hybrids did not match those of the other treatments. A total of 20 locations in six states were included in the study. Corn rootworm damage was generally higher in the 2004 locations than in the previous three years. Consequently, the low, moderate and high infestation levels were determined differently for purposes of analyzing the 2004 data:
Infestation Category |
Average Node Injury Score in Untreated Check at a Location |
Low |
0.0 –0.74 |
Moderate |
0.75-1.75 |
High |
> 1.75 |
2004 results for root damage, root lodging and grain yield are shown in Figures 4 to 6.
|
Figure 4. 2004 CRW damage ratings for three corn rootworm technologies, 20 locations. |
Figure 4 shows that the ranking of treatments for root feed-ing damage was identical across all three infestation levels. Under low and moderate infestations, Pioneer YGRW hybrids and both insecticide treatments provided adequate protection against root damage. At high infestation levels, only Pioneer YGRW and Aztec provided adequate control.
Figure 5 shows percent root lodged plants for each treatment under low, moderate and high infestation levels in 2004.
|
Figure 5. Percent root lodging for three corn rootworm technologies, 2004. 20 locations. |
As Figure 5 indicates, root lodging was non-existent at low infestation levels, and largely prevented by all three treatments at moderate infestations. At high infestation levels, no treatment was able to prevent root lodging, but Pioneer YGRW hybrids and Aztec reduced it the most.
Grain yield under CRW pressure is the most important measure of treatment effectiveness. As Figure 6 demonstrates, Poncho 1250 plots were highest yielding under moderate CRW pressure. Under high pressure, Aztec plots yielded the most, followed by Pioneer YGRW and Poncho 1250.
|
Figure 6. Grain yield (bu/acre) for three corn rootworm technologies, 2004. 20 locations. |
On-Farm Strip Comparisons
Pioneer also conducted on-farm strip trials throughout the Corn Belt in 2004 to study corn rootworm control strategies. In a total of 262 strip trial locations, corn rootworm feeding pressure was rated after maximum larval feeding had occurred (early July to early August). Plots were rated by digging 10 plants in the strip with the untreated elite hybrid and scoring the roots using the Iowa State 0 to 3 node injury scale (Table 2). Grain yield and moisture were measured at harvest. Grain yield at low, moderate and high infestation levels is shown in Figure 7.
As the graph above indicates, Poncho 1250 was slightly higher yielding than the check, and significantly higher yielding than Pioneer YGRW hybrids at both low and moderate infestation levels. At high infestations, both Poncho 1250 and Pioneer YGRW equally out-yielded the untreated check, but the difference was not significant due to high variability in the results. (Note that the genetic backgrounds of hybrids with Poncho 1250 and YGRW were not the same.)
|
Figure 7. Grain yield (bu/acre) for Poncho 1250 and Pioneer YGRW hybrids* at various infestation levels. 141 on-farm strip trial locations, 2004. |
Corn rootworm management is essential to prevent root damage and yield loss in many areas of the U.S. and Canada. Several options are available to growers including rotation, soil insecticides, Poncho 1250 seed-applied insecticide and Pioneer hybrids with the YGRW trait.
Rotation
Growers in most areas of the U.S. can still manage CRW effectively by simply rotating to a crop other than corn for one season. Unfortunately, that simple system is no longer an option for growers in other areas where extended diapause of northern corn rootworm or the eastern variant of western corn rootworm are now predominant.
Extended diapause refers to an attribute of northern corn rootworm eggs which results in their hatching after two winters instead of just one. In parts of northern Iowa, southwestern Minnesota, and southeastern South Dakota, an estimated 40 to 60% of northern corn rootworm eggs exhibit this trait. Research studies and observations indicate that the problem is expanding in those states, putting even more corn in a two-year rotation at risk to CRW damage. Growers in affected areas must now use a different means of CRW management.
|
|
Left: Northern corn rootworm adult on corn leaf 7. |
|
Eastern variant: In an expanding area centered in eastern Illinois and western Indiana, western corn rootworms have developed the ability to deposit eggs in soybeans or other crops rather than only laying eggs in corn fields. In this region, corn is at risk regardless of the rotation, so another management system must be implemented.
Soil Insecticides
Soil-applied insecticides have been used effectively for corn rootworm control for decades. Granular products such as Aztec, Force, Counter4 and Lorsban5 are generally applied by metering boxes mounted to the planter row units. The effectiveness of Aztec and Force on corn rootworms, combined with their lower toxicity compared to some older products, have made these insecticides a popular choice among growers.
Liquid products like Capture6 and Regent4 are also applied through metering devices at the planter. University and Pioneer studies have shown that these products can be effective under low and moderate levels of corn rootworm pressure, but are often inadequate under high corn rootworm infestations.
Poncho 1250 Seed-Applied Insecticide
Poncho 1250 is a seed-applied insecticide first introduced on Pioneer corn hybrids in 2004. Because it comes on the seed and in the bag, growers will find advantages in convenience, handling and safety compared to soil-applied insecticides. Poncho 1250 also has advantages in grain marketing flexibility and hybrid availability compared to transgenic rootworm control products. Poncho 1250 has proven effective in low and moderate corn rootworm infestations, but is not recommended under high infestations.
In addition to CRW, several problematic secondary soil insects are listed on the Poncho 1250 label including wireworm, white grub, grape colaspis, and seedcorn maggot. Above-ground insects listed on the label are billbug, chinch bug, flea beetle and black cutworm.
YieldGard Rootworm Trait
Hybrids with the YieldGard Rootworm trait are another option to help manage corn rootworms. In Pioneer and university studies, these hybrids have performed as well as or better than soil insecticides in protecting corn roots from feeding damage by CRW. Yields of YGRW hybrids have been variable, depending on hybrid. Scientists caution growers to not expect absolute control of CRW with transgenic corn rootworm products.
Choosing a CRW management option should begin with an accurate understanding of the risk of CRW damage in each field. Where field history and geographic location indicate that a low or moderate risk of CRW damage exists, growers have choices of crop rotation, Poncho 1250 seed-applied insecticide, soil insecticides, or hybrids with the YGRW trait. A two-year rotation would not be an option in extended diapause or eastern variant areas.
Where field history and geographic location indicate that the field is at high or very high risk of CRW damage, growers should consider granular soil insecticides, hybrids with the YGRW trait, or crop rotation. However, in many high-risk areas with extended diapause or eastern variant problems, a rotation that includes at least two years away from corn is necessary for control.
Managing CRW is a challenging issue made even more complex by changes in insect biology that now make crop rotation ineffective in some areas. Because of this complexity, growers are encouraged to consult with their Pioneer sales professional about CRW management options. Knowledge of your field history, local CRW biology, and infestation levels in your area are instrumental to determine an effective management solution.
1 Poncho and Aztec are registered trademarks of Bayer AG.
2 Force is a registered trademark of Syngenta.
3 YieldGard Rootworm. YieldGard is a registered trademark used under license from Monsanto Co.
4 Counter and Regent are registered trademarks of BASF.
5 Lorsban is a trademark of Dow AgroSciences LLC.
6 Capture is a registered trademark of FMC Corporation.
7 Image of CRW larva and northern CRW beetle courtesy of Donald Specker.
8 Image of CRW-damaged roots courtesy of Jim Boersma.
9 Image of western corn rootworm beetle courtesy of Jim Kalisch, University of Nebraska.
