Cooperative
Extension
Service


University of Illinois
at
Urbana-Champaign


No. 15/July 3, 1998

Corn Rootworm Larval Injury and Hybrid Compensation

We have had quite a few calls recently concerning the ability of hybrids to respond to rootworm larval injury. Most calls were generated after folks noticed root pruning and larvae in first-year corn fields of east-central Illinois and continuous corn fields elsewhere. To make matters worse, the wet fields this season have negated the possibility of applying rescue treatments during cultivation in many areas. So, many people are wondering how their hybrids will hold up.

From 1993 through 1996, we conducted experiments to improve our understanding of the compensatory responses of different corn hybrids to corn rootworm larval injury at the Northern Agronomy Research Center located near DeKalb and at our research farm near Urbana. Details from the 1996 experiments provide a snapshot of how different hybrids respond to rootworm damage.

On May 3 (DeKalb) and May 20 (Urbana), 1996, we planted the following corn hybrids in a corn rootworm trap-crop area (continuous corn, about 4 acres): Asgrow RX707, Burrus BX58, Crows 401, DeKalb 591, FS 6774, Garst 8501, Hughes 5500, Northrup King N6560, Pioneer 3394, Renk RK839, and Wyffels W707. Each hybrid was planted in plots that were four rows wide and about 140 feet (DeKalb) and 132 feet (Urbana) long. Two rows in each four-row plot were treated with Counter 15G applied in a band at planting time. The experimental design was a split plot with 8 and 10 replications at DeKalb and Urbana, respectively.

On August 2 (DeKalb) and July 16 (Urbana), we extracted five plants from each of one treated and one untreated row for all treatment combinations. We washed the soil from the roots and then rated them for rootworm injury, based on the 1-to-6 rating scale developed at Iowa State University. After rating them, we submerged each root system in large graduated cylinders to estimate root volume. On August 20 (DeKalb) and August 21 (Urbana), we measured root volumes again on the same number of roots for each insecticide and hybrid combination. All plots were mechanically harvested in the fall. Rainfall totals for DeKalb during May, June, July, and August were 6.7, 5.4, 8.5, and 3.5 inches, respectively. Total precipitation at Urbana for those months was 8.3, 5.7, 3.3, and 1.4 inches, respectively.

Root ratings and yields from DeKalb and Urbana are presented in Tables 1 and 2, respectively. At both sites, Counter 15G performed very well, keeping root injury below 2.5 for all hybrids. Conservatively, many entomologists suggest that ratings above 3.0 may result in economic yield loss. At DeKalb, differences in yield between treated and untreated plots were significant for only three hybrids--Burrus BX58, Garst 8501, and Pioneer 3394. Overall, root injury at DeKalb was moderate, and rainfall was plentiful in July and August. Interestingly, root volumes of hybrids in the plots treated with Counter 15G at DeKalb increased only slightly between August 2 and 20. By contrast, root volumes decreased from August 2 to August 20 in the untreated plots. We speculated that the excessive summer precipitation may have contributed to root diseases at the DeKalb site.

Table 1. Comparative root ratings and yields among 11 corn hybrids, treated with insecticides or not treated, DeKalb, Illinois, 1996.

InsecticideNo insecticide
HybridRoot
rating
Yield
(bu/A)
Root
rating
Yield
(bu/A)
Yield
difference
Asgrow
RX707
1.82167.883.31156.9510.93 NS
Burrus
BX58
2.03155.483.31141.0714.41*
Crows 4011.88153.813.23144.429.39 NS
DeKalb 5911.98152.612.87145.317.30 NS
FS 67742.15147.363.54141.445.92 NS
Garst 85012.10156.643.18137.2619.38**
Hughes 55002.03155.523.33150.654.87 NS
Northrup
King N5560
2.05125.173.38118.906.27 NS
Pioneer
3394
1.78168.713.51150.9317.78**
Renk RK8392.00145.123.45137.068.06 NS
Wyffels
W707
2.05162.533.54157.255.28 NS

* Significant at 0.05 level
** Significant at 0.01 level
NS not significant

Table 2. Comparative root ratings and yields among 11 corn hybrids, treated with insecticides or not treated, Urbana, Illinois, 1996.

InsecticideNo insecticide
HybridRoot
rating
Yield
(bu/A)
Root
rating
Yield
(bu/A)
Yield
difference
Asgrow
RX707
2.30140.914.04 124.8716.04*
Burrus
BX58
2.22157.194.36118.9838.21**
Crows 4012.12156.394.12116.8339.56**
DeKalb 5912.38154.634.40131.1923.44**
FS 67742.38159.974.28128.0731.90**
Garst 85012.00156.314.02112.6443.67**
Hughes 55002.18155.974.26129.4926.48**
Northrup
King N5560
2.20158.444.24121.9136.53**
Pioneer
3394
2.24152.663.78125.2027.46**
Renk RK8392.08142.004.22104.6737.33**
Wyffels
W707
2.18169.564.46152.7616.80*

* Significant at 0.05 level
** Significant at 0.01 level
NS not significant

A different story emerged from Urbana, with significant declines in yield for all hybrids in the untreated plots. The level of rootworm injury at Urbana was greater than at DeKalb, and rainfall in July and August at the Urbana site was significantly less than rainfall during the same months at DeKalb. Only 3.3 and 1.4 inches of rain fell at the Urbana location during July and August, respectively. Despite the lack of moisture during these months, most of the hybrids compensated for rootworm injury by growing abundant root tissue between July 16 and August 21. In the untreated plots, the percentage increases in root volume among the hybrids were: Asgrow RX707, 44.57%; Burrus BX58, 31.80%; Crows 401, 22.85%; DeKalb 591, 16.19%; FS 6774, 38.23%; Garst 8501, 4.72%; Hughes 5500, 27.68%; Northrup King N6560, 41.98%; Pioneer 3394, 40.42%; Renk RK839, 9.77%; and Wyffels W707, 24.86%.

Many questions remain unanswered about larval injury, root compensation, and final yields for this season. As our data sets from 1996 indicate, precipitation patterns, corn hybrid, amount of larval injury, and location interact in a complex fashion to determine final yield. Let us know how your "rootworm season" progresses and how your hybrid holds up underattack.

Mike Gray (m-gray4@uiuc.edu) and Kevin Steffey (ksteffey@uiuc.edu), Extension Entomology, (217)333-6652