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Issue No. 23, Article 2/October 8, 2010

Preliminary Root Ratings for 2010 University of Illinois Corn Rootworm Trials

The western corn rootworm population was down significantly in 2010 across much of Illinois. Potential reasons include back-to-back years of saturated soil conditions at the time of corn rootworm larval hatch, increasing use of Bt corn rootworm hybrids, and aerial treatments of corn and soybean fields with tank-mixes of fungicides and insecticides. All three factors are likely contributors to the low densities of western corn rootworm adults reported by many producers, researchers, crop consultants, and certified crop advisers.


Stacks of rated roots from corn rootworm trials. (Photo courtesy of Joe Spencer, Illinois Natural History Survey).

Results from our annual corn rootworm trials also revealed very low pressure in our checks (control treatments). In fact, injury in the checks at Monmouth and Perry was so low--approaching an average of zero, with no pruned roots observed--that the other experimental treatments were not rated. In my 23 summers at the University of Illinois, I've never observed this low level of pressure in our standard corn rootworm trials that followed trap-crop conditions. Consequently, Table 1 shows only the results from our experiments in DeKalb and Urbana.

Table 1. Preliminary Node-Injury Ratingsa,b for Corn Rootworm Control Products in University of Illinois Research Trials near DeKalb and Urbana, 2010.

Product

Rate

Placementc

DeKalbd

Urbanae

Soil insecticides

Aztec 2.1G (DKC 61-22)

6.7 oz/1,000 row ft

Band

0.27 bcd

0.02 f

Aztec 4.67G (Pioneer 35F40, Dekalb) & (DKC 61-22, Urbana)

3 oz/1,000 row ft

SB furrow

0.16 cd

0.06 ef

Force CS (DKC 61-22)

8 fl oz/acre

Band

0.09 d

0.13 ef

Rootworm Bt corn hybrids

Agrisure RW (84U96 3000GT)

-

-

0.54 b

0.27 e

Agrisure RW (H-8577 3000GT)

-

-

0.5 bc

0.27 e

HxXTRA (P1162XR)

-

-

0.08 d

0.1 ef

HxXTRA (35F44)

-

-

0.04 d

0.02 f

SmartStax (DKC 61-21)

-

-

0.01 d

0.0 f

SmartStax (2D692)

-

-

0.03 d

0.02 f

YieldGard VT3 (DKC 61-19)

-

-

0.23 bcd

0.01 f

Soil insecticides + rootworm Bt corn hybrids

Counter 20G + (HxXTRA, 35F44, DeKalb) and (SmartStax, DKC 61-21, Urbana)

4.5 oz/1,000 row ft

SB furrow

0.02 d

0.0 f

Force CS + Agrisure RW (84U96 3000 GT)

8 fl oz/acre

Band

0.02 d

0.03 f

Force CS + Agrisure RW (H-8577 3000 GT)

8 fl oz/acre

Band

0.02 d

0.02 f

Force CS + HxXTRA (35F44)

8 fl oz/acre

Band

0.00 d

0.02 f

Force CS + YieldGard VT3 (DKC 61-19)

8 fl oz/acre

Band

0.02 d

0.0 f

Lorsban 15G + HxXTRA (2K662)

8 oz/1,000 row ft

Band

0.02 d

0.02 f

SmartChoice 5G + HxXTRA (35F44 at DeKalb) & (SmartStax, DKC 61-21, Urbana)

3.5 oz/1,000 row ft

SB furrow

0.02 d

0.0 f

Untreated checks

DKC 61-22f

-

-

0.63 b

0.56 d

Garst 85W95 GT/CB/LLg

-

-

2.17 a

1.13 c

Golden Harvest H8577GT/CB/LLg

-

-

1.90 a

1.83 b

Mycogen ST-6808g

-

-

2.13 a

2.31 a

Pioneer 35F40g

-

-

2.49 a

2.09 ab

aNode-injury ratings are based on the 0-to-3 root-rating scale developed by Oleson et al. (2005): 0.00, no feeding damage; 1.0, one node (circle of roots), or the equivalent of an entire node, pruned back to within approximately 1.5 in. of the stalk (or soil line if roots originate above ground nodes); 2.0, two complete nodes pruned; 3.0, three or more complete nodes pruned (greatest rating that can be given).
bMeans followed by the same letter within a column do not differ significantly (P = 0.05, Duncan's New Multiple Range Test.)
cBand: insecticide applied in a 5-in. band over the planted row; furrow: insecticide directed into the seed furrow; SB furrow: insecticide applied through a SmartBox insecticide delivery system and directed into the seed furrow.
dDeKalb: Planted on May 10 into an area planted to a trap crop in 2009 (late-planted corn interplanted with pumpkins). Roots were evaluated on July 14.
eUrbana: Planted on May 14 into an area planted to a trap crop in 2009 (late-planted corn interplanted with pumpkins). Roots were evaluated on July 12.
fSeed treated with Poncho 250, 0.25 mg a.i. per seed.
gSeed treated with Cruiser 250, 0.25 mg a.i. per seed.

Even at those locations, the overall average for our five checks was only about 2.0 (two nodes of roots destroyed). The check with the lowest injury ratings was DKC 61-22, which was treated with the low rate of Poncho (0.25 mg a.i. per seed). The other checks were treated with the low rate of Cruiser (0.25 mg a.i. per seed). It has become increasingly difficult to obtain completely untreated seed (no insecticidal seed treatment) for our experiments. Neonicotinoid insecticidal seed treatments on seed corn have become the industry standard.

The soil insecticides at both locations protected roots very well, with only minor scarring on most root systems. For those producers who are choosing seed for next season, don't forget non-Bt hybrids as an option. Soil insecticides have proven reliable management tools for corn rootworms for many decades, so long as they are properly applied. Rescue treatments always remain an option for European corn borer control should this insect emerge at economic levels in isolated fields. Not surprisingly, when soil insecticides were applied in combination with Bt rootworm hybrids, very little root injury was detected. However, in my estimation, it seems doubtful that this extra investment in corn rootworm protection was worth the added cost. We are finishing our harvests and will provide yield results in the upcoming winter meetings to confirm this speculation. The stand-alone Bt rootworm hybrids provided acceptable levels of root protection at DeKalb and Urbana. The Agrisure RW treatments at DeKalb did have more root injury than some of the other Bt treatments, with approximately 1/2 node of roots pruned.

I hope this information is useful to producers making decisions on seed for 2011. Please let me know if you have any questions about these results. I extend my thanks to Ron Estes and Nick Tinsley, University of Illinois Department of Crop Sciences, for their leadership in establishing these trials. The work could not have been completed without their dedication.--Mike Gray

Author:
Mike Gray

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