Cooperative
Extension
Service


University of Illinois
at
Urbana-Champaign


No. 19/August 1, 1997

Preliminary Corn Rootworm Soil-Insecticide Efficacy Results from Urbana

On July 23, we evaluated the performance of registered and experimental soil insecticides against corn rootworm larvae on first-year and continuous corn plots at our Urbana experimental site. In the continuous corn experimental plot, the rootworm pressure in our control (no insecticide) averaged a root rating of 5.25 (5 = two nodes of roots destroyed) on the Iowa State rating scale of 1 to 6 (Table 1). This degree of root pruning is quite severe; however, many of the on-farm "digs" this season have shown that producers' fields are not immune from this level of root injury. What did these preliminary results reveal? Fortress 5G applied through the SmartBox system performed much better when applied in the furrow. This finding meshes with the on-farm observations we have witnessed this season. Secondly, Furadan 4F continued to perform very poorly, with a consistency rating of only 10 percent. It should be mentioned that over 60 percent of the plants in the Furadan 4F treatment were severely lodged. Based upon the lack of performance of Furadan 4F in our trials this season, as well as in previous years, it seems almost certain that this product will be removed from our recommendations for the 1998 season. Finally, in general the registered products held up reasonably well to the ferocious rootworm feeding in the Urbana trial. The performance was not stellar; however, it was better than in many growers' fields we've walked this summer.

In addition to the continuous corn research plot, we evaluated the performance of several registered and a few experimental insecticides in a first-year corn experiment (Table 2). Root ratings in the control averaged 4.18 (one node of roots destroyed) on the Iowa State 1-to-6 scale, continuing evidence of the first-year corn problem plaguing growers in east-central Illinois. Several treatments failed to keep root injury below a rating of 3.0; however, some popularly used products performed satisfactorily under moderate rootworm pressure.

Interest regarding insecticide performance at reduced rates continues to persist among growers throughout Illinois, particularly in east-central Illinois. Provided in Table 3 are the results from our reduced-rate trial located just north of Urbana. The plot was devoted to late-planted corn intercropped with pumpkins in 1996. This trap-crop procedure set the stage for significant rootworm pressure in 1997. Of interest was the number of times products, when applied in the same manner (for example, band, furrow, or SmartBox) and at the same formulation, failed to be rate responsive, such as (1) Aztec 4.7G applied as a band through the SmartBox system, (2) Counter 20CR applied in furrow, (3) Aztec 2.1G applied in a band, (4) Lorsban 30G applied in a band, (5) Force 3G applied in a band, (6) Chlorofos 15G applied in a band, and (7) Thimet 20G applied in a band. These results suggest that certain formulations of some products applied (for example, band or furrow) in the same manner were not responsive to the rates we used in our trials. This observation surfaced previously in our on-farm trials in the early 1990s. In essence, during those trials we found that if a reduced rate of a product failed to protect roots adequately, so did the labeled rate. Preliminary results from our 1997 reduced-rate Urbana trial tend to support this idea.

Next week we will report on the root-rating results from experimental trials located in DeKalb and Monmouth. In the mean time, don't forget to check your fields for silk clipping!

Table 1. Preliminary root-rating and consistency-rating results from continuous corn soil-insecticide efficacy trial, Urbana. (Experimental site was a trap crop in 1996.)

InsecticideRate1Placement2Root rating3Percent
consistency4
Fortress 5G0.16furrowSBX2.54100
Aztec 4.7G0.15band-SBX2.6590
Counter 20CR1.3furrow2.8595
Counter 20CR1.3band2.8590
Lorsban 15G1.3band2.9590
Chlorofos 15G1.3band3.0580
Aztec 2.1G0.15furrow3.190
Aztec 2.1G0.15band3.1580
Lorsban 30G1.3band3.275
Fortress 5G0.16band-SBX3.3560
Force 3G0.13band3.4555
Lorsban 30G1.3furrow3.4956
Thimet 20G1.3band3.650
Lorsban 4E1.3band3.7535
Furadan 4F1broadcast4.210
Control5.250

1All rates are specified as lb (AI)/acre, based upon a 30-inch row spacing.
2Microtube applications and SBX (SmartBox) applications were applied at planting.
3Five plants per treatment per replication (4) were evaluated for rootworm injury. The root-rating scale is as follows: 1, no visible damage or only a few minor feeding scars; 2, some roots with feeding scars but none pruned off to within 1.5 inches of the plant; 3, several roots eaten off to within 1.5 inches of the plant, but never an entire node; 4, one node of roots destroyed; 5, two nodes of roots destroyed; and 6, three nodes of roots destroyed.
4Consistency ratings are based upon the percent of the total number of roots examined with a root rating below 4.0.

Table. 2. Preliminary root-rating and consistency-rating results from the first-year corn soil-insecticide efficacy trial, Urbana. (Experimental plot was soybean field in 1996.)

InsecticideRate1Placement2Root rating3Percent
consistency4
Aztec 4.7G0.15band-SBX2.3100
Counter 20CR1.3band2.35100
Aztec 2.1G0.15band2.55100
Fortress 5G0.16furrowSBX2.6100
Fortress 5G0.16band-SBX2.7585
Lorsban 30G1.3band2.9585
Chlorofos 15G1.3band380
Lorsban 15G1.3band3.175
Force 3G0.13band3.265
Lorsban 4E1.3band3.2575
Thimet 20G1.3band3.6540
Control4.1830

1All rates are specified as lb (AI)/acre, based upon a 30-inch row spacing.
2Microtube applications and SBX (SmartBox) applications were applied at planting.
3Five plants per treatment per replication (4) were evaluated for rootworm injury. The root-rating scale is as follows: 1, no visible damage or only a few minor feeding scars; 2, some roots with feeding scars but none pruned off to within 1.5 inches of the plant; 3, several roots eaten off to within 1.5 inches of the plant, but never an entire node; 4, one node of roots destroyed; 5, two nodes of roots destroyed; and 6, three nodes of roots destroyed.
4Consistency ratings are based upon the percent of the total number of roots examined with a root rating below 4.0.

Table 3. Root-rating results from a reduced-rate insecticide trial in a continuous corn experimental plot near Urbana.

InsecticideRate1Placement2Root Rating3
Fortress 5G2.25furrow-SBX2.6
Counter 20CR6band2.65
Aztec 4.7G1.5band-SBX2.7
Aztec 4.7G3band-SBX2.75
Fortress 5G3furrow-SBX2.75
Aztec 4.7G2.25band-SBX2.8
Counter 20CR6furrow2.8
Counter 20CR4.5furrow2.9
Fortress 5G2.25band-SBX2.95
Aztec 2.1G5furrow3
Fortress 5G3band-SBX3
Fortress 5G1.5furrow-SBX3.05
Counter 20CR3furrow3.05
Aztec 2.1G6.7band3.05
Aztec 2.1G6.7furrow3.05
Aztec 2.1G5band3.1
Counter 20CR4.5band3.15
Fortress 5G1.5band-SBX3.3
Aztec 2.1G3.5furrow3.3
Chlorofos 15G6band3.4
Lorsban 30G3furrow3.45
Lorsban 15G6band3.45
Thimet 20G6band3.45
Lorsban 30G4band3.45
Lorsban 30G2band3.45
Force 3G2band3.5
Lorsban 30G2furrow3.5
Aztec 2.1G3.5band3.5
Force 3G4band3.5
Force 3G3band3.6
Lorsban 15G4band3.6
Counter 20CR3band3.65
Lorsban 30G4furrow3.65
Chlorofos 15G4band3.75
Chlorofos 15G8band3.8
Lorsban 15G8band3.85
Lorsban 30G3band4.2
Thimet 20G4.5band4.3
Thimet 20G3band4.5
Control5.32

1All rates are specified as ounces of product per 1,000 feet of row.
21SBX (SmartBox) applications were applied at planting.
3Five plants per treatment per replication (4) were evaluated for rootworm injury. The root-rating scale is as follows: 1, no visible damage or only a few minor feeding scars; 2, some roots with feeding scars but none pruned off to within 1.5 inches of the plant; 3, several roots eaten off to within 1.5 inches of the plant, but never an entire node; 4, one node of roots destroyed; 5, two nodes of roots destroyed; and 6, three nodes of roots destroyed.


Mike Gray and Kevin Steffey, Extension Entomology, (217)333-6652