Cooperative
Extension
Service


University of Illinois
at
Urbana-Champaign


No. 21/August 15, 1997

More Problems with Control of Rootworm Larvae with Soil Insecticides

Adding fuel to the fire described in the previous article is the rather impressive number of reports of soil insecticides that did not provide adequate root protection this year. Again, this issue has been discussed several times in articles in this Bulletin. Growers are beginning to wonder what to expect next year. Well, quite frankly, we don't know what to expect next year, but we are not ready to give up on soil insecticides. Yes, they had a tough year this year, and considerable stress was placed on their performance, or lack thereof: early planting; delayed rootworm egg hatch; cool, wet soils early, followed by dry soil conditions throughout June and July; very large densities of rootworm larvae. Under these circumstances, it is not surprising that reports of poor performance were numerous. However, I believe we have forgotten that reports of control problems with many soil insecticides occurred in the mid-1980s, again primarily due to very similar factors (although Furadan and Amaze were subject to enhanced microbial degradation). Since then we've had a decade or more of reasonably good results with soil insecticides protecting root systems from injury. Consequently, I think a lot of folks began believing that the soil insecticides were bulletproof. However, when environmental factors are not in our favor, occasionally soil insecticides don't do what they're supposed to do.

During the first week of August, John Shaw, Mike Gray, and a crew of tough and weary students finished digging roots from our insecticide-efficacy trials, both on-farm trials and our standard trials at the research farms near DeKalb and Monmouth. I include some preliminary root-rating results from the trials near DeKalb and Monmouth (Table 1 and Table 2, respectively).

Table 1. Preliminary root-rating and consistency-rating results from a soil-insecticide trial in continuous corn, DeKalb, 1997. (Experimental plot was a trap crop in 1996.)

InsecticideRate1Placement2Root rating3Percentage consistency4
Aztec 2.1G0.15band3.2560
Aztec 2.1G0.15furrow3.5545
Aztec 4.7G0.15band-SBX3.4055
Counter 20CR1.3furrow3.3565
Counter 20CR1.3band3.4053
Force 3G0.13band3.5550
Fortress 5G0.16band-SBX3.4060
Fortress 5G0.16furrow-SBX3.1075
Lorsban 30G1.3band3.5545
Lorsban 15G1.3band4.0720
Lorsban 4E1.3band4.2027
Untreated control------4.680

1All rates are specified as lb a.i. per acre, based upon a 30-inch row spacing.
2SBX: SmartBox application 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; 6, 3 or more 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 a soil-insecticide trial in continuous corn, Monmouth, 1997. (Experimental plot was a trap crop in 1996.)

InsecticideRate1Placement2Root rating3Percentage consistency4
Aztec 2.1G0.15band4.3015
Aztec 2.1G0.15furrow3.8020
Aztec 4.7G0.15band-SBX3.3540
Chlorofos 15G1.3band5.000
Counter 20CR1.3furrow3.9333
Counter 20CR1.3band4.470
Force 3G0.13band3.6055
Fortress 5G0.16band-SBX4.800
Fortress 5G0.16furrow-SBX4.755
Lorsban 30G1.3band4.785
Lorsban 15G1.3band4.7510
Lorsban 4E1.3band4.930
Untreated control------5.180

1All rates are specified as lb a.i. per acre, based upon a 30-inch row spacing.
2SBX: SmartBox application 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; 6, 3 or more 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.

These trials were in locations that were planted to trap crops last year to ensure rootworm pressure this year. The trap crop idea worked. The results are not pretty. You can see that the rootworm pressure at both locations was severe (average root rating of 4.68 and 5.18 in the untreated control at DeKalb and Monmouth, respectively). My understanding is that the root ratings at DeKalb would have been higher; however, the plants were so stressed that they could not grow another node of roots. Therefore, the highest root rating given in that trial was 5.0.

None of the products performed very well at either location, regardless of rate and placement. The results from these trials mirror some of the control problems experienced by growers this year. What message can you take away from trials like these? Well, it was a tough year. Soil insecticides do what they're supposed to most of the time, but certain circumstances may result in root protection that is less than satisfactory. After we have the opportunity to see results from trials conducted at other universities this year, we may have a clearer picture of what happened with rootworm control in 1997.
Kevin Steffey and Mike Gray, Extension Entomology, (217)333-6652