Integrated Crop Management

Corn rootworm insecticide results

Labeled rates of corn rootworm insecticides were evaluated across Iowa during the past three years in six locations per year. Five of the fields were conventional-till (chisel plowed in the fall and field cultivated in the spring). The sixth field was no-till. Tests were conducted in various soil types and under various moisture conditions. All products were evaluated in replicated side-by-side tests for their ability to protect corn roots from corn rootworm larval injury.

[1] Heavy feeding injury caused by rootworm larvae.
[2] Corn rootworms can injure corn roots.

Corn roots were rated for rootworm injury by using the Iowa 1-6 scale:

Insecticide consistency ratings were applied to the root ratings. To prevent yield loss, an insecticide should protect the roots so that no more than approximately one-third of a node of roots is eaten. If an insecticide gave this level of protection every time, it would be 100 percent consistent, indicating that the root rating was always at 2.99 or below. Basically, the higher the consistency, the better the performance.

Roots from untreated check rows, in both tillages, suffered moderate to heavy injury and averaged about 1.5 nodes (circles) of roots destroyed. Any tests that did not challenge an insecticide's performance (no obvious root pruning in the untreated rows) were excluded from the analysis.

All granular products were evaluated in-furrow or a T-band except Thimet. Thimet is the only granular insecticide in these tests that cannot be placed in-furrow because of possible seed or seedling phytotoxicity. Performance results are shown in the tables.

Table 1. Granular corn rootworm insecticide performance (1997-1999).

Insecticide Placement % Consistency Root Rating

(1-6 scale)
Aztec 2.1G Furrow 91a 2.1a
Force 3G Furrow 88a 2.2a
Force 3G T-band 86a 2.2a
Counter 20CR Furrow 84a 2.3a
Counter 20CR T-band 80ab 2.2a
Aztec 2.1 G T-band 78ab 2.3a
Fortress 5G Furrow 65bc 2.6b
Lorsban 15G T-band 63bc 2.7b
Lorsban 15G Furrow 57c 2.8b
Fortress 5G T-band 57c 2.9b
Thimet 20G T-band 56c 2.9b
Untreated Check -- 0d 4.4c

Consistency equals the percentage of times an insecticide treatment prevented potential yield loss (insecticide root rating less than or equal to 2.99 when untreated check root rating greater than or equal to 3.00).

Data from 17 tests (81 replications) at various locations throughout Iowa. Means within a column followed by the same letter are not significantly different (P < 0.05, Ryan's Q test).

Table 2. Corn rootworm insecticide performance that includes Regent (1997-1999).

Insecticide Placement % Consistency Root Rating

(1-6 scale)
Aztec 2.1G Furrow 90a 2.1a
Force 3G Furrow 88a 2.1a
Force 3G T-band 85ab 2.2a
Counter 20CR Furrow 82abc 2.3a
Aztec 2.1 G T-band 76abcd 2.3a
Counter 20CR T-band 75abcd 2.2a
Lorsban 15G T-band 65bcde 2.7b
Fortress 5G Furrow 63cde 2.6b
Fortress 5G T-band 59de 2.8b
Regent Furrow 57de 2.8b
Lorsban 15G Furrow 57de 2.8b
Thimet 20G T-band 52e 2.9b
Untreated Check -- 0f 4.4c

Data from 13 tests (60 replications) at various locations throughout Iowa. Regent 80WG formulation was tested in 1997 and 1998 (water carrier of 4 qt/acre); Regent formulation changed to 4SC in 1999 (water carrier of 8 qt/acre).

Table 3. Corn rootworm insecticide performance that includes Furadan (1997-1999).

Insecticide Placement % Consistency Root Rating

(1-6 scale)
Aztec 2.1G Furrow 93a 2.2a
Force 3G Furrow 90a 2.2a
Counter 20CR Furrow 81ab 2.4ab
Furadan 4F Broadcast 77ab 2.4ab
Fortress 5G Furrow 65bc 2.6bc
Lorsban 15G T-band 58c 2.7c
Thimet 20G T-band 57c 2.9c
Untreated Check -- 0d 4.5d

Data from 14 tests (69 replications) at various locations throughout Iowa.

Furadan 4F treatments were applied during the first 2 weeks of June and received no incorporation.

This article originally appeared on pages 13-15 of the IC-484 (2) -- February 28, 2000 issue.

Source URL: