Corn rootworm insecticide consistency 7-year tests

Labeled rates of corn rootworm insecticides have been tested in conventional-till (chisel plowed and field cultivated) fields over the last 7 years. For the last 4 years, these same insecticides have been examined in no-till fields. In all data presented, insecticides went head-to-head in their ability to protect corn roots from corn rootworm larval feeding. Tests were conducted throughout the state in various soil types and under various moisture conditions. Any test that did not challenge an insecticide's performance (no obvious root pruning in the untreated rows) was not included in the analyses. Lorsban has only recently been labeled for in-furrow placement and was not included in the early tests; thus, data for Lorsban in-furrow are not presented.

Corn rootworm larval injury.

Evaluations

Corn root systems from insecticide-treated rows and untreated rows were evaluated for corn rootworm feeding damage by using the Iowa 1-6 root-rating scale with 1 indicating no visible damage or only a few minor feeding scars; 2 indicating some roots with feeding scars but none eaten off to within 1.5 inches of the plant; 3 indicating several roots eaten off to within 1.5 inches of the plant but never the equivalent of an entire node of roots destroyed; 4 indicating one node of roots destroyed or the equivalent; 5 indicating two nodes of roots destroyed or the equivalent; and 6 indicating three or more nodes of roots destroyed. Insecticide consistency ratings (percentage of time ratings were kept at 3 or below) are presented in Tables 1-3.

Table 1. Insecticide consistency in conventional-till and no-till (1992-1998).

Percentage consistencya
Insecticide Placement Combined

tillages
Conv.-Till No-Till
Force 3G T-band 95a 99a 86a
Force 3G Furrow 95a 95a 97a
Aztec 2.1G Furrow 94a 95a 93a
Counter 20CR Furrow 93a 90a 100a
Counter 20CR T-band 93a 92a 93a
Aztec 2.1G T-band 92a 97a 76a
Fortress 5G Furrow 92a 90a 96a
Lorsban 15G T-band 91a 91a 90a
Thimet 20G T-band 73b 69b 86a
Fortress 5G T-band 70b 75b 55b
Untreated check -- 0c 0c 0c

Means within a column followed by the same letter are not significantly different (P < 0.05, Ryan's Q test).

aConsistency equals the percentage of times an insecticide treatment prevented potential yield loss. Data are from 24 tests (80 conventional-till and 29 no-till replications) at various locations throughout Iowa. Untreated check root ratings were combined tillages, 4.4; conventional-till, 4.5; and no-till, 4.4.

Table 2. Consistency data set that includes Regent 80WG (1996-1998).

Insecticide Placement Percentage

consistency
Root

rating
Force 3G T-band 98a 2.1a
Force 3G Furrow 93ab 2.2a
Aztec 2.1G Furrow 93ab 2.3a
Aztec 2.1G T-band 93ab 2.3a
Lorsban T-band 91ab 2.5abc
Counter 20CR T-band 86abc 2.2ab
Fortress 5G Furrow 86abc 2.5abc
Counter 20CR Furrow 84abc 2.5ab
Fortress 5G T-band 72bc 2.5abc
Regent 80WG Furrow 68c 2.8c
Thimet 20G T-band 68c 2.9c
Untreated check -- 0d 4.1d

Data are from 10 tests (44 replications). Means within a column followed by the same letter are not significantly different (P < 0.05, Ryan's Q test).

Table 3. Consistency data set that includes Furadan 4F (1993-1998).

Insecticide Placementa Timing Percentage

consistency
Root

rating
Force 3G T-band or F At-plant 97a 2.2a
Counter 20CR T-band or F At-plant 94ab 2.2a
Furadan 4F Broadcast Post-emerge 85b 2.2a
Untreated check ---- ---- 0c 4.4b

Data are from 16 tests (64 replications). Means within a column followed by the same letter are not significantly different (P < 0.05, Ryan's Q test).

aCounter 20CR and Force 3G were applied T-band in 1993-1996, and Furrow in 1997-1998; Furadan 4F treatments were applied during the first 2 weeks of June and received no incorporation.

This article originally appeared on pages 187-188 of the IC-480(24) -- November 9, 1998 issue.

Updated 11/08/1998 - 1:00pm