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).

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).

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).

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.