During each of the last seven years, Iowa State University has conducted tests that have evaluated the performance of granular soil insecticides as it relates to root protection from corn rootworm larvae. The Iowa 1-6 root rating scale (given below) was used to measure insecticide performance. A rating of 3.0 or less was considered as providing adequate root protection; a rating higher than 3.0 indicates that there was a potential for economic yield loss. The following rating scale was used:

1 = no damage or only a few minor feeding scars

2 = feeding damage evident, but no roots eaten off to within 1 1/2 inches of the plant

3 = at least one root eaten off to within 1 1/2 inches of plant, but never an entire node (circle) of roots

4 = one node (circle) of roots completely removed

5 = two nodes (circles) of roots completely removed

6 = three nodes (circles) of roots completely removed

Although root ratings are a very common method of evaluation, we believe that by examining the consistency of performance over a number of years, then the root rating information is easier to appreciate. We measure consistency as a percentage of the tests where the plants were exposed to moderate or heavy feeding pressure (root ratings greater than 3.0) and the insecticide gave good root protection. Roots in the untreated plots would have to average greater than 3.0 before the root ratings from the insecticide plots could be included in the seven-year summary.

The table below presents root damage ratings and percentage consistency values for a range of moderate to heavy rootworm feeding pressure, and a variety of environmental conditions (dry springs of 1988, 1989, and 1992; wet springs of 1990 and 1991). All insecticides were applied in a T-band in front of the press wheel, except for those indicated with an (f), which are in-furrow applications. Four insecticides were consistent in providing good root protection: Counter, Dyfonate, Force, and Lorsban. Force applied in-furrow, however, did not perform as consistently as when applied in a T- band. Counter 20CR was not included in the table below, primarily because we did not have data for seven years.

However, in ISU tests over the past couple of years its performance has been equal to that of Counter 15G, no better and no worse. Furadan 15G was not included because it is being phased out of the market, but sale and use will be allowed through August 31, 1994.

                   Labeled Rates of Soil Insecticides
                   7-Year Performance (1986-1992)*

                              Root          Percentage
          Insecticide        Rating         Consistency

          Counter 15G           2.2           94 a
          Dyfonate II 20G       2.5           94 a
          Force 1.5G            2.4           92 a
          Counter 15G (f)       2.5           91 a
          Lorsban 15G           2.6           89 a
          Force 1.5G (f)        2.8           75   b
          Thimet 20G            3.0           58     c
          untreated check       4.6           0       d

          *171 replications; numbers followed by the same letter are  not significantly different (P=0.05, Ryan's Q-test).