Farmers throughout the Midwest are using soil insecticides for corn rootworm control at a lower rate than the amount stated on the insecticide label. This is called a reduced rate. Their primary motivations for using a reduced rate are to save money by lowering the cost of growing corn and to reduce the amount of insecticide entering the environment.

Farmers need to know if a reduced rate will provide the same amount of protection as the labeled rate. Answers to reduced rate insecticide performance questions, based on the latest available research, follow.

Has Iowa State University tested the performance of reduced rates? Yes, from 1988 through 1992, five insecticides were evaluated under a variety of environmental conditions, ranging from dry in 1988 and 1992 to very wet in 1991. Insecticides were t-banded at the full-label rate (8 oz./1,000 row feet for 15G and 1.5G material; 6 oz./1,000 row feet for 20G material) and at 34 and 12 these rates. Counter and Force also were tested in furrow. Roots were examined in July for root injury. Each insecticide was given a consistency rating based on the percentage of tests in which adequate root protection was achieved.

How did the reduced rates perform? All five insecticides were just as consistent in providing root protection at the 34-label rate as at the full rate, except Force in furrow (see table). Consistency at the 12-label rate, compared to the full rate, was also good for Counter and Lorsban, but not for Dyfonate and Thimet. The 12-label rate was not tested for Force. Similar results have been found by entomologists at other midwestern universities.

Two other items are worth noting. First, no insecticide applied at the full rate gave complete protection all the time; if it had, a 100% would appear in that column. When an insecticide failed at the full rate, the 34 rate failed an equal number of times. Second, you will notice that Lorsban at the 34-rate had a consistency percentage that was numerically larger than Lorsban at the full-label rate. This is a result of experimental variation over the years and test sites, but statistically there was no difference in root protection between the two rates.

An asterisk (*) occurring beside a number in the table indicates that the rate was statistically different from the full rate and did not provide the same level of root protection.

What is the position of the insecticide manufacturers? No manufacturer has reduced the rate normally stated on their insecticide labels during recent years. Therefore, no chemical manufacturer is promoting reduced rates. If you decide to use a reduced rate, the manufacturer would not be legally obliged to warranty the performance of their product.

What should I consider if I use a reduced rate? First, you must calibrate all insecticide boxes on the planter before planting so the exact rate can be applied. When you cut the rate, there is less room for error. Second, select an insecticide and application method that performs well at the 34 rate (see table). Third, start out on a few acres to determine if this fits into your farming operation. Plant strips at both the full rate and a reduced rate. Then dig roots in July and make comparisons in performance. Fourth, remember that it is not illegal to use a reduced rate of an insecticide.