Reduced rates of rootworm pesticides

Farmers throughout the Corn Belt are using soil insecticides for corn rootworm control at a rate below the amount stated on the insecticide label.

This is called a reduced rate. There two primary reasons for using reduced rates: 1) to save money by lowering the cost of growing corn; and 2) to reduce the amount of insecticide entering the environment. Farmers also are asking if a reduced rate will provide the same amount of protection as the labeled rate. This is a good question to ask.

Has Iowa State University tested the performance of reduced rates? Yes, from 1988 to 1992, five insecticides were evaluated by Jim Oleson and Jon Tollefson, Department of Entomology, 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 ft. for 15G and 1.5G material; 6 oz./1,000 row ft. for 20G material) and at 3/4 and 1/2 of the full rate. 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 3/4-label rate as at the full rate, except Force in furrow (see table). Consistency at the 1/2-label rate, compared to the full rate, also was good for Counter and Lorsban, but not for Dyfonate and Thimet. The 1/2-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, even at the full rate, no insecticide gave complete protection all the time, otherwise a 100 percent would appear in that column. But when an insecticide failed at the full rate, the 3/4-label rate failed an equal number of times. Second, you will notice that Lorsban at the 3/4-label rate had a consistency percentage that was numerically larger than Lorsban at the full-label rate. This is a result of experimental variation over years and test sites, but statistically there was no difference in root protection between the two rates.

An asterisk (*) 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 position do insecticide manufacturers take? In recent years, no manufacturer has reduced the normal rate stated on insecticide labels, therefore, no chemical manufacturer is promoting reduced rates. If a farmer decides to use a reduced rate, it should be noted that the manufacturer would not have a legal obligation to warranty the performance of a product.

What should a farmer consider before using a reduced rate? First, all insecticide boxes on the planter must be calibrated 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 3/4-label 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 it is not illegal to use a reduced rate of an insecticide.

Updated 04/28/1996 - 1:00pm