Many farmers throughout the Midwest are applying soil insecticides for rootworm control at a rate below that stated on the insecticide label. This is called a reduced rate. The motivation behind using a reduced rate is primarily to save money by lowering the cost of growing corn. Farmers also are asking if a reduced rate will provide the same amount of protection as the labeled rate. That~s a good question.
Have Iowa State University entomologists tested the performance of reduced rates?
Yes. During the last five years, Counter, Dyfonate, Force, Lorsban, and Thimet were all tested under a variety of environmental conditions ranging from dry soil conditions in 1988 and 1992 to very wet soils 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), and at 3~4 and 1/2 these rates. Counter and Force also were tested in furrow. Insecticide performance was evaluated based on the amount of root damage and then rated on a percent consistency. Tests were conducted under conditions of both moderate and heavy rootworm feeding pressure.
How did the reduced rates perform?
Test results indicate that all five insecticides were just as consistent in providing root protection at the 3 ~4-label rate as they did at the full-label rate, except Force in furrow. Consistency at the 1/2-label rate compared to the labeled rate was also good, but only for Counter and Lorsban. Similar results have been found by entomologists in Illinois, Indiana, Minnesota, and South Dakota.
What is ISU Extension's official position?
We do not recommend the use (of) soil insecticides at reduced rates. Iowa State University is not responsible for the performance of insecticides used at less than the labeled rate.
What is the position of the insecticide manufacturers?
There are under no legal obligation to guarantee the performance of their products when used below the minimum labeled rate.
What if a farmer wants to use reduced rates anyway?
It is not illegal to use less pesticide. But in addition to the statements above, farmers should consider several other factors before experimenting with the idea. First, all insecticide boxes on the planter must be calibrated before planting so that the exact rate can be applied. Second, reducing the rate to 1/2 the labeled rate may be pushing the performance expectations too far. Third, if reduced rates are used, always plant some test strips at the full- labeled rate, and untreated strips, so that root protection and yields can be evaluated. Fourth, start out small and test it on only a few acres to determine if it fits into the farming operation.
What is the future of reduced rates?
One insecticide is already being used at a reduced rate. Force is a 1.5G material and has 1/10 the active ingredient relative to other 15G insecticides. But until one of the chemical manufacturers takes the lead in reducing their labeled rate of insecticide, the conservative approach is to follow the directions on the label.