Soil insecticide and herbicide interaction potential

For western corn rootworm control in continuous corn, we have been recommending that growers either use a transgenic hybrid containing a Bt protein active against rootworm larvae, or make use of a granule or liquid soil insecticide and a hybrid not containing the protein.

In Ohio, we do not believe that it is necessary to combine both a Bt hybrid plus a soil insecticide to achieve economic control. Our rootworm populations are seldom severe enough to consider both tactics together, nor do we feel it is appropriate to use a soil insecticide in addition to the Bt protein to prevent resistance from occurring with the Bt tactic; rotation is a much better way to prevent resistance from occurring.

However, we do know that there will be some growers that will apply a soil insecticide to the Bt hybrid, which in addition to those using a soil insecticide as a standalone method of rootworm control, is a tactic that they probably have not used for many years. This article is a reminder, a strong reminder, that if growers use an organophosphate soil insecticide, that they need to pay attention to possible phytotoxic interactions between the soil insecticide and the various herbicides they might use.

Knowing that different combinations of soil insecticides and herbicides can negatively interact and cause phytotoxicity to corn plants was better known years ago prior to the use of Bt for rootworm control, when soil insecticides were the primary tactic that growers used. However, most growers have not used them in many years and perhaps need a reminder of the potential danger from applying certain herbicides with the numerous soil insecticides.

Grower are urged to look at Table 8 in the “2013 Weed Control Guide for Ohio and Indiana”  Click here to see a list of herbicides that if used with certain organophosphate soil insecticides can potentially result in crop injury. Growers should read herbicide and insecticide labels closely to determine possible interactions, especially for some of the newer herbicides.