University of Illinois

No. 7/May 9, 1997

Insecticide/Corn Herbicide Interactions

Many producers in central Illinois remain concerned about the potential for rootworm injury in first-year corn. Utilization of soil insecticides will likely increase this season, which raises the concern for corn injury from the interaction of certain organophosphate insecticides and ALSinhibiting herbicides. Keep in mind that the labels of several ALSinhibiting corn herbicides contain precautionary statements about applying the product to corn previously treated with certain soil insecticides. Injury caused by this interaction is often expressed as stunted corn development, which can range in severity from barely noticeable to very damaging.

Why do combinations of certain soil insecticides and ALS-inhibiting herbicides cause injury to corn? Both insecticides and herbicides are foreign compounds to the corn plant. When foreign compounds enter the plant, the plant responds in a defensive manner and attempts to render the foreign compounds inert so they cannot cause any phytotoxic effects. This process is often referred to as metabolism of foreign compounds. A plant has several pathways that can metabolize different foreign compounds, but certain organophosphate insecticides and ALS-inhibiting herbicides share a common metabolic pathway. When one or the other is present in the plant, the pathway is able to metabolize the compound before it can cause much harm; but, when both are present, the metabolism pathway is "overloaded" and cannot effectively process both compounds. When this happens, the corn injury may become evident.

What can be done to minimize the potential for crop injury from this type of insecticide/herbicide interaction? Insecticide selection and placement (banded application) are two possible methods to alleviate this concern. Selection of an imidazolinone-resistant corn (IR) hybrid can also help to reduce or eliminate injury potential from this interaction.

Labels of the ALS-inhibiting herbicides listed in Table 2 carry precautionary statements about this type of injury potential. These labels also recommend methods to avoid this problem. Producers are encouraged to consult the herbicide label before applying an ALS-inhibiting herbicide to corn previously treated with soil insecticides.

Aaron Hager, and Marshal McGlamery, Department of Crop Sciences, (217)333-4424

Table 2. ALS-inhibiting herbicides (used in corn) that carry insecticide/herbicide interaction precautionary statements.

AccentBroad-strike + Dual
Basis GoldLightning