Corn Herbicide/Insecticide Precautions

The increased occurrence of corn rootworm injury in cornfields rotated with soybeans, especially in east-central Illinois, has many producers considering applying a soil-insecticide at planting. Numerous insecticide choices are available to producers, but several of these could potentially restrict some corn herbicide options. In particular, use of certain organophosphate (OP) insecticides could restrict the use of herbicides that inhibit the acetolactate synthase (ALS) enzyme, commonly referred to as ALS-inhibiting herbicides. The precautions and restrictions are due to concerns for corn injury following use of certain OP insecticides and ALS-inhibiting herbicides.

Why do combinations of certain OP insecticides and ALS-inhibiting herbicides cause injury to corn? Most ALS-inhibiting herbicides used in corn-production systems are systemic in nature; that is, after these materials enter the plant they are able to move extensively (translocate) from the site of uptake. Often, translocated compounds accumulate in areas of the plant undergoing active cell division (meristems). Both insecticides and herbicides are considered foreign compounds to the corn plant. When a foreign compound enters a plant, the plant attempts to "defend itself" against any potential injury the foreign compound could cause, by rendering the foreign compound inert or nonphytotoxic. This process is commonly referred to as metabolism or breakdown of foreign compounds. A corn plant has several different pathways to "detoxify" foreign compounds, but the OP insecticides and many ALS-inhibiting herbicides share a common metabolic pathway. When one or the other is present within the plant, the plant is able to metabolize the compound before it may cause any deleterious effects; but if both insecticide and herbicide are present, the metabolism pathway can become overwhelmed and cannot effectively metabolize both compounds. When this happens, corn injury can result.

What can be done to help minimize the potential for corn injury from this insecticide/herbicide interaction? Selection and placement of the insecticide, as well as corn-hybrid selection, can all be helpful. Table 1 is a summary of herbicide label information with respect to the potential for corn injury caused by various OP/ALS-inhibiting herbicide interactions. As indicated in the table, placement of the insecticide (in-furrow or T-band), selection of insecticide (Counter, Lorsban, etc.), and corn hybrid (normal, IT hybrids) can influence the potential for corn injury from this interaction. The data in this table are for "normal" corn hybrids, except for products containing imazethapyr or imazapyr, which require an IT or IR corn hybrid. (We selected label information for IT hybrids for these herbicides.) Most of these herbicide labels have few restrictions about insecticide selection or placement when the respective herbicide is applied toIR corn hybrids.

Table 1. Herbicide label restrictions for application to "normal" corn previously treatedwith certain organophosphate insecticides

No: Do not use this herbicide if this soil-applied insecticide was previously applied.
UCI: Using this herbicide on corn previously treated with this insecticide may result in Unacceptable Crop Injury.
TCI: Using this herbicide on corn previously treated with this insecticide may result in Temporary Crop Injury.
*Do not place Lorsban in-furrow if Python or Broadstrike is to be used.
Foliar OP: Cygon, Diazanon, Disyston, Imidan, Lorsban, malathion, Penncap-2FM.

Aaron Hager (hagera@idea. ag.uiuc.edu) and Marshal McGlamery (mmcglame@uiuc.edu), Extension Weed Science, (217)333-4424