Corn Herbicide/Insecticide Precautions

A recent article published in this Bulletin by Dr. Mike Gray (“Soil Insecticide Use on Bt Corn Expected to Increase this Spring Across Much of Illinois”) provides insights and data that suggest the use of at-planting soil-insecticides in corn is likely to increase this season.  Many insecticide choices are available to farmers, but several of these could restrict the option to use certain corn herbicides.  Specifically, using an organophosphate (OP) insecticide at planting or after corn emergence could restrict the use of herbicides that inhibit either the ALS or HPPD enzymes.  The precautions and restrictions most often appear on the herbicide label and are due to the increased potential for corn injury following use of OP insecticides and ALS- or HPPD-inhibiting herbicides.

Why do certain combinations of OP insecticides and ALS- or HPPD-inhibiting herbicides increase the risk for corn injury?  Both insecticides and herbicides are foreign compounds to the corn plant.  The corn plant tries to “defend itself” against any potential injury a foreign compound might cause by attempting to render it 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- and HPPD-inhibiting herbicides share a common metabolic pathway.  When an insecticide or herbicide is present within the plant, the plant is usually able to metabolize the compound before it may cause any deleterious effects.  However, if both insecticide and herbicide are present, the metabolism pathway cannot effectively metabolize both compounds.  When this happens, corn injury can result.

The following table is a summary of herbicide label statements about the potential interaction with organophosphate insecticides.  As always, be sure to consult the most current product labels for additional information.

Table 1.  Corn herbicide label statements: interactions with organophosphate (OP) insecticides.

No = Do not use this herbicide on corn if this insecticide was previously applied

Yes = The herbicide may be applied to corn previously treated with this insecticide

Abbreviations:

UCI = unacceptable crop injury

TCI = temporary crop injury

SCI = severe crop injury

–          = no information on label

^aDo not apply within 60 days of crop emergence where an OP insecticide was applied.

^bDo not apply within 45 days of crop emergence where Counter was applied.

^cDo not apply within 45 days of crop emergence where an OP insecticide was applied.

^dSoil-applied OP insecticides should not placed in furrow, but rather applied in a band or T-band.