Many fields may require unplanned postemergence applications to control grass escapes due to the dry weather. We have received several questions concerning restrictions on Accent and related herbicides in fields that have been treated with insecticides. This article describes the interaction between organophosphate (OP) insecticides and ALS-herbicides, risks associated with these combinations, and current restrictions or warnings present on the herbicide labels.

Organophosphate insecticides and ALS-herbicides are degraded by the same enzyme system in corn plants. If an OP insecticide is present in the corn plant at the time of herbicide application, the rate at which the herbicide is metabolized may be reduced and this reduced rate of metabolism may allow the herbicide to accumulate in the corn to injurious levels. Injury symptoms range from chlorosis within the corn whorl (yellow flash) and development of malformed bands of tissue on leaves. In rare situations, the main tiller of the corn plant may be killed.

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Corn injury from ALS-herbicide and insecticide interaction.

Several factors affect the potential for interactions between these two classes of pesticides. Most important is how much insecticide is absorbed and translocated within the corn plant. Counter is more systemic than other commonly used soil-applied OP insecticides, thus Counter has a greater risk of interaction and herbicide labels specify more restrictions with Counter than with other insecticides. Corn tolerance to the herbicide also influences the potential for a significant interaction. DuPont products containing rimsulfuron (Basis, Basis Gold, Accent Gold) carry the most restrictive labels concerning insecticide use.

Other factors that influence interactions between soil-applied insecticides and herbicides include application method, formulation, soil type, and rainfall. In-furrow insecticide applications increase the amount of insecticide absorbed by seedling corn plants compared with band applications, thereby increasing the potential for interactions. Counter CR (controlled release) is designed to limit the concentration of terbufos in the soil solution, thereby reducing insecticide availability to corn. Soil organic matter ties up much of the insecticide, making it less available to the corn plant. Some herbicide labels base their recommendations for use following Counter on soil organic matter levels. Finally, soil water availability influences the availability of insecticides to corn. As soils dry the insecticide becomes more tightly bound to soil colloids and less is absorbed by the corn. Significant rainfall before or soon after the herbicide application may increase insecticide absorption and increase the potential for crop response.

Labels for most ALS-inhibitor herbicides registered for use on corn include comments or restrictions for use on fields treated with a soil insecticide (Table 1). Although interactions between OP insecticides and ALS-inhibitor herbicides have not been a major problem in Iowa, it is important to be aware of and follow recommendations on the product label.

Table 1. Label-indicated interactions between ALS-herbicides and OP insecticides.

	Soil-Applied OP Insecticide						Non-OP	Application of Foliar-OPa
	Counter CR				Aztec or Fortress			Insecticide (days)
Herbicide	Furrow	T-Band	Thimet	Lorsban	Furrow	Band	Regent or Force	Before	After
Nicosulfuron and Rimsulfuron
Accent, Basis Gold, Celebrity, Celebrity Plus	No	Caution	Caution	Caution	Yes	Yes	Yes	7	3
Accent Gold, Basis	No	No	No	Caution	Yes	Yes	Yes	7	3
Primisulfuron and Prosulfuron
Beacon, Exceed, NorthStar, Spirt	No	No	Caution	Caution	Caution	Caution	Yes	10	7
Flumetsulam
Bicep Magnum TR	No	No	No	Caution	Caution	Yes	Yes	—	—
Broadstrike+Dual	No	No	No	Caution	Caution	Yes	Yes	—	—
Hornet (preemergence)	No	No	No	Caution	Caution	Yes	Yes	—	—
Hornet (postemergence)	No	No	No	Caution	Caution	Caution	Yes	10	10
Python	No	No	No	Caution	Caution	Yes	Yes	—	—
Imazethapyr and Imazapyr
Pursuit, Lightning (IT/Clearfield hybrids)	Yes	Yes	Yes	Caution	Yes	Yes	Yes	—	—
Halosulfuron
Permit (postemergence)	-- no restrictions on label --

Yes= herbicide label permits this combination or does not preclude its use.

Caution = herbicide label indicates potential for temporary crop injury, depending upon soil conditions and organic matter. Consult product label(s) for specific recommendations.

No = herbicide label indicates that use of this combination may cause unacceptable levels of crop injury or this use is not permitted by product labeling.

^a Foliar-applied OP insecticides = Cygon, Diazinon, Dimethoate, Di-Syston, Imidan, Lorsban, or Penncap-M.