Applications of both terbufos and nicosulfuron can result in a synergistic interaction that injures corn. Field studies were conducted at two Illinois locations in 1991 to determine the effect of insecticide formulation and application method on the serverity of this interaction. Terbufos, applied to the soil at planting, interacted with nicosulfuron applied postemergence to injure corn and reduce grain yield. In-furrow-applied terbufos caused a more injurious interaction to corn than terbufos placed in a surface band; the 15G formulation was more injurious than the 20CR formulation. Simulated rainfall timing experiments were conducted in 1992 and 1993 by treating corn with terbufos 15G and terbufos 20CR and applying rainfall (3.8 cm) 1, 7, or 14 d before nicosulfuron application. No visual injury or yield reductions were noted in corn treated with nicosulfuron alone. Terbufos 15G caused greater corn injury than did terbufos 20CR at each rainfall timing when sprayed with nicosulfuron. Corn injury and subsequent grain yield that can result from the interaction between nicosulfuron and terbufos was greater when simulated rainfall occurred 1 and 7 d before nicosulfuron application than when rainfall occurred 14 d before application.

Field experiments were conducted in Michigan, Minnesota, and Wisconsin in 1990 to explore interactions between nicosulfuron applied POST and terbufos insecticide at 0.06 or 0.11 g ai/m of row applied in-furrow on ‘Pioneer 3751’ field corn and ‘Jubilee’ sweet corn. Nicosulfuron at 0, 70, and 140 g ai/ha plus nonionic surfactant and 28% nitrogen fertilizer was applied to both corn types. Field corn response to nicosulfuron and terbufos was similar at all locations, whereas sweet corn injury varied with location. Nicosulfuron injured field corn more when applied at the four-leaf than the three-leaf stage. Injury to both corn types increased as nicosulfuron rate increased or when applied following terbufos. Nicosulfuron at 140 g/ha without terbufos did not reduce yield of either corn type; however, corn previously treated in-furrow with terbufos reduced yield.

Field experiments were conducted in Michigan, Minnesota, and Wisconsin to evaluate injurious interactions between nicosulfuron herbicide applied POST and nine insecticides applied at planting on ‘Pioneer 3751’ field corn in 1991. Insecticides were applied in-furrow, T-band, and surface band at planting. Corn injury from nicosulfuron and terbufos applications was more severe in Wisconsin than in Michigan. No corn injury was observed in Minnesota. Terbufos 15% ai granule (15G) or 20% ai controlled release granule (20CR) increased nicosulfuron injury to corn in Wisconsin. Nicosulfuron at 35 g ai/ha caused the greatest vigor reduction following terbufos 15G, intermediate vigor reduction following terbufos 20CR, phorate 20G, or phorate 20CR, and the least vigor reduction following fonofos, chlorpyrifos, chlorethoxyfos, tefluthrin, or carbofuran in Wisconsin.

Greenhouse and laboratory experiments were conducted to determine the effect of soil organic matter on the synergistic interaction between nicosulfuron, a sulfonylurea herbicide, and the insecticide terbufos in corn. Terbufos was applied infurrow at planting to soils containing 1, 3, and 5% organic matter and nicosulfuron was applied POST to corn at the three-leaf growth stage. The combination of terbufos and nicosulfuron reduced corn fresh weight 75, 45, and 41 % in soils containing 1, 3, and 5% organic matter, respectively. Terbufos increased [14C]nicosulfuron uptake into corn grown in both 1 and 5% organic matter soils. Terbufos decreased epicuticular leaf wax deposition on corn leaves by 35 and 18% in 1 and 5% organic matter soils, respectively. [14C]Nicosulfuron was completely metabolized after 24 h in the absence of terbufos. In the presence of terbufos, 19 and 11% of intact [14C]nicosulfuron remained 24 h after treatment while 11 and 6% remained after 48 h in the 1 and 5% organic matter soils, respectively. In vivo acetolactate synthase activity was reduced to 3 and 20% of control by nicosulfuron at 24 h and to 6 and 38% of control at 48 h, respectively, in the 1 and 5% organic matter soils that contained terbufos. These experiments indicated that soil organic matter is an important factor in determining the level of injury expected from the synergistic interaction between nicosulfuron and terbufos in corn.