INSECTICIDE-OIL DROPLET SIZE EFFECT ON FALL ARMYWORM LARVAE CONTROL VIA CHEMIGATION

A mixture of an insecticide (chlorpyrifos) and immiscible oil was appl ied through a laboratory-simulated irrigation system onto corn and cot ton plants that contained fall armyworm larvae (FAW), Spodoptera frugi perda (J. E. Smith). Effects of insecticide-oil (I/O) droplet size, ac tive chemical rate, and crop canopy architecture on percent larval con trol were evaluated. Small I/O droplets (D(V.5) = 6 mum) were less eff ective in controlling FAW larvae than were large droplets (D(V.5) = 74 mum). The percent larval control reached 90.4% and 83.6% on corn and cotton, respectively, at half the low recommended dose. The result con firmed that chemigation was effective on insect control. Percent larva l control was higher on corn plants than on cotton plants because of a n architectural advantage of the corn canopy which directed insecticid e to the larval feeding location. The influence of I/O droplet size on larval control was more apparent on cotton at the lower chemical rate than on corn at the higher chemical rate. Applying optimal I/O drople t sizes in chemigation significantly reduced the use of insecticide ne eded to achieve improved efficacy.