Effect of malathion on resistance to soil-applied herbicides in a population of rigid ryegrass (Lolium rigidum)

Abstract

The effect of the organophosphate insecticide malathion on the response of resistant rigid ryegrass population SLR 31 to the herbicides trifluralin, pendimethalin, clomazone, and triallate was investigated. The insecticide and herbicides were soil-applied prior to emergence of plants grown under controlled conditions. In the absence of malathion, the resistant population exhibited significant resistance to the four herbicides compared with a susceptible population. Levels of resistance, as determined by comparison of herbicide rates required to inhibit growth by 50%, were 35, 11, 2.4, and 2.4 for pendimethalin, trifluralin, triallate, and clomazone, respectively. Malathion had a synergistic effect on pendimethalin in the resistant population but not in the susceptible population. Malathion had no synergistic effect on trifluralin, triallate and clomazone. Resistance to triallate and clomazone was found despite the fact that the resistant population had never before been selected with these herbicides. This resistance, selected by other herbicides, further indicates that the use of alternative herbicides to control multiple resistant weeds is unlikely to be a successful resistance management approach.