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

The effect of the organophosphate insecticide malathion on the response of resistant rigid ryegrass population SLR 31 to the herbicides trifluralin, p endimethalin, clomazone, and triallate was investigated. The insecticide an d herbicides were soil-applied prior to emergence of plants grown under con trolled conditions, In the absence of malathion, the resistant population e xhibited significant resistance to the four herbicides compared with a susc eptible population. Levels of resistance, as determined by comparison of he rbicide rates required to inhibit growth by 50%, were 35, 11, 2.4, and 2.4 for pendimethalin, trifluralin, triallate, and clomazone, respectively. Mal athion had a synergistic effect on pendimethalin in the resistant populatio n but not in the susceptible population. Malathion had no synergistic effec t on trifluralin, triallate and clomazone. Resistance to triallate and clom azone was found despite the fact that the resistant population had never be fore been selected with these herbicides. This resistance, selected by othe r herbicides, further indicates that the use of alternative herbicides to c ontrol multiple resistant weeds is unlikely to be a successful resistance m anagement approach.