PARAQUAT RESISTANCE IN HORSEWEED (CONYZA-CANADENSIS) AND VIRGINIA PEPPERWEED (LEPIDIUM-VIRGINICUM) FROM ESSEX COUNTY, ONTARIO

Populations of horseweed and Virginia pepperweed resistant to the herb icide paraquat were found in several fruit orchards in Essex County, O ntario, Canada, where paraquat had been applied four to five times per year for at lease 10 yr. Dose-response experiments were conducted in the greenhouse for each species, using 2- to 4-mo-old planes from popu lations that had received repeated exposure to paraquat and those that had not. The paraquat dose that reduced shoot dry weight by 50% (ED50 ) was estimated for each population. Resistant populations of horsewee d had ED50 values 25 to 35 times higher than susceptible populations. A resistant population of Virginia pepperweed had an ED50 10 times hig her than susceptible populations.