Resistance to methomyl in populations of the spotted tentiform leafminer (Lepidoptera: Gracillariidae) from southern Ontario

Laboratory bioassays with spotted tentiform leafminer, Phyllonorycter blancardella (F.), moths indicated the occurrence of 4-9-fold resistance to methomyl. Cross-resistance to oxamyl occurred at approximately the same levels. Resistance also occurred in larval stages. In the field, applications of methomyl timed for a predominance of early instars overcame resistance and were as effective as pyrethroids or diflubenzuron. Surveys in 1984 and 1986 indicated resistance was largely restricted to production areas in Norfolk County, Ontario, Canada