In Ontario, some populations of the spotted tentiform leafminer, Phyllonorycter blancardella (F.), have become resistant to all insecticides used for their control between 1976 and 1990. Resistance to organophosphorous (OP) insecticides reached 160-fold by 1977 but has since declined by 2- to 4-fold. An altered acetylcholinesterase (AChE), which is less sensitive to inhibition by OPs, is likely responsible for the resistance. Resistance to pyrethroids, probably due to target-site insensitivity, also is widespread. This resistance (< 10 fold) has been overcome by synchronizing sprays with susceptible life stages. A few leafminer populations are resistant to the carbamoyl oximes, methomyl and oxamyl. Although < 10-fold resistance to carbamates can be overcome by timing, growers usually curtail use until resistance declines. In one population, resistance has declined from 9- to 2.3-fold in 6 yr. A combination of insensitive AChE and enhanced metabolism, probably by aliesterases, may cause the resistance. Insecticidal applications suppress parasitism of leafminers. Permethrin and fenvalerate are more toxic to susceptible and resistant leafminers than to Pholetesor ornigis (Weed); however, cypermethrin and deltamethrin have only narrow selectivity. Methomyl and oxamyl are also toxic to P. ornigis and Sympiesis marylandensis Girault, but methomyl is not persistent in the field. Azinphosmethyl appears most damaging to P. ornigis, not only because of direct toxicity, but also because applications for other pests coincide with adult flights. Recent studies have not identified increasing levels of pyrethroid or methomyl resistance in P. ornigis, and none has indicated any increased tolerance to azinphosmethyl.