Synergism of permethrin and fenvalerate by piperonyl butoxide for fourth instars of the Colorado potato beetle,Leptinotarsa decemlineata(Say) fed on different host plants was used as an indicator of allelochemical-mediated induction of the cytochrome P-450. Beetles collected from Parry Sound, Ontario, were fed on potato (Solanum tuberosumL.), tomato (Lycopersicon esculentumMill.), buffalo bur (S. rostratumDunal), and climbing nightshade (S. dulcamaraL.). Combined LD50's were used to calculate and compare synergistic ratios. The mean weight of the larvae in each bioassay (replication) was used as a covariate in calculation of combined 72-h LD50's and the approximate 95% CI for the synergistic ratios was calculated. For permethrin, the only significant difference found was between synergistic ratios of larvae fed on buffalo bur (10.5) and climbing nightshade (5.9,P<0.001). For fenvalerate, larvae fed on climbing nightshade showed the highest resistance factor, but larvae fed on buffalo bur showed the highest synergistic ratio (29.2), which was significantly different from that of potato (14.7,P<0.01). LD50's for larvae fed on potato were lowest for both insecticides, but feeding on the other host plants increased the LD50. Implications of this response to allelochemicals in alternate host plants to breeding for resistance to Colorado potato beetle is discussed.