Virtually all plant species produce a diversity of secondary plant compounds or allelochemicals. Some of these chemicals are known to modulate the activities of cytochrome P450 monooxygenases (P450s) by acting either as P450 inducers and/or inhibitors. In the same way that plant allelochemicals consumed in the human diet alter the bioavailability and therefore the efficacy of certain drugs by modulating drug-metabolizing P450 activities, modulation of xenobiotic metabolizing P450s in herbivorous insects by host plant allelochemicals have the potential to affect xenobiotic toxicity. We show here that consumption of plant allelochemicals such as coumarin and flavonoids can ameliorate the toxicity of mycotoxins and insecticides to Helicoverpa zea and the toxicity reduction is due to the induction of P450s that metabolize these xenobiotics.