Mixed‐Function Oxidase Activity in a Generalist Herbivore in Relation to Its Biology, Food Plants, and Feeding History

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Abstract
A general—purpose detoxification enzyme system, known as the mixed—function oxidase (MFO) system, apparently protects generalist herbivores from defensive plant allelochemics. Previous studies limited to lepidopterous larvae show that the MFO system has the attributes of an effective biochemical defense, mainly because this system has been shown to occupy a key position in the primary degradation and inactivation of a wide variety of exogenous lipophilic substances, and its level increases rapidly via introduction by plant allochemics. The gut microsomal MFO system of the highly polyphagous Japanese beetle, Popilia japonica, is induced rapidly, attaining maximal level within 24 h of feeding on host plants. The order of induction is: field polyphagouf beetles > laboratory—simulated polyphagy > single—plant feeding. However, apparently the induction is not related to host preference of this herbivore, but probably occurs in response to host allelochemics. Single—plant feeding of three host plants: sassafras, broccoli, and phlox, as a group, produced MFO induction that was 83 and 67%, respectively, of the level achieved in mixed plant feeding on a natural polyphagy. The Implication of this mode of substantial induction in single—plant feeding, in relation to the beetles' feeding habits immediately after emergence, is discussed. The beetle larvae clearly show that under faculatative monophagy, the MFO level is significantly lower than in polyphagy. Taken together, these data show that the MFO system in the Japanese beetle is highly flexible and is responsive not only to overall breadth of the diet, but also to the immediate feeding history.