FALL ARMYWORM SENSITIVITY TO FLAVONE - LIMITED ROLE OF CONSTITUTIVE AND INDUCED DETOXIFYING ENZYME-ACTIVITY

We used inhibition and induction of detoxifying enzymes to determine W hether these enzymes allow a generalist species (Spodoptera frugiperda ; fall armyworms) to cope with ingestion of the flavonoid, flavone. Fl avone induces polysubstrate monooxygenases (PSMO), general esterases ( GE), and glutathione S-transferases (GST) in S. frugiperda, yet this s pecies is affected deleteriously by low dietary concentrations of this allelochemical. First, in a series of experiments, larvae were fed ar tificial diets containing increasing concentrations of flavone, either alone or with known inhibitors of either PSMO, GE, or GST enzymes. In an additional treatment, flavone and inhibitors of all three enzyme s ystems were administered in diets simultaneously. PSMO and GE activiti es were reduced in vivo by their respective inhibitors, whereas that o f GST was induced or unchanged. Significant synergism of flavone's gro wth-reducing activity occurred at the highest concentration tested (0. 125% fresh mass, fm) when the PSMO inhibitor, piperonyl butoxide, or t he GST inhibitor, diethyl maleate, was added to the diet, and at 0.08% fm flavone, when combined with the GE inhibitor, tri-tolyl phosphate. In many cases, however, the additive effect (i.e., reduction in growt h owing to flavone alone + inhibitor alone) was greater than the syner gistic effect, and no synergism occurred in the treatment with the thr ee inhibitors combined. In the second approach, caterpillars were pree xposed to a concentration of flavone (0.02% fm) that induced these enz ymes ca. 1.5- to 2.5-fold, prior to switching larvae to a diet contain ing a higher (growth-reducing) flavone concentration (0.125% fm). The relative growth rates (RGR) of induced larvae were significantly great er (14%) than those of the uninduced larvae on the 0.125% fm flavone d iet. Additionally, in two of the three experiments, relative consumpti on rate (RCR) was significantly greater (7-24%) in induced compared wi th uninduced larvae. The variable responses to inhibitor treatment and the relatively small benefit of enzyme induction suggest that these e nzyme systems have minimal impact on the detoxification of flavone in S. frugiperda, even though this allelochemical induces enzyme activity and has been reported to be metabolized in vitro.