NADPH-dependent monooxygenase that detoxifies senecionine and similar plant alkaloids that are ingested by the larvae. Is active towards a narrow range of related substrates with highest activity towards senecionine, followed by seneciphylline, retrorsine, monocrotaline, senecivernine, axillarine and axillaridine. NADPH + O2 + senecionine = H2O + NADP(+) + senecionine N-oxide Optimum pH is 7.0. Optimum temperature is 40-45 degrees Celsius. Homotetramer. Hemolymph. Larvae store pyrrolizidine alkaloids from their host plant S.jacobaeae as non-toxic N-oxides. This serves as protection against insectivores; after ingestion of the larvae, the N-oxides are reduced in the gut of the insectivores to form toxic alkaloids (PubMed:11972041). Belongs to the FMO family.