Plant secondary metabolites can affect insect feeding but responses are species-specific. Sideroxylonal-A (a formylated phloroglucinol) has been shown to inhibit feeding in several vertebrate and invertebrate herbivores. To investigate whether sideroxylonal-A affected feeding in sawfly larvae, Perga affinis affinis, we fed larvae eucalypt leaves containing various concentrations of sideroxylonal-A, and measured frass production as an indicator of consumption. We found that: (1) at least 80% of the sideroxylonal-A ingested by larvae was stored in the diverticulum; (2) less than 1% was excreted in frass; (3) feeding was unaffected by the concentration of sideroxylonal-A; and (4) larvae produced more frass on natal host leaves than on non-natal host leaves.