AbstractRecent studies on insect interactions on plants have revealed that herbivorous insects indirectly interact with each other through changes in plant traits following herbivory. However, less attention has been given to plant biomass relative to plant quality in relation to indirect interactions among herbivores. We explored the extent to which the larval food demand of two specialist butterflies (Sericinus montelaandAtrophaneura alcinous) explains their interaction on a host plant,Aristolochia debilis. A laboratory experiment showed that plant mass consumption byA. alcinouslarvae was 2.6 times greater than that byS. montela. We predicted thatA. alcinous, which requires more food, is more vulnerable to food shortages thanS. montela. In a cage experiment, an asymmetric interspecific interaction was detected between the two specialist butterflies;S. montelalarval density significantly decreased the survival and prolonged the development time ofA. alcinous, butA. alcinousdensity affected neither the survival nor the development time ofS. montela. The prediction based on the food requirement was partly supported by the fact that increasingA. alcinousdensity likely caused a food shortage, which more negatively affectedA. alcinoussurvival thanS. montelasurvival. Conversely, increasing the density ofS. monteladid not reduce the remaining food quantity, suggesting that the negative effect ofS. monteladensity onA. alcinouswas unlikely to be due to food shortage. Although aristolochic acid I, a defensive chemical specific toAristolochiaplants, did not influence the food consumption or growth of either butterfly larva, unmeasured attributes of plant quality may have mediated an indirect interaction between the two butterflies. Consequently, our study suggests that not only the quality but also the quantity of plants should be considered to fully understand the characteristics, such as symmetry, of interspecific interactions among herbivorous insects on the same host plant.