Asymmetric interactions between two butterfly species mediated by food demand.

Recent 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 ( and ) explains their interaction on a host plant, . A laboratory experiment showed that plant mass consumption by larvae was 2.6 times greater than that by . We predicted that , which requires more food, is more vulnerable to food shortages than . In a cage experiment, an asymmetric interspecific interaction was detected between the two specialist butterflies; larval density significantly decreased the survival and prolonged the development time of , but density affected neither the survival nor the development time of . The prediction based on the food requirement was partly supported by the fact that increasing density likely caused a food shortage, which more negatively affected survival than survival. Conversely, increasing the density of did not reduce the remaining food quantity, suggesting that the negative effect of density on was unlikely to be due to food shortage. Although aristolochic acid I, a defensive chemical specific to plants, 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.