1. Frugivorous and seed-feeding insects may alter the traits of fruits, such as shape and size, which may influence fruit attractiveness to frugivorous birds. Consequently, trait-mediated interactions may occur in systems where plants, seed-dispersing frugivorous vertebrates, and frugivorous or seed-feeding insects interact. We investigated colour manipulation in Ilex integra Thunb. berries caused by the seed parasitoid wasp Macrodasyceras hirsutum Kamijo and how that manipulation relates to fruit attractiveness for frugivorous birds.
2. In winter, the colour of I. integra berries varied from green to red, but most berries were greenish, indicating that the berries were immature. Berry dissection indicated that the number of live parasitoid larvae present within each berry was closely related to berry colour – the greater the number of live larvae, more intense is the green colour of the berry. However, the wasp larvae did not modify the shape or size of the berries. More than 98% of berries that were protected from the insects by gauze bags ripened and turned red. In the present study, berries with unfertilised seeds alone turned red. Field-feeding preference tests showed that the brown-eared bulbul Hypsipetes amaurotis Temminck preferred red berries to green berries.
3. We demonstrated that the seed parasitoid wasp manipulates the berry colour, but not its shape or size, in a density-dependent manner. Because green berries suffered less from bird foraging, we believe that this colour manipulation helps the wasps to avoid being killed by the birds. The present study indicates that manipulation by wasps may reduce the level of mutualism between the tree and seed-dispersing birds.