Apparent competition between parasitoids mediated by a shared hyperparasitoid

Cocoons of the specialist parasitoid Cotesia melitaearum, which attacks the Glanville fritillary butterfly in the Åland islands of SW Finland, are parasitized by the generalist hyperparasitoid Gelis agilis. We added experimentally to the system a second host species for G. agilis, C. glomerata, with which C. melitaearum does not compete for resources. After the one‐time addition of the second parasitoid the natural populations of C. melitaearum declined in the treatment, as predicted by the apparent competition theory.