Apparent competition between parasitoids mediated by a shared hyperparasitoid

Cocoons of the specialist parasitoid Cotesia melitaearum, which attacks the Glanville fritillary butterfly in the Aland islands of SW Finland, are par asitized by the generalist hyperparasitoid Gelis agilis. We added experimen tally 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 ad dition of the second parasitoid the natural populations of C. melitaearum d eclined in the treatment, as predicted by the apparent competition theory.