Field experiments testing for apparent competition between primary parasitoids mediated by secondary parasitoids

1. Two field manipulation experiments were performed to look for apparent c ompetition between primary parasitoids of aphids, mediated by shared second ary parasitoids. In each case the frequency of secondary parasitism on a 'f ocal' species was compared in the presence or absence of a 'treatment' spec ies and its aphid host. 2. In both experiments the focal species was Aphidius ervi (Haliday) attack ing Acyrthosiphon pisum (Harris) (pea aphid). In the first experiment the t reatment species was Aphidius rhopalosiphi (De Stefani-Perez) attacking Sit obion avenae (Fabricius) (a grass aphid) and in the second Aphidius microlo phii (Pennachio & Tremblay) attacking Microlophium carnosum (Buckton) (nett le aphid). 3. No significant effects of either the presence of A. rhopalosiphi or of i ts host on secondary parasitism of A. ervi were detected in the first exper iment. 4. In the second experiment, rates of secondary parasitism were estimated t wice: a significant effect of treatment was found on the first occasion but not the second. In the former, parasitism was highest in the presence of t he treatment species and its host, intermediate when just the host was pres ent, and lowest when both were absent. 5. We discuss the evidence for apparent competition in host-parasitoid comm unities.