Effect of SS infection on susceptibility of A. pisum to parasitism by A. ervi. Each value is the mean number of mummies (= successful parasitism) out of a possible 30, ± SE. In all experiments, the same aphid clonal lineage served as a source of uninfected aphids and as a recipient for artificial infection with the U-, R-, or T-type SS. Aphids infected with either the R- or the T- type SS were significantly less likely to succumb to parasitism. Numbers above columns are replicates (N). *, P < 0.05; ***, P < 0.001.