Aphidius ervi; NUTRITION-; Hemipteran host bacterial endosymbionts role in parasite development; DEVELOPMENT-; Nutritional role of hemipteran host bacterial endosymbionts; HEMIPTERAN-HOSTS; Acyrthosiphon pisum; Host bacterial endosymbionts role in parasite nutrition & development Acyrthosiphon pisum; MUTUALISM-; Buchnera (bacteria); Endosymbionts role in hymenopteran parasite nutrition & development; HYMENOPTERAN-PARASITES; Aphidius ervi; Host bacterial endosymbionts role in parasite nutrition & development This study aims at evaluating whether the bacterial endosymbionts of the pea aphid, Acyrthosiphon pisum (Harris), may be of any nutritional importance to the endophagous braconid parasitoid Aphidius ervi Haliday. Aposymbiotic aphids, obtained by rifampicin treatment on artificial diet, were parasitized, and both parasitoid development and major changes in the nitrogen-derived nutrients in the host hemolymph were observed. A. ervi larvae developing in aposymbiotic pea aphids showed a significant delay in reaching the adult stage and the emerging adults showed a 50% weight reduction compared to those emerging from symbiotic aphids. In the hemolymph of parasitized symbiotic aphids, parasitism-specific proteins of approximately 15 and 45 kD were detected, and their level increased on day 5 and 6 after parasitoid oviposition. At that stage, a significant increase in the free amino acid level was observed, with glutamine showing the highest relative abundance. In contrast, the hemolymph of aposymbiotic aphids did not contain the two parasitism-specific proteins and no increase in free amino acids was observed. The results indicate that the bacterial endosymbionts in parasitized pea aphids are of pivotal importance for the development of A. ervi. Aposymbiotic aphids have a lower nutritional suitability, showing a significant reduction in the hemolymph of nitrogen-derived compounds of nutritional importance to the developing parasitoid larva. The possible role of Buchnera in nitrogen recycling is also discussed as a possible strategy for optimizing the utilization of dietary nitrogen by the parasitoid larva.