Host age and fitness-related traits in a koinobiont aphid parasitoid

1. Trade-offs play a key role in species evolution and should be found in host-parasitoid interactions where the host quality may differ between host age categories. 2. The braconid wasp, Aphidius ervi, is a solitary endoparasitoid that allows its aphid hosts to continue to feed and grow after parasitisation. The hypotheses that host age influences their quality and that female parasitoids exploit their hosts based on that quality were tested under laboratory conditions using no-choice tests. 3. Aphidius ervi females accepted the aphid Myzus persicae for oviposition and their progeny developed successfully in all host ages. The fitness-related traits of parasitoids did not increase linearly with the host age in which they developed. Host quality was found to be optimal at intermediate host ages and the females preferred to parasitise these hosts. The shortest progeny development time and a more female-biased sex ratio were observed in hosts of intermediate age. 4. This study suggests the existence of multiple interactive trade-offs occurring during host-parasitoid interactions according to host age related quality.