Cascading bottom‐up effects of invasive alien plants on the parasitoid Aphidius colemani mediated by the aphid host Aphis fabae

BACKGROUND Invasive alien plants (IAP) can threaten biodiversity by disrupting trophic interactions and, consequently, affect agroecosystems by providing alternative resources for phytophagous insects at crop field margins. This may influence pest populations and parasitoid performance through bottom‐up effects. We examined the cascading effects of three IAPs common at agroecosystem interfaces ( Galinsoga quadriradiata , Reynoutria japonica , Senecio inaequidens ) on a major pest, the black bean aphid ( Aphis fabae ), and its parasitoid Aphidius colemani , a widely used biological control agent. We assessed key life‐history traits to evaluate both aphid host and parasitoid fitness, choosing Beta vulgaris as the control crop‐plant, a cultivated host plant commonly infested by Aphis fabae , used here to compare the other effects relative to it. The traits evaluated here were aphid host mass and larviposition rate, parasitism ratio, parasitoid emergence ratio, parasitic success, female ratio, development time, mass, tibia length and egg load. RESULTS We found contrasting suitability among the IAPs as host plants for Aphis fabae hosts and their effects on Aphidius colemani . On R . japonica , aphids showed high larviposition rates and developed successfully, reaching the adult stage with optimal mass after 9 days, and this IAP was also associated with the highest parasitoid performance. Conversely, S . inaequidens negatively impacted larviposition rates and the subsequent development of the offspring (which remained at a nymphal stage after 9 days and were lighter), but resulted in similar parasitoid performance compared to the control crop‐plant, despite reduced aphid host performance. Finally, aphids failed to establish on G . quadriradiata , and consequently parasitoids did not establish either. CONCLUSION Differences in host plants’ suitability drove variations in both aphid host and parasitoid performance, which could ultimately influence potential top‐down regulations. These results highlight the need to consider the ecological roles of IAPs when assessing their agronomic impact. Although they could act as reservoirs for aphid pests, some IAPs might also benefit pest management by supporting parasitoid populations, and contribute to broader ecosystem functions.