Host morphs, population density, and symbiont status shape parasitism success of <i>Aphelinus asychis</i> (Hymenoptera: Aphelinidae) in <i>Acyrthosiphon pisum</i> (Hemiptera: Aphididae)

The parasitoid wasp Aphelinus asychis Walker (Hymenoptera: Aphelinidae) has been shown to effectively control parthenogenetic Acyrthosiphon pisum Harris (Hemiptera: Aphididae) populations. However, its effectiveness against the sexual generation of aphids remains unclear. Additionally, secondary symbionts in aphids, such as Hamiltonella defensa and Regiella insecticola (both Enterobacterales: Enterobacteriaceae), may influence parasitoid success. This study investigated the parasitism performances of A. asychis among third-instar oviparous female, male, and viviparous females of A. pisum, as well as the effects of aphid density, parasitoid density, and aphid symbiont infections on parasitoid emergence and aphid mortality. The results showed that A. asychis exhibited high parasitism performance on third-instar male aphids, with parasitoid emergence and host mortality that were significantly greater than in viviparous and oviparous females. Increasing aphid density initially enhanced parasitoid emergence per-female, but efficacy plateaued at higher aphid densities, likely due to superparasitism and larval competition. Moreover, higher parasitoid densities increased aphid mortality but reduced per-female parasitoid emergence. Symbiont infection significantly reduced parasitoid success. Aphids uninfected with symbionts exhibited the highest parasitoid emergence, followed by those infected singly with H. defensa. Aphids singly infected with R. insecticola or co-infected exhibited the lowest parasitoid emergence. No additive protective effect was detected in aphids co-infected with H. defensa and R. insecticola, potentially due to shared defense mechanisms, a lack of synergistic benefits between coexisting symbionts, or competitive interactions. Our findings highlight the need to integrate knowledge of aphid reproductive biology, parasitoid behavior, and symbiont-mediated defenses to optimize biological control strategies targeting A. pisum.