Plant water stress decreases milkweed aphid (Aphis nerii) host suitability for parastoid wasps

Aphids (Hemiptera: Aphididae) are widespread herbivorous insects found on a variety of host plants which form complex interactions with parasitoid wasps. Here we investigate the relationships between milkweed host plant (Asclepiadaceae) water stress, milkweed aphid (Aphis nerii; Hemiptera: Aphididae) cardenolide sequestration, and parasitoid wasp (Aphidius colemani; Hymenoptera: Braconidae) developmental success. We expected that host plant water stress would cause variation in the concentration of cardenolides sequestered by A. nerii, altering the ability of parasitoid wasps to utilize them as hosts.  To test this we applied water stress treatments to two species of milkweed plants (Asclepias tuberosa and Asclepias viridis) colonized by A. nerii. We found that: (1) plant cardenolide concentrations were higher in water stressed plants than in plants that were well-watered (2) plant cardenolide concentrations were affected by the presence of aphids and (3) aphid cardenolide concentrations appeared to be correlated with plant cardenolide concentration.  Next, in order to assess the developmental success of A. colemani parasitoid wasps, we released A. colemani into enclosures of A. nerii on host plants that were well-watered or water stressed.  Our results suggest that more A. colemani emerge from aphids reared on well-watered A. tuberosa than water stressed A. tuberosa.  Overall, these experiments indicated that milkweed host plant water stress can affect aphid-parasitoid interactions.  Experiments to further understand the direct effects of aphid sequestered cardenolides on parasitoid wasps is a next priority.