Population growth and sequestration of plant toxins along a gradient of specialization in four aphid species on the common milkweed Asclepias syriaca

Dietary specialization in insect herbivores has long been hypothesized to predict tolerance of plant defences, with more specialized herbivores being highly tolerant of and sometimes sequestering plant secondary compounds. Plant variation in secondary compounds should thus play an important and predictable role in shaping the performance and distribution of insect communities. We compared the performance of four naturally co-occurring aphid species on twenty genotypes of the common milkweed Asclepias syriaca. Genotypes of milkweed consistently differed in functional traits, including concentrations of toxic cardenolides, while the diet breadths of the four aphids ranged from broadly generalized to monophagous. The two more generalized species had the highest population growth rate overall, while growth rates decreased with increasing specialization. In contrast, honeydew exudation as a measure of phloem consumption increased with specialization; thus, resource-use efficiency was lower in specialist aphids. The two more generalized aphids grew best on genotypes with the highest plant growth rate (as an approximation for resource availability), while specialist aphids were not affected by plant growth. All four species contained apolar cardenolides in their bodies and excreted polar cardenolides, but only the most specialized aphid Myzocallis asclepiadis was negatively affected by increasing cardenolide concentrations of the host plant. Sequestration of cardenolides increased with diet specialization, with M.asclepiadis accumulating twice as much as any other species, perhaps explaining its susceptibility to plant cardenolides. Heritable plant traits differentially impacted co-occurring insect herbivores within the same guild. Generalist aphids were susceptible to variation in plant vigour but not defensive compounds. Increased host specialization resulted in lower resource-use efficiency, increased phloem throughput and ultimately higher cardenolide sequestration. Variation in these traits is thus likely to determine the relative distribution of generalist and specialist herbivores on plants in natural communities.