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...