Biology and trophic interactions of lucerne aphids

Lucerne or alfalfa Medicago sativa is the most important temperate forage legume worldwide. Only one or two varieties of lucerne were grown in the U.S.A. and Australia (the two leading exporters of lucerne) before the late 1950s and late 1970s, respectively. These dates coincided with the arrival of aphid species, which devastated lucerne stands and prompted the development of aphid resistant cultivars. Lucerne-feeding aphids, including bluegreen aphids Acyrthosiphon kondoi, pea aphids Acyrthosiphon pisum, spotted alfalfa aphids Therioaphis trifolii maculata and cowpea aphids Aphis craccivora, however, still present significant risks for the lucerne industry worldwide and account for 25% of global production losses. Moreover, increased production costs, negative environmental effects and emerging aphid resistance to insecticide applications have led to a narrowing of management options against lucerne aphids. Understanding lucerne aphid biology and trophic ecology will be needed to develop future management practices, including biological control. We review and synthesize research on the four lucerne aphid species, focussing on cultivar resistance and their interactions with other organisms, including predators, parasitoids, entomopathogens and bacterial symbionts. The effects of global climate change are considered, with a particular emphasis on the potential for compromised aphid resistance in lucerne cultivars under future climates. We conclude by identifying future research questions and perspectives for the sustainable management of lucerne aphids. These include the characterization of plant secondary metabolites associated with natural enemy recruitment, an understanding of the role of endosymbionts in cultivar resistance and a better comprehension of multi-trophic interactions of lucerne aphids, both with other herbivores and higher trophic groups.