Aphids have evolved a symbiotic relationship with the bacterium Buchnera aphidicola, which reside in specialized cells called bacteriocytes and provide nutrients lacking in the insects phloem diet. In addition they occasionally harbor other vertically transmitted bacteria (secondary symbionts), but the benefit of these infections has been unclear. In the January 29 early edition of the Proceedings of the National Academy of Sciences, Kerry M. Oliver and colleagues at the University of Arizona, Tucson, USA, show that facultative bacterial symbionts in aphids offer resistance to parasitic wasps (PNAS, DOI:10.1073/pnas.0335320100, January 27, 2003).

Oliver et al. examined the consequences of artificially inoculating facultative symbioses in Acyrthosiphon pisum (the pea aphid). They observed that infection with symbionts caused a high mortality in developing Aphidius ervi parasitoid larvae and offered resistance against parasitoid attack. Compared with uninfected controls, experimentally infected aphids were as likely to be attacked by ovipositing...

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