Variation in learning of herbivory-induced plant odours by parasitic wasps: from brain to behaviour

Abstract

Two closely related parasitic wasp species, Cotesia glomerata and Cotesia rubecula, lay their eggs in first-instar caterpillars of Pieris brassicae and/or Pieris rapae hosts. They find their hosts by responding to secondary plant metabolites, induced by herbivory. Both wasp species have an innate preference for the odours of infested cabbage, common host plants of these Pieris caterpillars, but they can also learn to respond to the odours of other host plants, after they have found suitable host caterpillars on that plant. This experience results in an association of the odours of that plant with the presence of suitable hosts. The two wasp species differ profoundly in olfactory learning; C. glomerata instantly changes its innate preference for cabbage odours towards the odours of another plant after a single experience, whereas C. rubecula never changes its innate preference for cabbage odours. Both wasps show an increase in flight response to a previously unattractive host plant after a single oviposition experience on that plant, but this memory wanes in C. rubecula after a day, and remains unchanged for at least 5 days in C. glomerata. In this paper, ultimate factors are discussed that may have contributed to the evolution of the observed differences in learning in these two wasp species. Furthermore, hypotheses on the possible neural mechanisms and genes underlying these differences are given, based on current knowledge on the cellular mechanisms of learning as determined for genetic and neurobiological model species like the fruit fly Drosophila melanogaster and the honeybee Apis mellifera