Associative learning in two closely related parasitoid wasps: a neuroecological approach

Insects are useful model organisms to study learning and memory. Their brains are less complex than vertebrate brains, but the basic mechanisms of learning and memory are similar in both taxa. In this thesis I study learning and subsequent memory formation in two parasitoid wasp species that differ in associative learning of the odours of plants on which they have encountered a host caterpillar. After ovipositing in a caterpillar on a certain plant species C. glomerata shifts its preference to the experienced plant odour, whereas C. rubecula does not shift plant odour preference after a similar experience. This difference in learning between these two closely related wasp species provides an attractive model to study physiological and ecological factors that could influence learning.As a first step to analyse possible physiological differences that could influence learning, I describe morphological, anatomical and histochemical aspects of the neural pathways that mediate associative learning of odours in these wasps. The two wasp species display a high degree of similarity in morphology of the olfactory pathway at both the level of the sensilla, and the level of the glomeruli, the primary olfactory neuropile. I furthermore identify the octopaminergic neurons that could mediate the reward stimulus in the two wasp species, but the results did not allow us to distinguish possible dissimilarities between the species.In addition I redefined the difference in preference learning between the two species in terms of associative and non-associative learning and analysed the temporal dynamics of the memory trace. Both wasps display associative learning after an oviposition reward conditioning, but the temporal dynamics differ. C. glomerata displays a stable memory for the experienced odour that lasts for at least five days, whereas in C. rubecula the memory starts to wane after one day.Finally, I studied the effect of physiological and ecological traits of hosts as possible factors influencing memory formation. For this I used two geographically disjunct populations of C. glomerata that differ in their host use. Both populations only change preference after an oviposition reward on their preferred host species, suggesting that physiological factors exert a major influence on learning in these two populations. I discuss the ultimate factors that could have contributed to a difference in learning in C. glomerata and C. rubecula