Of moths, mites and microbes - The role of bacteria in the life history of two arthropod herbivores

Bacteria are omnipresent in nature and a multitude of associations between bacteria and higher organisms (plants, animals and fungi) have been described. The fitness effects of bacteria on organisms with which they are associated can vary from beneficial to neutral to detrimental. In this thesis, I investigated the bacterial communities as well as pathogenic and non-pathogenic effects of bacteria on life history traits of two generalist herbivorous arthropods. In the noctuid moth Heliothis virescens, I found that bacterial communities are highly variable depending on diet, rearing background and developmental stage, suggesting that this moth has not evolved a long-term, stable symbiosis with the detected bacteria. I further investigated the effect of bacterial immune challenge on reproductive traits in H. virescens and found that the type of immune response is sex-specific: while females show higher immunity after bacterial challenge than males, males have a higher baseline immunity. In line with this result, bacterial challenge negatively affects female attractiveness and mating success but does not affect male mating success. Immune challenge also affects oviposition strategy of H. virescens females: females increase oviposition rate, but become less selective for oviposition site. In the two-spotted spider mite, Tetranychus urticae, I show that Wolbachia, Cardinium and Spiroplasma affect mite survival and oviposition, mite gene expression and the induction of tomato responses after mite feeding. While Wolbachia seems to benefit the mites, Cardinium has the characteristics of a parasite.