16For parasitoids, the host represents the sole source of nutrients for the developing immature. 17Subsequently, host quality is an important factor affecting immature development and the 18 resulting fitness of the emerging parasitoid, with impacts on fecundity, longevity and offspring 19 sex ratio. Host age is an integral component of host quality and a key factor in host selection by 20 the female parasitoid. The current study aimed to investigate the effect of decreasing host quality 21 (determined by increasing host age) on adult life history traits (size, wing loading, longevity, and

Upon encountering a host, a female parasitoid wasp has to decide whether to learn positive or negative cues related to a host. The optimal female decision will depend on the fitness costs and benefits of learned stimuli. Reward quality is positively related to the rate of behavioral acquisition in processes such as associative learning. Wolbachia, an endosymbiotic bacterium, often plays an impressive role in the manipulation of its arthropod host's biology. Here we studied the responses of two natural Wolbachia infected/uninfected Trichogramma brassicae populations to theoretically high- and low- reward values during a conditioning process and the consequences of their responses in terms of memory duration. According to our results, uninfected wasps showed an attraction response to high value rewards, but showed aversive learning in response to low value rewards. Memory span of uninfected wasps after conditioning by low-value rewards was significantly shorter compared to high-value rewards. As our results revealed, responses to high quality hosts will bring more benefits (bigger size, increased fecundity and enhanced survival) compared to low-quality hosts for uninfected wasps. Infected wasps were attracted to conditioned stimuli with the same memory duration after conditioning by both types of hosts. This was linked to the fact that parasitoids emerging from both types of hosts present the same life-history traits. Therefore, these hosts represent the same quality reward for infected wasps. According to obtained results it can be concluded that Wolbachia manipulates the learning ability of its host resulting in the wasp responding to all reward values similarly.

15The Hopkin's host-selection principle (HHSP) suggests that organisms at higher trophic levels 16 demonstrate a preference for the host species on which they developed during larval stage. 17Although investigated in many herbivorous and predatory insects, the HHSP has, to our 18 knowledge, never been tested in the context of insects hosting selfish endosymbiotic passengers 19 such as the maternally inherited bacterium Wolbachia pipientis. Here, we investigate the effect 20 of Wolbachia infection on host pre-imaginal learning in the parasitoid wasp Trichogramma 21 brassicae (Hymenoptera: Trichogrammatidae). We compare host-choice in Wolbachia-infected 22 and uninfected adult female parasitoids after rearing them on two different Lepidopteran hosts, 23 namely the flour moth Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) or the grain moth 24Sitotroga cerealella (Lep.: Gelechiidae). We show that in T. brassicae, Wolbachia affect the pre-25 imaginal learning ability of female wasps. Wolbachia infected wasps do not show any host 26 preference and easily switch hosts in the laboratory, while uninfected wasps significantly prefer 27 to lay eggs on the host species they developed on. We discuss how the facilitation of a generalist 28 strategy by Wolbachia may allow T. brassicae to escape intraspecific competition with their 29

Nutritional quality during early life can affect learning ability and memory retention of animals.Here we studied the effect of resource quality gained during larval development on the learning ability and memory retention of two sympatric strains of similar genetic background of the parasitoid Trichogramma brassicae: one uninfected and one infected by Wolbachia. Wolbachia is a common arthropod parasite/mutualistic symbiont with a range of known effects on host fitness. Here we studied, for the first time, the interaction between resource quality and Wolbachia infection on memory retention and resource acquisition. Memory retention of uninfected wasps was significantly longer when reared on high quality hosts when compared to low quality hosts. Furthermore, uninfected wasps emerging from high quality hosts showed higher values of protein and triglyceride than those emerging from low quality hosts. In contrast, the memory retention for infected wasps was the same irrespective of host quality, although retention was significantly lower than uninfected wasps. No significant effect of host quality on capital resource amount of infected wasps was observed, and infected wasps displayed a lower amount of protein and triglyceride than uninfected wasps when reared on high quality hosts.