Microbial associates are widespread in insects, some conferring a protection to their hosts against natural enemies like parasitoids. These protective symbionts may affect the infection success of the parasitoid by modifying behavioral defenses of their hosts, the development success of the parasitoid by conferring a resistance against it or by altering life-history traits of the emerging parasitoids. Here, we assessed the effects of different protective bacterial symbionts on the entire sequence of the host-parasitoid interaction (i.e. from parasitoid attack to offspring emergence) between the pea aphid, Acyrthosiphon pisum, and its main parasitoid, Aphidius ervi and their impacts on the life-history traits of the emerging parasitoids. To test whether symbiont-mediated phenotypes were general or specific to particular aphid-symbiont associations, we considered several aphid lineages, each harboring a different strain of either Hamiltonella defensa or Regiella insecticola, two protective symbionts commonly found in aphids. We found that symbiont species and strains had a weak effect on the ability of aphids to defend themselves against the parasitic wasps during the attack and a strong effect on aphid resistance against parasitoid development. While parasitism resistance was mainly determined by symbionts, their effects on host defensive behaviors varied largely from one aphid-symbiont association to another. Also, the symbiotic status of the aphid individuals had no impact on the attack rate of the parasitic wasps, the parasitoid emergence rate from parasitized aphids nor the life-history traits of the emerging parasitoids. Overall, no correlations between symbiont effects on the different stages of the host-parasitoid interaction was observed, suggesting no trade-offs or positive associations between symbiont-mediated phenotypes. Our study highlights the need to consider various sequences of the host-parasitoid interaction to better assess the outcomes of protective symbioses and understand the ecological and evolutionary dynamics of insect-symbiont associations.

The development of integrated pest management strategies becomes more and more pressing in view of potential harmful effects of synthetic pesticides on the environment and human health. A promising alternative strategy against Delia radicum is the use of trap crops. Chinese cabbage (Brassica rapa subsp. pekinensis and subsp. chinensis) is a highly sensitive Brassicaceae species previously identified as a good candidate to attract the cabbage root fly away from other crops. Here, we carried out multi-choice experiments both in the laboratory and in field conditions to measure the oviposition susceptibilities of different subspecies and cultivars of Chinese cabbages as compared to a broccoli reference. We found large differences among subspecies and cultivars of the Chinese cabbage, which received three to eleven times more eggs than the broccoli reference in field conditions. In laboratory conditions, the chinensis subspecies did not receive more eggs than the broccoli reference. We conclude that D. radicum largely prefers to lay eggs on the pekinensis subspecies of Chinese cabbage compared to the chinensis subspecies or broccoli. Some pekinensis cultivars, which received over ten times more eggs than broccoli in the field, appear especially promising candidates to further develop trap crop strategies against the cabbage root fly.Insects 2020, 11, 127 2 of 12 or to lay eggs on unsuitable ones, resulting in poor larval development [12]. This "optimal bad motherhood" hypothesis [13] appears for example when the potentially most suitable plant host for larvae is unsuitable for adult fitness and thus unattractive to mothers. Oligophagy increases the likelihood that the same plant species is best for the fitness of adults and larvae, so that oligophagous insects are less likely to be "optimal bad mothers" than polyphagous ones [14,15].Knowledge about insect host selection and preferences can be used to develop integrated pest management (IPM) strategies where the crop to be protected from a pest can be cultivated in combination with a far more attractive host plant used as a trap crop [16,17]. The trap crop diverts the pest from the main crop, which decreases economic damage. Moreover, if the trap crop is unsuitable for larvae (dead-end trap cropping), bad mother choice can be exploited to reduce the local population of the pest [18], limiting future crop damage. Numerous studies have revealed which plant hosts are preferred by major phytophagous insect pests and which are less appropriate for their larvae, with potential use in trap crop strategies [19][20][21]. However, preferences of insect pests have often been studied at the supraspecific or specific taxonomic levels while the wide infraspecific diversity created by plant breeding and varietal creation remains largely unexplored. This lack of information comes from the fact that conventional plant protection strategies based on pesticides do not require an extensive knowledge of plant susceptibility to insect pests and accordingly this trait is rarely considered i...

1. Selective pressures exerted on the feeding behaviour of animals have been extensively studied to understand their foraging patterns. In herbivores, specific within-plant patterns of resource exploitation have been reported, but their determinants remain poorly understood.2. Here, we describe and decipher the determinants of the foraging pattern of the pollen beetle Brassicogethes aeneus, a pollinivorous insect that is a pest of oilseed rape Brassica napus. This insect feeds from flowers for almost all of its life cycle, except for a couple of weeks preceding blossoming. During this period, only flower buds are available and the insect destroys them to feed from the pollen they contain, causing serious yield losses.3. We found that during this critical period, pollen beetles exhibit a stereotypic intrainflorescence feeding pattern that depends on flower bud maturity. To explain this pattern, we first deciphered the selective pressures driving pollen beetles' feeding behaviour. Using a set of manipulative laboratory experiments, including behavioural experiments on plant tissues and artificial substrates, chemical characterization of plant tissues and performance experiments, we show that the pollen beetles' feeding behaviour does not seem to be driven by specialized metabolites or an attempt to reach an optimal nutrient balance, but rather by a process of maximization of total macronutrient intake. Next, using optimal diet choice models, we found that one aspect of the intra-inflorescence feeding pattern, the preference for young over old flower buds, could be well-explained through the lens of total macronutrient intake maximization per unit of time to access the resource. 4. Our study provides new insights into small-scale foraging patterns and highlights the need to characterize and assess the relative influence of several components of diet quality when deciphering selective pressures driving foraging patterns.