Olfactory information use for foraging in "Microplitis mediator", a parasitoid of the cabbage moth "Mamestra brassicae"

My PhD project was at the interface between behavioural ecology and biological control, two disciplines that do not easily intertwine because of their divergent aims. On one hand, behavioural ecology is a fundamental science that seeks to understand animal behaviour from an evolutionary perspective and generally works from the point of view of the individual. On the other hand, as an applied science, biological control seeks to elaborate concrete strategies to improve pest control and works from population and community perspectives. However, the success of biological control methods depends on the behaviour of biological control agents, i.e. natural enemies of target pests, which creates a perfect opportunity for behavioural ecology and biological control to work hand in hand. In this work, I studied the foraging behaviour of the larval parasitoid Microplitis mediator (Haliday) (Hymenoptera: Braconidae) with the idea that my findings could contribute to the improvement of the control of its host the cabbage moth Mamestra brassicae (Linnaeus) (Lepidoptera: Noctuidae), which is an important cabbage pest distributed throughout Europe and Asia. Information about the biology, the life cycle and the rearing method of M. mediator and M. brassicae is reviewed in chapter 1.
In a first study (chapter 2), I tested the olfactory attractiveness of flowers/inflorescences of five wildflowers species (bishop’s weed, cornflower, buckwheat, candytuft, and oregano) to naive female M. mediator. I conducted choice tests in a Y-tube olfactometer to test the olfactory attractiveness of flowers/inflorescences against air and the relative attractiveness of the flower species offered in paired choice. I showed that all the flower species were highly attractive to female M. mediator when tested against air, but that in paired choice tests cornflower and candytuft were as attractive as each other and both more attractive than buckwheat. This indicates that M. mediator is able to use olfactory cues to identify potential food sources and has evolved preferences that could be exploited in biological control. In particular, this study has shown that cornflower is a very promising floral subsidy.
However, in a patchy and unpredictable environment, not all food sources are equally abundant and/or near, and parasitoids are expected to have evolved capacities to detect the most promising food sources in terms of proximity and/or abundance. In a second study (chapter 3), I tested whether female Microplitis mediator foraging for food sources, i.e. flowers of cornflower and inflorescences of buckwheat, were able to use quantitative olfactory information to orient themselves towards the most promising (i.e. most abundant and/or closest) food sources. I conducted behavioural assays in a 6-arm olfactometer where groups of six wasps were released and faced with a gradient of volatile concentration created by using different numbers of flowers/inflorescences as odour sources. I also collected and analyzed the volatiles emitted by different numbers of flowers/inflorescences of the two flower species. The results showed that female M. mediator were able to use quantitative olfactory information. In general, they were most attracted to the highest numbers of flowers/inflorescences, which also emitted the highest volatile quantities. However, the response of the wasps towards the two flower species differed. The contrast between the different numbers of flowers/inflorescences was important for the wasps to be able to discriminate and had to be higher with cornflower than with buckwheat. With cornflower, the flower species emitting both substantially higher absolute volatile quantities and more potentially attractive types of volatile compounds (e.g. benzenoïds), the response of the wasps to single flowers was very strong already and showed saturation with increasing numbers of flowers. Conversely, with buckwheat, the flower species emitting low volatile quantities, the response of the wasps to few inflorescences was weak but accelerated with increasing numbers of inflorescences. This would suggest that a higher sensitivity at low volatile quantities than at high volatile quantities could have been selected in M. mediator, which would be adaptive. These results highlight the importance of taking flower density into account to optimize the use of floral subsidies for biological control purposes. In particular, these results suggest that cornflower should be attractive at low densities whereas higher densities of buckwheat could be needed to attract M. mediator in the field.
To conclude, my work has shown that M. mediator is well adapted for food foraging, because it can detect both the quality and the quantity of olfactory information to localize potentially rewarding food sources. I also demonstrated that studying the foraging behaviour of a parasitoid provides relevant information that can be exploited to improve its use for biological control.