Benefits of aggregation in Tetranychus urticae
(2011)
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- (en) In this PhD thesis, we investigate the aggregation and cooperation mechanisms in the phytophagous mite Tetranychus urticae and more particularly to how they adjust their social organization in function of the environmental conditions and their needs. This type of behavior is usually studied from a functional point of view while the mechanisms that are at the origin are however not well understood. Therefore, it is necessary to study both the behavioural relationships between individuals and the fitness of resulting collective productions, to understand the mechanisms behind the emergence of sociality. T. urticae is considered as a major pest and most of research done was on its control whereas its social behaviours is poorly understood. However, T. urticae has several interesting and necessary prerequisites to a social organization (group living, building of a common web, heterogeneous distribution). Our first step was to analyse the impact of group living compared to solitary individuals on fitness. We observed that grouped mites produced significantly more eggs and faecal pellets (silk production indicator) per mite per day. Moreover, single mites had a higher death rate than mites living in groups. We assumed that mites could benefit from the silk production of other individuals to invested resources in other activities such as eggs or faecal pellets production. Furthermore, we showed an impact of the genetic origin of individuals on this group; the productions of feces decreased in the presence of unrelated individuals. In the second part, we studied the mechanisms controlling the aggregate behaviour of T. urticae. Our work showed an influence of the genetic composition of the group on the spatial distribution of individuals. For instance, the interindividual distances increases with inbreeding. Furthermore, we observed a recruitment process for the whole plant infestation, that allows the mites to form large aggregates in a same part of the host-plant and to benefit from group living advantages (e.g. Allee effect, dilution effect, common web building). The last step of this study concerned the communication mechanisms between individual. We studied the influence of the presence of silk on the spatial distribution and behaviour of individuals. Our results showed a modulation of the attraction to silk according to its age and the level of starvation. Indeed, fed females were more attracted by fresh silk, while starved females were more attracted by zones without silk. This showed that the silk could be an indicator of the presence of conspecifics which would therefore, influence the spatial distribution. A fed idividuals would look for the presence of silk/conspecific whereas starved individuals would avoid silk to decrease the risk of competition. This work contributed to define the social status of T. urticae and to understand the group-living mechanisms present in this species. It underlines also the key role played by the genetic background of individuals on their spatial distribution and their life history traits, which let us suppose a kin discrimination ability in that species.
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Le Goff, G. (2011). Benefits of aggregation in Tetranychus urticae. https://hdl.handle.net/2078.5/154930