Heritability and Artificial Selection on Ambulatory Dispersal Distance in Tetranychus urticae: Effects of Density and Maternal Effects

Dispersal distance is understudied although the evolution of dispersal distance affects the distribution of genetic diversity through space. Using the two-spotted spider mite, Tetranychus urticae, we tested the conditions under which dispersal distance could evolve. To this aim, we performed artificial selection based on dispersal distance by choosing 40 individuals (out of 150) that settled furthest from the home patch (high dispersal, HDIS) and 40 individuals that remained close to the home patch (low dispersal, LDIS) with three replicates per treatment. We did not observe a response to selection nor a difference between treatments in life-history traits (fecundity, survival, longevity, and sex-ratio) after ten generations of selection. However, we show that heritability for dispersal distance depends on density. Heritability for dispersal distance was low and non-significant when using the same density as the artificial selection experiments while heritability becomes significant at a lower density. Furthermore, we show that maternal effects may have influenced the dispersal behaviour of the mites. Our results suggest primarily that selection did not work because high density and maternal effects induced phenotypic plasticity for dispersal distance. Density and maternal effects may affect the evolution of dispersal distance and should be incorporated into future theoretical and empirical studies

The Formation of Collective Silk Balls in the Spider Mite Tetranychus urticae Koch

Tetranychus urticae is a phytophagous mite that forms colonies of several thousand individuals. These mites construct a common web to protect the colony. When plants become overcrowded and food resources become scarce, individuals gather at the plant apex to form a ball composed of mites and their silk threads. This ball is a structure facilitating group dispersal by wind or animal transport. Until now, no quantitative study had been done on this collective form of migration. This is the first attempt to understand the mechanisms that underlie the emergence and growth of the ball. We studied this collective behaviour under laboratory conditions on standardized infested plants. Our results show that the collective displacement and the formation of balls result from a recruitment process: by depositing silk threads on their way up to the plant apex, mites favour and amplify the recruitment toward the balls. A critical threshold (quorum response) in the cumulative flow of mites must be reached to observe the emergence of a ball. At the beginning of the balls formation, mites form an aggregate. After 24 hours, the aggregated mites are trapped inside the silk balls by the complex network of silk threads and finally die, except for recently arrived individuals. The balls are mainly composed of immature stages. Our study reconstructs the key events that lead to the formation of silk balls. They suggest that the interplay between mites' density, plant morphology and plant density lead to different modes of dispersions (individual or collective) and under what conditions populations might adopt a collective strategy rather than one that is individually oriented. Moreover, our results lead to discuss two aspects of the cooperation and altruism: the importance of Allee effects during colonization of new plants and the importance of the size of a founding group

The Search Behavior of Omnivorous Thrips Larvae is Influenced by Spider Mite Cues

The western flower thrips is an omnivorous insect that consumes both leaf tissue and spider mite eggs. For this reason, these thrips are often described as ‘opportunistic predators’ of spider mites. Several studies have shown that western flower thrips are often found in association with spider mites, and the development time of thrips decreases and their survivorship increases when they consume spider mite eggs. We tested the hypothesis that thrips larvae may respond to chemical cues from spider mites, and that they may modify their prey-searching behavior when these spider mite-induced cues are present. We prepared hexane extracts from: 1) webbing of spider mites isolated from maize leaves, 2) webbing produced by spider mites inside an empty glass tube to exclude any plant-derived cues from the extract, and 3) spider mite cuticle extracts. These three extracts were subsequently applied in droplets to one-half of filter papers, and hexane alone was applied in droplets to the other half. We showed that residence time of thrips larvae was higher on filter paper with spider mite webbing extract, especially when the extract originated from spider mites isolated in glass tubes. In the presence of webbing extracts, we also observed: 1) a decrease in velocity, 2) an increase of angular velocity and 3) an increase in time immobile. Extracts from spider mite cuticle only increased velocity and proportion of time immobile of the thrips larvae. Our results suggest that chemical cues from spider mite webbing induce an arrestment response and play an important role in the non-random search behavior of thrips larvae searching for eggs on spider mite infested plant leaves

Web as a meeting point in the two spotted spider mite Tetranychus urticae

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Group effect on fertility, survival and silk production in the web spinner Tetranychus urticae (Acari: Tetranychidae) during colony foundation

In many vertebrates and invertebrates, individuals reared in isolation show biological modifications compared with those reared in groups of two of more. The spider mite Tetranychus urticae is characterised by a communal organization and displays some forms of cooperative behaviour (aggregation and common web spinning). To evaluate the potential fitness cost or gain of group living, we investigated the effect of being in group on life history parameters (silk production, fecundity, death rate, feeding rate). In this respect, virgin single females and grouped females (two to six individuals) were compared every day for 5 days. Grouped mites produced significantly more web/mite per day (since the second day of experiment) and more eggs/mite per day (since the fourth day of experiment) than single mites. Moreover, single mites had a higher death rate than mites living in groups. However, no difference was found concerning feeding rate. We assume that mites could benefit from the web production of other individuals and invested resources in other activities such as egg production. We showed that these group effects already exist in small groups. In natural conditions with a higher group size, the impact of group living might be stronger for the dynamics of T. urticae populations and the colony foundation. © 2010 Koninklijke Brill NV, Leiden.IF:1.47SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Spatial distribution and inbreeding in Tetranychus urticae

FI:1.71info:eu-repo/semantics/publishe

Aggregation behaviour in Tetranychus urticae

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Spatial and Social behaviour of two mite species: Tetranychus urticae and Dermatophagoides pteronyssimus

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Aggregation behaviour and genetic diversity in the two-spotted spider mite Tetranychus urticae

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