Modelling collective foraging in endemic bark beetle populations

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Tree-killing bark beetles are widely studied at epidemic population densities because of their significant impacts on forests. At endemic levels, these species are restricted to poorly defended resources, such as wind-felled, lightning-struck, or suppressed trees. It is poorly understood how these scattered and unpredictable resources are discovered and exploited. In this prospect, the collective foraging shown by most bark beetles, in the form of independent searching by individual beetles combined with mutual attention to each other’s chemical signals, represents an efficient strategy to increase the probability to discover the resources.In order to explore the role of host distribution and collective foraging on the discovery of and settlement on wind-felled trees by bark beetles at endemic population densities, we developed a spatially explicit foraging model for the major forest pest in Europe, Ips typographus. The model is based on the diffusion of the beetles, the landing of pioneers on wind-felled trees realistically distributed based on field observations in southern Belgium, and the response of flying beetles to pheromones emitted by these pioneers.The results show that social foraging effectively increases the proportion of beetles that landed on wind-felled resources compared to solitary foragers. Furthermore, the model highlights the parameters that influenced the foraging strategy of I. typographus and that need narrower quantification in further experiments. Finally, in the frame of the tested parameters, the model highlights the existence of thresholds in the reproduction rate and in the size of the starting population under which populations collapse. If confirmed, this prediction would shed new light on the understanding of foraging at low, endemic population levels.