Predators use volatiles to avoid prey patches with conspecifics

1. Simple models of optimal foraging, such as ideal free distribution models, are based on the assumption that foragers are omniscient with respect to the quality of all patches in the environment; they know how much food and how many competitors are present in each patch. 2. In contrast, simple population dynamic models treat predator-prey distributions in a phenomenological way, and do not take fitness consequences for individual foragers into account. Yet, the precise way in which these distributions come into being is what really matters to population dynamics. It is therefore necessary to study the behavioural mechanisms underlying the distributions of foragers over patches. 3. We studied the behaviour of a predatory mite, Phytoseiulus persimilis, in response to prey patches occupied by conspecifics. It is well known that high predator densities in prey patches promote dispersal of these predatory mites. Our question was to what extent predators can assess the presence of conspecifics from a distance. 4. Experiments with a Y-tube olfactometer showed that predatory mites avoid patches occupied by conspecifics. 5. This avoidance cannot be attributed to odours of conspecific predators, or of prey damaged by predation, as these odour sources both appear to be attractive. 6. Separating the prey patch from the conspecific predators in the odour source led to the avoidance response only when the predators in the odour source were positioned upwind from the prey patch, and not when they were positioned downwind. This suggests that predators release an odour that elicits the production of yet another odour by the prey. This was supported by the observation that removal of adult prey led to a quick disappearance of the avoidance response. 7. We argue that distant discrimination between patches with and without competing conspecifics may be quite common among predators and parasitoids, and that the use of odours instead of physical inspection of patches allows predators to instantaneously integrate information on the distribution of food and competitors. 8. This behavioural mechanism may bring predators and parasitoids closer to behaving as ideal free foragers than was previously thought possible.