Barometric pressure decrease induces density dependent changes in foraging behaviour in a parasitoid fly.

Barometric pressure is an environmental factor involved in the modulation of a variety of activities in insects. Generally, a drop in barometric pressure precedes the arrival of weather conditions that can affect insect activities and life expectancy. We simulated different scenarios of pressure drop in a modified hermetic chamber and studied their influence on the host-seeking behaviour of the larvae of the robber fly Mallophora ruficauda using air stationary olfactometers. In addition, we studied whether larval density modulates orientation to the host under the same scenarios of pressure drop. We found that motivation to search for hosts is affected by the same slope of pressure drop in both low and high density larvae. However, larval density modulates the onset of the responses to pressure decrease , as low density larvae stop searching for hosts more quickly than high density larvae. This result reflects an avoidance strategy according to which low density larvae would have a reduced host range and higher risks to die and less chances to find a suitable host under adverse pressure conditions. Low density larvae, known to prefer healthy hosts, do notsearch for parasitized hosts under normal pressure conditions nor under a range of pressure drop, strongly suggesting that host selectivity is not modulated by barometric pressure. This study paves the way to a better understanding of the changes in insect crucial behaviours induced by weather conditions and provides more knowledge about the risk factors likely to affect insect survival in a context of foraging ecology.