Adverse weather conditions impede odor-guided foraging of parasitoids and reduce their host-finding success

Biological pest control depends on the ability of natural enemies to find herbivore-infested plants, which is often guided by herbivore-induced plant volatiles (HIPVs). While foraging behavior of natural enemies has been extensively studied under controlled conditions, little is known on how odor-guided foraging behavior is influenced by fluctuating weather conditions in the field. Here we investigated how HIPV-guided foraging of Cotesia glomerata parasitoids is influenced by prevailing weather conditions in the field. C. glomerata females were released in 5 m radius circles of cabbage plants infested with Pieris brassicae caterpillars, and host finding success and direction of foraging activity was assessed by recapturing parasitoids on the host-infested plants. Wind speed and direction, humidity, barometric pressure and temperature was recorded and related to parasitoid foraging activity. Parasitoid foraging behavior was studied in more detail by observing individually released parasitoids in 0.5 m circles with host-infested cabbages. Recapture rates and movement direction of parasitoids was influenced by an interaction between humidity, wind speed, temperature and change in barometric pressure. High humidity and low wind speed was generally positively associated with host finding success. Direct observations of parasitoid flight behavior confirmed the influence of humidity and wind speed. Successful host finding in upwind direction was only observed at wind speeds below 3 m/s. Our study highlights the complex ways in which weather variables moderate parasitoid foraging behavior under field conditions. Since unfavorable weather conditions result in a strong reduction in parasitoid foraging efficiency, weather conditions are an important factor in the effectiveness of parasitoids as biocontrol agents.