How Trichogramma parasitoids use moth sex pheromones as kairomones: orientation behaviour in a wind tunnel

Abstract

Abstract The orientation behaviour of Trichogramma egg parasitoids (T.evanescens Westwood and T.pretiosum Riley) in response to the sex pheromone of their noctuid hosts {Mamestra brassicae L. and Heliothis tea (Boddie)) was investigated in a wind tunnel. Wasps were released on platforms which served as models of leaves, and were exposed to overhead light and an air stream that was either clean or loaded with host sex pheromone. The wasps exhibited: upwind anemotaxis, which was not affected by odour; odour‐modulated positive phototaxis; and odour‐induced inverse orthokinesis. Compared with clean air, residence times, walking times and path lengths on a platform were higher in pheromone‐loaded air than in clean air. In pheromone‐loaded air, walking velocity was reduced. During locomotion on a horizontal platform, net movement was upwind, regardless of the presence or absence of pheromone in the air. On a platform inclined at 45o, anemotaxis appeared offset by positive phototaxis. If wasps were released on top of a glass rod above a platform, host sex pheromone caused wasps to land shortly after take‐off. These results are used to explain higher rates of parasitism of moth eggs in pheromone‐treated plots in earlier field experiments.