Effects of host‐odour plume altitude and changing wind velocity on upwind flight manoeuvres of a specialist braconid parasitoid

Abstract

Abstract. Females of the specialist parasitoid, Microplitis croceipes (Cresson) (Hymenoptera: Braconidae), were released in a wind tunnel into host‐odour plumes dispersed by winds of three velocities and winds whose speed was changed while the wasps were engaged in upwind flight. In steady winds of 61, 122 and 183 cms‐‐1, wasps maintained similar ‘preferred’ ground speeds by adjusting their airspeed, while turning to a lesser degree as wind velocity increased. In winds of changing velocity (either increasing or decreasing within a 60–100 cm s‐1 range), wasps lowered their rate of upwind progress, leading to more tortuous tracks. During changing wind speeds longitudinal image flow decreased. Wasps flying in host‐odour plumes 10 cm and 20 cm above the flight tunnel floor in a 122 cm s‐1 wind had similar ground speeds; thus their rate of ventral visual image flow varied two‐fold. M.croceipes may ‘aim’ upwind by comparing how changes in the course angle vary with the direction of visual image flow. During changing wind velocities the relationship between changes in visual and flight muscle generated torque is ambiguous. Under these conditions most wasps cast, a manoeuvre characterized by wide lateral excursions across the wind without upwind progress. Once wind speed stabilizes, flight straightens out and upwind flight resumes.