Analysis of Anemotactic Flight Tendencies of the Spotted Lanternfly (Lycorma delicatula) during the 2017 Mass Dispersal Flights in Pennsylvania

Abstract

Video-recordings were made of adult spotted lanternflies, Lycorma delicatula, taking flight from apple trees in an orchard in northeast Pennsylvania in September, 2017 during a mass dispersal flight event involving thousands of adults. The trajectories of adults flying upwind in straight and level or gradually descending flight allowed them to traverse only up to ca. 40 m in a single flight-bout. Many did not make it to trees or bushes that were at even shorter distances than this and they landed in the grass. Flight tracks of 162 adults launching themselves into the wind from the upper branches of apple trees were video-recorded in plan view from below by a camera placed on the ground aimed straight up at the sky. The tracks were then digitized and analyzed using a triangle of velocities technique to determine the degree to which the adults were progressing in a directly upwind flight track, with the wind vector experienced by each adult calculated from the adults’ flight track itself. Average airspeeds of upwind-flying L. delicatula had been previously measured in another group of adults and shown to not vary with wind speed. The headings (direction of thrust) of adults in the video frames were determined by matching the image of the adult in each video frame with a template image of a pinned adult of a known distance from the camera and heading. Matching the body axis in this way works for this species because the adults flying in these elongated fairly straight flight paths did so with forewings spread out flat to the ground with little discernable roll. Having determined airspeed and heading plus ground speed and track for each set of images allowed the third side of the triangle of velocities — the wind velocity vector — to be calculated for each flying adult at whatever altitude or lateral location in the camera’s field of view it was flying. Adult L. delicatula were found to head upwind in flight at 10.7° off the wind line to produce resulting track angles of progression over the ground averaging 30.9° off the wind line due to this discrepancy between their headings and the wind velocities into which they were flying. The wind velocity vectors provided by a ground-based anemometer during the periods each adult was flying through the video frames deviated from the wind velocity vectors calculated using the triangle of velocities technique by nearly 22° and were 50% lower in wind speed than the calculated vectors taken at the higher altitudes and locations each adult was flying. The triangle of velocities technique might provide a new way of using certain species of insects as free-flying anemometers to take wind velocity readings at different heights and spatial locations that are not attainable through the use of ground-based anemometers.