Video analysis of aphid flight behaviour

Abstract

Video techniques have been used to study the flight behaviour of aphids over a number of years. Experimental approaches include: (1) two-dimensional analysis of flight responses to visual stimuli in a video-computer automated vertical wind tunnel; (2) three-dimensional analysis of flight tracks in horizontal wind tunnels and in the field; and (3) assessment of behaviour after landing and factors affecting the probability of subsequent take-off. Studies in the laboratory have concentrated on the host-alternating black bean aphid, Aphis fabae. Both crowd-induced long-day winged females (summer alate virginoparae) which fly to fresh summer hosts and short-day induced, winged autumn forms (gynoparae) have been investigated. Individual A. fabae can be flown for many hours in a vertical wind tunnel when changes in the rate of climb are countered by corresponding changes in downward air flow. Such control may be automated by coupling the video signal of the flying aphid, via a teletracker and through a computer, to servo control air flow. The computer is also used to effect the presentation of visual stimuli. The two-dimensional flight track of the insect can be saved to computer disc for later analysis and responses to visual stimuli presented during flight can be detailed. Studies have revealed major differences in initial flight behaviour between summer and autumn winged forms, as well as a maximum response to a visual target illuminated with green, monochromatic light at wavelengths of ≈550 nm. To obtain a more accurate analysis of flight, a technique was developed to examine three-dimensional aspects of aphid flight tracks. By superimposing the signals from two video cameras placed side-by-side with overlapping fields of view, two images of the insect appear on the monitor and the distance between them is proportional to the distance from the cameras. The x y z co-ordinates during flight can then be calculated at up to 50 Hz and flight track parameters computed. This technique was used with aphids flying in a horizontal wind tunnel in the laboratory and, when coupled to an ultrasonic anemometer, in the field. Studies show that aphids tend to control ground speed and to land into the wind. When young adult winged aphids, which have flown from their natal plant in culture cabinets, are placed on host or non-host plant material, they may stay and settle or fly off. The use of close-up video showed that gynoparae of A. fabae can discriminate between a summer host (broad bean, Vicia faba) and the winter, relevant host (spindle tree, Euonymus europaeus) within a 5-min period. On the unsuitable plant, 75% initiate flight. Plant factors detected during brief stylet penetration of epidermal cells appear to be responsible for this discrimination.