An infra-red remote sensing system for the active detection and automatic determination of insect flight trajectories (IRADIT)

Abstract

A non-technical description is given of a new, powerful, low cost field system (Infra-red Active Determination of Insect Flight Trajectories or IRADIT) for detailed and automatic remote sensing studies of natural insect flight behaviour. The special requirements and difficulties of the detection problem are defined. A series of examples of field devices and techniques are presented to illustrate the key factors of the optical sensing and tracking of insects in flight. In the finally adopted IRADIT system, flying insects are differentially illuminated, under all natural light conditions, by an intense beam of pulsed near-infra-red radiation and detected using a shuttered image intensifier linked with a video camera operating at a rate of 50 frames per second. Immediate fully-automatic determination of the flight trajectories of several simultaneously detected insects was achieved, at this same high rate and in the presence of sky background photon noise, by processing the video signals with electronic circuits and a microcomputer. Flight trajectories are influenced by the local wind, whose vector must be subtracted to study insect flight behaviour. This was achieved by the use of a specially developed sensitive three-vector vane anemometer, providing digital data to the microcomputer at a minimum rate of 5 Hz. In tests of the prototype IRADIT-anemometer system in the field, insects with wing area of only 1-5 mm 2 and flying against the midday sky were readily tracked at ranges up to 15 m. A range of at least 100 m is expected for nocturnal moth tracking.

Flying insects in the field

“…In some circumstances it is possible to increase contrast and hence observable trajectory length by the introduction of an artificially dark background, either a simple black cloth (Gibson and Brady, 1985), or more elaborate (and more effective) non-reflective structures (Schaefer and Bent, 1984). In other circumstances -when viewing against the sky for example -the only way of increasing contrast is to use an artificial illumination source to increase the brightness of the target.…”

“…Unfortunately, natural illumination contains a substantial component of near infra-red radiation, so this strategy yields only a limited advantage. Full invulnerability to twilight requires the use of more specialized lighting equipment of the type employed by Schaefer and Bent (1984). Their source, which was able to illuminate small targets brightly enough to make them show up in high contrast against the midday sky, used a filtered xenon lamp which was pulsed at high power in synchronism with an electronic shutter in their intensifier/video-camera detector.…”

“…Logging the image coordinates by repeatedly adjusting the position of cross-hairs is a time-consuming and rather tedious process, and can sometimes be avoided by the use of a target-following algorithm implemented on a small desktop computer (Schaefer and Bent, 1984), even in the presence of background images (J. Zeil, pers.…”

Radar and opto-electronic measurements of the effectiveness of Rothamsted Insect Survey suction traps

“…The standard aerofoil trap was situated at the same site as in 1979, and 15-min samples were obtained throughout the day (Schaefer & Bent, 1984). The specimens were categorized by wing area, calculated by multiplying the length of a wing by the maximum width.…”

“…A suitably sophisticated sensing device could then track those airborne objects entering through A, and measure the fraction actually entering the trap inlet, which is the efficiency E. The sensing method should meet the requirement of not interfering with the trapping by altering either the flow field or the behaviour of the dispersing objects, such as insects discussed here. One remote monitoring device, suitable even for small targets seen against the sky at midday, is the recently developed IRADIT system (infra-red active detection and automatic determination of insect trajectories; Schaefer & Bent, 1984).…”