Optical studies of insect flight at low altitude

The use of radar for the study of insect flight is described and the two major limitations of the technique are identified, namely low altitude coverage and target identification. A solution in the form of an optical system which is capable of measuring insect aerial density, speed and direction of flight and wing beat frequency up to 30 m above ground level (AGL) is described. A computer simulation of the signal received from a flying insect is shown to agree with laboratory measurements. Possible methods for measuring insect orientation and discriminating between moths and grasshoppers are deduced. The potential of the optical system is illustrated by a selection of results from a joint radar-optical field study of the flight behaviour of the African Armyworm Moth (spodoptera exempta). In particular, the density and size structure of the aerial population in the range 10 - 30 m AGL are shown to be similar to those observed by the radar at 70 m AGL, whereas activity at ground level is seen to follow a different pattern, which is closely related to the emergent behaviour of the moth. Field results are also presented which suggest strongly that the wing beat frequency of armyworm moths is dependent on ambient air temperature.