THE CHANGING NUMBERS OF APHIS FABAE SCOP., FLYING AT CROP LEVEL, IN RELATION TO CURRENT WEATHER AND TO THE POPULATION ON THE CROP

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Abstract
Aphis fabae Scop, flying over bean crops in the summer usually show a double peak of aerial density during the day; scarcely any flight occurs at night. The first peak is thought to be composed mainly of alatae moulted since the previous evening and the decline of the peak to their depletion as they fly away from the crop. The second peak is probably composed mainly of alatae moulted during the same day. Lack of flight at night is due partly to low temperature, partly to lack of alatae old enough to fly and probably also to low light intensity.Contrary to expectation changes in aerial density from hour to hour are only very weakly correlated with weather factors, especially wind‐speed. The total numbers in each of the two peaks do show a low but significant correlation with both wind‐speed and temperature in one case, but even then only a relatively small amount of the variation in aerial numbers is associated with weather changes. Other factors, particularly rapid changes in the numbers of alatae on the crop caused by moulting, accumulation before flight and depletion by flight evidently obscure changes due simply to varying flight behaviour. Thus because of large populations on the crop, it is possible to have quite large numbers in the air even when the weather is relatively unfavourable for flight.In addition to these observations, the relation between total numbers caught at different wind‐speeds shows that most of the migration occurred in winds when the aphids could have had no control over the general direction of flight. The current view that most migration takes place only in calm weather can therefore no longer be held.