Host finding by aphids in the field

SUMMARY The spectral energy distribution of the light reflected from various leaves and soils has been measured and corrected for ‘standard’ sunlit conditions, and then compared on the basis of Moericke's evidence that aphids of many species distinguish two parts of the spectrum as colours, alighting and probing in response to wavelengths greater than 500μ. Alightments by Brevicoryne brassicae and Myzus persicae in the field occurred preferentially on leaves reflecting a greater proportion of long‐wave energy, with little or no regard for the ‘botanical’ (plant‐taxonomic) host status of the plants for each aphid. The colour attraction of such ‘yellow‐sensitive’ aphids serves rather to bias their alightments towards plants of the appropriate physiological type. Laboratory experiments confirmed the existence, inferred from field observations, of a second, non‐specific mechanism of visual attraction normally involved in alightments on plants, namely a close‐range optomotor reaction with both orienting and arresting components. The light reflected from various soils embodied maximum energy at the extreme long‐wave end of the spectral range affecting aphids, while the leaf energy peak was 540–560 mμ. The predominantly long‐wave emission from both leaves and soils contrasts sharply with the light from clear or clouded skies, wherein the peak energy region lies below 500 mμ. The aphids' type of colour vision thus provides for a primary discrimination between sky and ground, beside any subsidiary discriminations among plants and soils. Low‐flying Aphis fabae were attracted to white surfaces as well as to yellowing leaves in the field. The change in the migrant's behaviour from ‘distance flight’ in the upper air to low‐level ‘alighting flight’ appears to be due, not to a reversal of the response to short‐wave sky light from positive to negative, but simply to a relative strengthening of the positive response to long‐wave light from the ground.