THE DISTRIBUTION OF APHID INFESTATION IN RELATION TO LEAF AGE

Abstract

The leaf‐by‐leaf distributions of free infestations of viviparous Myzus persicae (Sulz.) and Aphis fabae Scop, were recorded on sugar‐beet plants and spindle bushes in pots in the greenhouse, and the distributions of the various seasonal forms of A. fabae were recorded on the same plants growing naturally outdoors. The aphid distributions were related to the ages of the leaves estimated on an arbitrary scale of ten types of leaf distinguished by degree of unfurling and colour. The diverse types of distribution recorded were all reducible to a common general pattern: growing and senescing leaves were more susceptible to colonization than maturing, mature and dying leaves. The resultant two‐peaked curve of aphid density on a series of leaves at successive stages of development was found in its entirety on certain sugar‐beet plants, but part only of the complete curve was usually found, owing to the incomplete range of leaf ages present on any one plant, particularly on the spindle tree.Fundatrigeniae were found colonizing sugar beet, and spring and summer migrants were found starting colonies on the spindle tree. The normal association of each seasonal form of the aphid either with the ‘primary’ winter host, or with a ‘secondary’ summer host, but not with both, is therefore not obligatory. It is attributed to the normal inaccessibility of summer hosts to fundatrigeniae and the normal unsuitability of the winter host when the ‘alienicolae’ are migrating and its leaves are mature.When M. persicae and A. fabae were infesting the same plant at the same time, their distributions were broadly alike, but M. persicae extended on to both younger and older leaves than did A. fabae and more strictly avoided the more mature leaves. Hence it is suggested that the degree of adaptation of a given aphid to a given plant may be gauged by the extent to which the aphid can colonize the plant's leaves not only when they are growing and senescing but also when they are mature and fully functional.