ONSET OF A DECLINE IN THE QUALITY OF SUGAR-BEET AS A HOST FOR THE APHID MYZUS-PERSICAE

The results presented in this paper suggest that sugar beet became les s suitable as a host for aphids after late June and early July, when t he plants were at the 10-12 leaf stage. This was indicated by a faster rate of aphid mortality and greater incidence of dark stomach deposit s after this time. Increased aphid mortality coincided with a change i n physiology of sugar beet leaves from being net sinks to sources of a ssimilate, Aphids which fed on older leaves, irrespective of plant age , suffered greater mortality than those feeding on young heart leaves. The incidence of dark deposits in aphid stomachs associated with this mortality was greater on outer than on inner leaves of old, mature an d young plants. It is suggested that the incidence of the dark deposit is an indicator of declining plant quality. The consequences of this for the spread of beet yellowing viruses are discussed.