Abstract.1To determine whether an aphid is ‘aware’ of which plant it is on, clones of a host alternating aphid,Cavariella aegopodiiScop., reared on its primary (Salix) or secondary (Daucus) host, were subjected to autumnal conditions.2Old clones produced males and egg laying females when exposed to autumnal conditions on the primary host but autumnal migrants and males when on the secondary host.3Unlike some other host alternating aphids,Cavarielladid not have to spend a period on the secondary host or go through the spring migrant stage before producing sexuals.4The adaptive significance of a host alternating aphid being ‘aware’ of which host plant it is on is discussed in the light of these