Geographic variation in host specificity reveals host range evolution in Uroleucon ambrosiae aphids

Some herbivorous insect taxa that exhibit otherwise conservative patterns o f host association include species or populations with atypical host specif icities. These species provide special opportunities to study the evolution of host range. Uroleucon ambrosiae aphids present an example. Most Uroleuc on species are monophagous on particular host plants from the Asteraceae, a nd U. ambrosiae appears to specialize on the giant ragweed, Ambrosia trifid a, in eastern North America. In the American Southwest, however, U. ambrosi ae uses a variety of asteraceous taxa as hosts. For the present study, we a ssayed host-associated behaviors of U. ambrosiae from both eastern and sout hwestern regions of the United States on each of four asteraceous genera. D ata from choice and no-choice experiments and electrical penetration graph analyses revealed highly significant differences in the acceptability of th e four test plants. Plant taxa were ranked in the same order across multipl e behavioral assays by aphids from both regions. However, eastern and south western aphids exhibited significantly different patterns of behavior. Ambr osia was the most highly accepted plant by aphids from both regions, but so uthwestern aphids accepted the other test plants more readily than did east ern aphids, indicating geographic variation in degree of host specificity. This regional differentiation held for aphids reared under controlled condi tions for the EPG studies, indicating a genetic basis for population diverg ence in U. ambrosiae host-use traits. We speculate that the generalism with in U, ambrosiae is an evolutionarily derived trait and represents an ecolog ical adaptation to the scattered and unpredictable distribution of the pref erred A. trifida host in the arid American Southwest.