Seasonal shifts in host usage in Uroleucon gravicorne (Momoptera: Aphididae) and implications for the evolution of host alternation in aphids

1. Uroleucon gravicorne (Patch) feeds on annual Erigeron species and on perennial Solidago species. 2. To test whether Solidago and Engeron are analogous t o winter and summer hosts of typical host-alternating aphids, reproductive performance, host preferences, and host associations were measured for E.strigosus, S.juncea and S.nernoralis during spring, summer and autumn. 3. Caged individuals can reproduce on both genera throughout the season, though colonies are rare on Solidago during summer. 4. Developmental rate, adult weight, and fecundity decline between May and August on all hosts; however, the drop is least o n E.strigosus, intermediate on S.nemoralis, and greatest o n Sjuncea. 5 . Throughout the season, reproductive performance is a t least as great o n Erigeron as on Solidago, the difference being least in spring. 6 . Soluble nitrogen content shows the same seasonal trends as reproductive performance, declining in all hosts, but declining least in E.strigosus. 7. Preference tests and field records show that Ugravicorne moves from Solidago t o Erigeron when the first alatae mature in late spring, feeds on Erigeron during the summer, and, in late summer, returns to Solidago where overwintering eggs are laid. 8. Solidago is not nutritionally superior during spring or autumn; its use during those seasons may result from the need for good oviposition sites.