What evolutionary advantage does the Woolly Alder Aphid's obligatory host alternation provide?

It reduces resource competition and possibly aids in escaping specialized predators associated with a single host plant

The complex, two-host strategy employed by the Woolly Alder Aphid during its holocyclic cycle presents a significant evolutionary advantage by diversifying its ecological niche. By thriving on alder trees during the growing season and then migrating to a distinct secondary host, such as maple, to complete the sexual phase, the aphid effectively spreads its resource utilization across two different plant types. This temporal and spatial separation reduces direct competition for necessary nutrients that would occur if the entire population remained on a single host throughout its life. Moreover, this movement allows the species to potentially evade specialist predators that have co-evolved closely with the primary host, thereby increasing overall species fitness and survival probability across the annual cycle.