EVOLUTION OF COMPLEX LIFE-CYCLES IN APHIDS

1. Two hypotheses have been proposed to account for host alternation in aphids: (i) it is maladaptive and a consequence of the specialization of the fundatrix for life on a woody (primary) host plant, which prevents the aphid transferring its whole life cycle to a nutritionally superior herbaceous (secondary) host plant; and (ii) it is adaptive and enables aphids to exploit the complementary growth patterns of herbaceous and woody plants. These hypotheses are tested experimentally using the host-alternating willow-carrot aphid Cavariella aegopodii. 2. The fundatrices of three species of Cavariella can survive on their secondary host plants and although they do not do as well as on their primary host plant at bud burst, their survival on the secondary host plant is comparable to that on the mature and senescent foliage of the primary host plant. 3. Overall, of the two host plants the primary is the better in spring and autumn, and the secondary in summer. This is so for both the fundatrices and summer apterae. 4. Although no species of Cavariella is known that completes its life cycle on a herbaceous plant in the field, in the laboratory the sexual females of C. aegopodii can develop, mate and lay eggs on their secondary host plant, most of which survive to hatch and give rise to fundatrices. 5. Following the growth of the primary and a secondary host of C. aegopodii in the field revealed that these two plants showed complementary growth patterns. 6. The relative success of life cycles with and without host alternation is dependent on the number of host transfers, the success of each transfer and the number of generations the aphid can complete on the herbaceous host. 7. The ecological significance of these results are discussed and it is concluded that they favour the view that host alternation is adaptive and is the means by which aphids exploit the complementary growth patterns of woody and herbaceous plants.