EVOLUTION OF COMPLEX LIFE-CYCLES IN APHIDS

1. Two hypotheses have been proposed to account for host alternation i n aphids: (i) it is maladaptive and a consequence of the specializatio n of the fundatrix for life on a woody (primary) host plant, which pre vents the aphid transferring its whole life cycle to a nutritionally s uperior herbaceous (secondary) host plant; and (ii) it is adaptive and enables aphids to exploit the complementary growth patterns of herbac eous and woody plants. These hypotheses are tested experimentally usin g the host-alternating willow-carrot aphid Cavariella aegopodii. 2. Th e fundatrices of three species of Cavariella can survive on their seco ndary host plants and although they do not do as well as on their prim ary host plant at bud burst, their survival on the secondary host plan t is comparable to that on the mature and senescent foliage of the pri mary 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 pla nt 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 fiel d revealed that these two plants showed complementary growth patterns. 6. The relative success of life cycles with and without host alternat ion is dependent on the number of host transfers, the success of each transfer and the number of generations the aphid can complete on the h erbaceous host. 7. The ecological significance of these results are di scussed and it is concluded that they favour the view that host altern ation is adaptive and is the means by which aphids exploit the complem entary growth patterns of woody and herbaceous plants.