On the mechanistic basis of plant affiliation in the black bean aphid (Aphis fabae) species complex

Abstract

A number of aphid species comprise multiple taxa (biotypes, races, subspecies etc.) with distinct but often overlapping plant ranges. These species complexes are of considerable evolutionary interest, with a long-standing debate as to whether they may have evolved sympatrically through disruptive selection arising from trade offs in the capacity to utilise different plants (e.g., Futuyma & Mayer, 1980; Diehl & Bush, 1989; Via, 1991; Bush, 1994). However, little is known about the behavioural or physiological processes that may underpin the negative correlations between the capacity of closely-related aphids to utilise different plant species [but see Wilkinson & Douglas (1998) and Caillaud (1999)]. This study concerns the black-bean aphid Aphis fabae Scop., a heterecious aphid with ca. 80 summer (secondary) hosts and 1–3 winter (primary) hosts. The differential capacity of the four recognised subspecies (A. f. cirsicanthoides, A. f. fabae, A. f. mordwilkoi and A. f. solani) to utilise secondary hosts may have selected for assortative mating of the sexual generation on the primary hosts (Mackenzie & Guldemond, 1994), and a negative correlation has been demonstrated in aphid utilization of Vicia faba and Tropaeolum majus, specific host plants of A. f. fabae and A. f. mordwilkoi respectively (Mackenzie, 1996). However, this trade off is not symmetrical (Douglas, 1997): A. f. fabae perform more poorly on T. majus than A. f. mordwilkoi on V. faba, and it was suggested that T. majus may be unsuitable for feeding by A. f. fabae. The purpose of this study was to use the electrical monitoring technique (EPG) of Tjallingii (1988) to test the prediction that the capacity of aphids to penetrate to the sieve elements and exhibit sustained feeding on phloem sap is reduced for A. f. fabae on T. majus, but not for A. f. mordwilkoi on V. faba.