Comparative performance of spotted clover aphid and spotted alfalfa aphid on annual medic cultivars

Twenty-three annual medic cultivars, hybrids and accessions were tested for their suitability as hosts of 2 genetically different biotypes of Therioaphis trifolii (Monell) [spotted alfalfa aphid (SAA) and spotted clover aphid (SCA)I which occur in Australia. Aphids were reared on excised trifoliate leaves on agar containing a soluble fertiliser. Parameters measured were initial host acceptance by adult aphids, mortality of Ist generation nymphs, developmental time of apterous aphids, incidence of alate adults, and 4-day fecundity of apterous adults. Host suitability indices were calculated for the 2 biotypes on each of the accessions. Spotted clover aphid clearly outperformed SAA on the majority of the medics tested. It accepted most host plants more readily, and SCA nymphs nearly always developed more rapidly, suffered lower mortality and produced fewer alatae than those of SAA. A number of cultivars which had been bred for resistance to SAA proved to be highly unsuitable hosts for SAA but relatively suitable hosts for SCA. Spotted clover aphid has the potential to be a serious pest on susceptible medics, and to be able to survive on resistant cultivars until more favoured pasture legumes become available.