BIOLOGICAL AND GENETIC-CHARACTERIZATION OF MORPHOLOGICALLY SIMILAR THERIOAPHIS-TRIFOLII (HEMIPTERA, APHIDIDAE) WITH DIFFERENT HOST UTILIZATION

The aphid Therioaphis trifolii (Monell) is indigenous to the western P alaearctic region, where it colonizes lucerne (alfalfa), clovers and r elated legumes. A form of the species, T. trifolii f. maculata (the sp otted alfalfa aphid, SAA), has been known in Australia since 1977, fee ding almost exclusively on Medicago sativa (lucerne). Since 1989, T. t rifolii has been found in Australia colonizing Trifolium subterraneum (subclover). We have compared samples of T. trifolii in Australia coll ected on lucerne and subclover using several techniques: survival and reproduction on different hosts, morphology, cuticular hydrocarbon pro file, karyology, and a combination of RAPD-PCR and mitochondrial DNA ( cytochrome oxidase) genetic markers. Whereas there were no distinct di fferences in cuticular hydrocarbons and karyology, we found significan t correlated differences between the host on which aphids were collect ed, some morphological characters, and the genotypes of the aphids. Ap hids collected from lucerne were always of one genetic type, and nearl y all those from subclover were of another. Both groups of aphids are morphologically distinguishable from the yellow clover aphid (YCA), th e other form of the species recognized so far. We conclude that the cl over-colonizers (spotted clover aphid, SCA) and the lucerne-colonizers (SAA) are each host-restricted forms (biotypes) of T. trifolii. Estim ates of mtDNA divergence are in the range of conspecific to closely co ngeneric, as compared with other insects including aphids. We develope d simple diagnostic DNA tests to distinguish the two pest aphids.