Three taxa were detected by allozyme markers within the mealy aphids of the Hyalopterus pruni complex, having different cultivated Prunus species as main primary hosts. The genetically closer H. pruni and H. amygdali A (DNei= 0.10) never share primary hosts, whereas H. amygdali A and B (DNei= 0.32) may occasionally share them, producing few F1 hybrids. The three species proved reproductively isolated in the field, with no gene exchange. Their speciation seem to have occurred long before the agricultural revolution, crop colonization representing a host range expansion rather than a host shift, as in sympatric speciation.
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Mosco et al. (1997) studied this question.