Distribution, host plants, and reproductive biology of the Sitona humeralis Stephens group of species (Coleoptera, Curculionidae)

Four taxa have hitherto been confused under the specific name of Sitona humeralis. Their present distribution and host plant relationship are described, based on the analysis of over 60000 specimens, field collected from 16 countries of the Western Palaearctic. Results show that the true S. humeralis achieved high levels of both abundance and dominance on cultivated lucerne, whereas S. bicolor bicolor was most successful on annual Medicago species. S. bicolor concavirostris and S. discoideus were able to reach similar degrees of dominance on both categories of host plants, the distribution of the latter species in southwest Europe, however, being correlated with that of annual medics.

There is a wide overlap in the distribution of S. humeralis and that of the other species of the group, and rearing in the laboratory demonstrated that interbreeding was likely to occur in some field situations. Combination of S. discoideus with S. humeralis resulted in a reduced percentage of mated females and depressed fecundity and fertility as compared with conspecific matings. The average fecundity of S. humeralis (1098 eggs per mated female) did not differ significantly from that of S. discoideus (1009 eggs), but oviposition lasted distinctly longer and showed different peak periods. In temperate areas adult S. humeralis emerge in late summer and undergo hibernation, but adults of the same species emerge in spring and undergo aestivation in the Mediterranean region.

As with their annual Medicago spp. hosts, species of the S. humeralis group seem to form a succession along the northern coast of the ancient Mediterranean Sea, indicating that they most probably evolved together over the whole region.