Host shift capability of a specialist seed predator of an invasive plant: roles of competition, population genetics and plant chemistry

Acanthoscelides macrophthalmus is a seed predator that has become widely distributed along with its native host, Leucaena leucocephala (Mimosoideae), which is a neotropical leguminous tree and one of the most invasive plants worldwide. Previous studies revealed that A. macrophthalmus is able to host-shift to several mimosoid species. Here, we aim to test the host-shift potential to other mimosoid and non-mimosoid plants and possible roles of interspecific competition, genetic background, and plant chemistry in host-shift. First, we found that A. macrophthalmus predator completed development on two new hosts: pigeon pea Cajanus cajan and Cajanus scarabaeoides (Faboideae), by rearing from seeds collected in South/Southeast Asia and Hawaii. In contrast, in most regions, both Cajanus species were infested only by other beetle species. Second, we performed no-choice tests using 11 leguminous plants, covering all three subfamilies as potential hosts, including the two new hosts. A Taiwanese A. macrophthalmus population reared in the laboratory on Leucaena did not deposit eggs on any of the seeds of each tested species. To compare host-shift responses between populations, we also used a Hawaiian A. macrophthalmus population that had completed its development on freshly collected Leucaena seeds from the field. This population deposited eggs onto and hatching larvae burrowed into C. cajan seeds, although none developed beyond the larval stage. Third, the surface chemical composition of seed-pods of L. leucocephala and the two Cajanus species was dissimilar, although that of seeds was highly similar. Finally, all of the host-shifting A. macrophthalmus populations shared the same haplotypic group.