Human-mediated plant dispersal changes insect distributions with potentially important evolutionary implications

Human introductions of plant species often lead to secondary invasions by non-native insects or range expansions of native ones. For instance, native insects may undergo extralimital spread when they adopt non-native plants that are closely related to their native host plants. Leptocoris soapberry bugs feeding on plants in the soapberry family (Sapindaceae) provide prime examples of such host shifts. In Australia and South Africa, the introduction of non-native sapindaceous plants, as well as human-mediated changes in the distributions of native ones, has the potential to impact the ecology and evolutionary dynamics of native soapberry bugs. Observations indicated that, in both countries, the ranges of some native Leptocoris species have expanded substantially due to the human-mediated movement and introduction of native and non-native sapindaceous species. In Australia, the native L. rufomarginatus has established in the Sydney region, where native Alectryon species have been introduced and naturalised well outside their historical ranges, i.e., extralimital host plant species, and the introduced non-native L. vicinus was found to be localised at its initial introduction site in Darwin on the non-native Schleichera oleosa . Species distribution models showed that vast areas in Australia currently have a suitable climate for both the native and non-native host plants of soapberry bugs. This suggests that range expansions by these host plants and insects are likely to continue. In South Africa, the native L. mutilatus now occurs ~1,000 km beyond its previously described range, feeding on the invasive balloon vine Cardiospermum grandiflorum and the non-native Australian tree A. connatus . Evidence is provided for differentiation in the proboscis length of L. mutilatus insects feeding on different non-native, invasive, and native host plants. In Australia, the native L. tagalicus was also successfully hybridised and backcrossed with the non-native L. vicinus , providing the first report of interspecific hybridisation for soapberry bugs under laboratory conditions. The shared evolutionary history between soapberry bugs and sapindaceous host plants suggests that further changes in these insects’ distributions and adaptive responses to novel host plants are likely to occur. Taken together, these results demonstrate how human-mediated plant dispersal affect the ecology, distribution, and evolution of native insects.