The rapid spread of invasive species in non-native regions is one of the main drivers of global biodiversity deprivation. Diverse invader species from different geographic origins, which have never previously interacted, often co-occur in introduced regions due to accelerated biological invasions in the Anthropocene. However, how novel interactions among exotic species from different regions are shaped remains poorly understood. Unraveling these dynamics is crucial to an understanding of the ecological consequences of accelerated species invasions. This study examined the host range expansion of an exotic insect associated with the invasive species Solidago altissima, which spreads worldwide. The invasion of S. altissima facilitated the spread of an exotic aphid, Uroleucon nigrotuberculatum, a specialist on Solidago spp. in North America, feeding exclusively on goldenrod shoots. In Japan, this aphid also feeds exclusively on S. altissima. However, we recently reported the aphid began to utilize other exotic Asteraceae species, such as Taraxacum officinale, in Japan. In this study, we aimed to further investigate how genetic variation in S. altissima promotes colonization success onto other exotic Asteraceae species (T. officinale). Because genetic variation largely influences aphid population size, which in turn may result in differences in spill-over intensity of aphids from goldenrods to dandelions. Specifically, we hypothesized that genetic homogeneity in S. altissima facilitates aphid spillover and host expansion.
We conducted a common garden experiment. The experiment included 30 plots, each containing six individual plants (four goldenrods and two dandelions). The plots were divided into two groups: 10 mixed-genotype plots (i.e., dandelions with four different genotypes of goldenrods) and 20 single-genotype plots (i.e., dandelions with four goldenrods of an identical genotype). Aphid densities were recorded biweekly to track their colonization and reproduction.
We found that the aphids increased on the goldenrods and then evidently colonized the focal dandelions. Moreover, aphids successfully reproduced and increased on the experimental dandelions. Furthermore, aphid colonization on dandelions was significantly greater in single-genotype plots. Thus, genetic homogeneity in the goldenrods resulted in greater recruitment on the dandelions. However, there were no differences in aphid abundance on the goldenrods between mixed and single genotype plots. Thus, greater recruitment on the dandelions did not result from different abundance on the goldenrods. Genetic homogeneity in S. altissima could create uniform plant defenses, reducing ecological barriers to promote aphid dispersal and facilitating host range expansion. These findings suggest that genetic homogeneity in invasive plants can promote exotic aphid host expansion, exacerbating the ecological impacts of invasions.