Identifying new associations between invasive aphids and Pinaceae trees using plant sentinels in botanic gardens.
Despite progress in prevention and management of biological invasions, challenges remain, including difficulties with assessing future invasion risks. Predicting the identity of potentially damaging invaders is complex because they are often unknown as pests in their natural range. We used a plant sentinel approach to assess host ranges of invasive aphids across 62 conifer species from around the world in Christchurch Botanic Gardens, as a model for risk assessment such as for the 'International Plant Sentinel Network'. Using standardised sampling, we obtained ca. 600 observations of aphid abundances (4731 individuals), mainly of the pine aphids Eulachnus brevipilosus and Essigella californica and the spruce aphids Cinara pilicornis and Elatobium abietinum. These aphids were highly genus-specific, despite the spatially mixed distribution of genera. A phylogenetic analysis of pine aphid host preferences showed that abundances of species varied among pine subgenera and the geographic origin of trees, with significantly more aphids on pines in the subgenus Pinus than the subgenus Strobus. Essigella californica occurred abundantly on many pines across most subsections in the subgenus Pinus whereas E. brevipilosus was largely restricted to a few species in the subsection Pinus. Our study revealed several new host records (previously unknown aphid-host relationships) including the abundant occurrence of E. brevipilosus on Japanese red pine, P. densiflora, and of E. californica on P. yunnanensis, P. serotina, P. brutia, and P. nigra, among others. Our study identified numerous novel insect-plant interactions that are likely to materialise if these aphids colonise new host plants, confirming the utility of the plant sentinel approach.