Pollinators of the invasive plant, yellow starthistle (Centaurea solstitialis), in north-eastern Oregon, USA

The potential pollinators of yellow starthistle (Centaurea solstitialis) were surveyed at six sites in north-eastern Oregon, USA, between May and September from 2000 to 2002. The objective of the study was to determine the species composition and relative abundance of the insects that visited yellow starthistle throughout the flowering season and to assess the pollen loads on their bodies in order to infer which species might be the most effective pollinators of this invasive plant species in north-eastern Oregon. A total of 1923 individual flower visitors were collected at the six sites over the 3 year study period, comprising four orders, 41 families, and 203 species of insects. The 20 most commonly collected species represented nearly 59% of the individuals and just ten of these species could be considered the key pollinators, judging by the combination of abundance and pollen carriage (the megachilids, Megachile apicalis (introduced) and Megachile perihirta, the apids, Apis mellifera (introduced), Bombus bifarius, Bombus centralis, Svastra obliqua, and Melissodes lutalenta, the halictids, Halictus tripartitus and Halictus ligatus, and the tachinid, Peleteria malleola. Over the 3 year study period, the six sites were consistently distinct in their flower visitor fauna, with the metropolitan Pendleton sites having a species composition distinct from the four mountain sites. Consistent patterns of interannual variation also were observed over the 3 year study. These patterns of flower visitation are interpreted in the context of the plant community within which yellow starthistle grows in north-eastern Oregon.