Novel plant-insect interactions in an urban environment: enemies, protectors, and pollinators of invasive knotweeds

Relationships between invasive plants and other species in their introduced ranges may facilitate or hinder the process of invasion. Fallopia japonica (Japanese knotweed), Fallopia sachalinensis (giant knotweed), and their hybrid Fallopia x bohemica (Bohemian knotweed) are widespread invasive plants in North America and Europe. These species possess extrafloral nectaries (EFNs) that attract mutualist insect protectors in their native ranges. Popillia japonica Newman, 1841 (Japanese beetle) is a primary herbivore of invasive Fallopia spp. in these species' native range in Japan. These natural enemies are reunited in North America, where Fallopia spp. have been repeatedly introduced as ornamental plantings of residential and commercial properties since the 1800s, and the Japanese beetle has become a widespread insect pest. Spread of Fallopia spp. and their hybrids along linear features of urban landscapes such as roads, railways, and waterways make their performance in urban environments important to these invasive species' impact. To test the role of insect interactions in the success of Fallopia japonica and F. x bohemica in urban conditions in their invasive range, we examined ant visitation, Japanese beetle abundance and herbivory, ant-Japanese beetle interactions, pollinator visits, and seed production. All active herbivory observed was by Japanese beetles. Leaf area loss to herbivory was much lower than levels reported in Japan, suggesting partial enemy release. New Fallopia leaves were more frequently visited by ants than mature leaves, while Japanese beetle herbivory was observed on mature but not on new leaves. All ant species observed visiting invasive Fallopia EFN were native to North America. Active physical defense by these ants against Japanese beetles was observed but was rare. Native and non-native insects visited Fallopia flowers, followed by seed production. Our results indicate that both native and non-native species may facilitate invasion of Fallopia through pollination and protection mutualisms.