A first step towards the biological control of Iris pseudacorus L. (Iridaceae)

Native to Europe, North Africa and western Asia, the yellow flag (Iris pseudacorus L.) has long been exported globally as a valuable ornamental and pond plant (Sutherland 1990), occasionally escaping cultivation and establishing itself as an invasive species. Outside of its native range, this emergent macrophyte tends to grow dense monospecific stands, displacing the local vegetation and altering the hydrology of aquatic ecosystems (Jacobs et al. 2011). To date, this species is considered invasive in Canada, part of the U.S., Chile, Argentina, Uruguay, South Africa, Japan, Australia and New Zealand (USDA 2013). Due to its potential ecological and economic impacts, in South Africa, it was recently listed as a category 1a invader for which management actions need to be prioritized (NEMBA 2014, Jaca & Mkhize 2015). Because mechanical and chemical control methods are regarded as unsustainable in the long term, a biological control program was initiated to tackle its invasion (Hill & Coetzee 2017). In this regard, we conducted a preliminary survey of yellow flag’s native populations in Europe in order to identify its naturally co‐evolved enemies and prioritize a set of candidate biocontrol agents to be investigated. A total of 12 locations (36 sampling sites) were visited between 2017 and 2018, covering different seasons and accounting for a variety of habitats across Belgium and Northern Italy. At each site, we collected invertebrates found on the plant and recorded their feeding behaviour in relation to our target as well as to the co‐occurring native vegetation. Of the 61 species identified from our sampling, only two met the criteria applied for selecting the candidates. Further investigations focused on the iris flea beetle, Aphthona nonstriata Goeze (Coleoptera: Chrysomelidae), in an attempt to fill the knowledge gap regarding its biology. Controlled life‐history observations highlighted the potential of this species to impair the growth and survival of I. pseudacorus. Adults are voracious leaf‐miners, whereas the larvae bore and develop within the plant rhizomes, generating wounds that may be a vector for bacterial and fungal infections. Finally, a short‐term no‐choice feeding assessment was carried both on cut leaves and live plants. The results showed adult flea beetles feeding significantly more on our target compared to the other plants tested. A population of beetles are currently undergoing host‐specificity testing within the quarantine facility of the Centre for Biological Control at the Rhodes University (South Africa), where it will soon be joined by our second candidate. Altogether, the information gathered through this research constitute a first fundamental step towards the biological control of I. pseudacorus in South Africa, and possibly elsewhere in the world.