Prospects for biological control of Ambrosia artemisiifolia in Europe: learning from the past

The recent invasion by Ambrosia artemisiifolia (common ragweed) has, like no other plant, raised the awareness of invasive plants in Europe. The main concerns regarding this plant are that it produces a large amount of highly allergenic pollen that causes high rates of sensitisation among humans, but also A. artemisiifolia is increasingly becoming a major weed in agriculture. Recently, chemical and mechanical control methods have been developed and partially implemented in Europe, but sustainable control strategies to mitigate its spread into areas not yet invaded and to reduce its abundance in badly infested areas are lacking. One management tool, not yet implemented in Europe but successfully applied in Australia, is biological control. Almost all natural enemies that have colonised A. artemisiifolia in Europe are polyphagous and cause little damage, rendering them unsuitable for a system management approach. Two fungal pathogens have been reported to adversely impact A. artemisiifolia in the introduced range, but their biology makes them unsuitable for mass production and application as a mycoherbicide. In the native range of A. artemisiifolia, on the other hand, a number of herbivores and pathogens associated with this plant have a very narrow host range and reduce pollen and seed production, the stage most sensitive for long-term population management of this winter annual. We discuss and propose a prioritisation of these biological control candidates for a classical or inundative biological control approach against A. artemisiifolia in Europe, capitalising on past experiences from North America, Asia and Australia.