Distribution and host preference of a potential biocontrol agent with a new association for the alien water lily Nymphaea mexicana in South Africa

Abstract

Although classical biological control makes use of natural enemies from the native range of an invasive alien species, there are occurrences in which species in the invaded range expand their host range and form new associations with the invader. Bagous longulus Gyllenhal (Coleoptera: Curculionidae) is a South African native weevil hosted by the native water lilies Nymphaea nouchali Burm. f. and Nymphaea lotus L. (family Nymphaeaceae), and it has formed a new association with the invasive alien water lily Nymphaea mexicana Zuccarini. Host range expansions of more-specialist native herbivores on alien invasive plant species occur relatively infrequently; therefore, we investigated the potential of this weevil as another means of managing N. mexicana in South Africa through augmentative biological control. Field surveys were conducted in 2021 and 2022 to determine the distribution of the weevil, and host-specificity tests were conducted under greenhouse conditions to understand its preference for its natural host as compared with N. mexicana and an exotic Nymphaea hybrid. The weevil was widely distributed at 31 surveyed sites across the country but was found feeding and completing lifecycles on only Nymphaea species, including the native hosts and the alien invasive N. mexicana. Additionally, there were no major differences in feeding preference for its natural host when compared with N. mexicana, but the Nymphaea hybrid tested was less preferred. The narrow host range of this weevil, its presence and impact on N. mexicana at three invaded sites, and the fact that a classical biological control agent has not yet been approved, all suggest a potential for B. longulus to be released augmentatively to manage N. mexicana in South Africa. Even so, possible risks such as increased herbivory on native Nymphaea species with augmented populations of its natural enemy should be carefully considered and tested before implementation.