Action: Floating pennywort: Biological control using co-evolved, host specific herbivores
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- A replicated laboratory and field study in South America found that the South American weevil caused more feeding lesions on floating pennywort than on any other plant species, but field results found that the weevil did not reduce floating pennywort biomass.
One-off introduction of a co-evolved, host-specific species from the area of origin of the invasive pest can potentially provide sustainable control without affecting non-target native plants. For example, a laboratory study in Argentina, found that the South American weevil Listronotus elongatus appears host specific on floating pennywort Hydrocotyle ranunculoides, exhibiting both feeding and reproductive behaviour (Cordo et al. 1982). Observations have also been reported of extensive damage caused by this same weevil to floating pennywort (Newman & Duenas 2010). In addition, observations in Germany showed that the introduced Coypus Myocastor coypus can eat floating pennywort (Hussner & Lösch 2007).
Cordo, H. A., DeLoach C. J. & Ferrer R. (1982) The weevils Lixellus, Tanysphiroideus, and Cyrtobagous that feed on Hydrocotyle and Salvinia in Argentina. Coleopterists Bulletin, 36, 279–286.
Hussner, A. & Lösch, R. (2007) Growth and photosynthesis of Hydrocotyle ranunculoides L. fil. in Central Europe. Flora (Jena), 202, 653-660.
Newman J.R. & Duenas M.A. (2010) Information sheet: control of floating pennywort (Hydrocotyle ranunculoides). Centre for Ecology & Hydrology, Wallingford, UK. 3pp.
Supporting evidence from individual studies
A replicated laboratory and field study in 2011 in South America (Walsh & Maestro 2011) found that the South American weevil Listronotus elongatus, was the most common herbivore on floating pennywort and caused more feeding lesions on floating pennywort than on any other plant species, but field results found that the weevil did not reduce floating pennywort biomass. Other species found to feed on floating pennywort included mining flies of genus Monochaetoscinella and Hydrellia, and the larvae of the moth Paracles quadrata. When the weevils were allowed to invade a mixed patch containing floating pennywort in the field, the highest numbers of larvae and adult weevils were found on patches of floating pennywort with the highest biomass, indicating that weevils perhaps move away from damaged plant sections and concentrate in the denser sections of the plant patch. Studies of adult weevil plant choice involved a three-stage process, beginning with a simple no-choice, cut-leaf feeding test on many plant species, followed by a whole-plant against cut-leaf no-choice test on selected species. Finally, South American weevils were allowed to invade a mixed patch containing floating pennywort, where after two months, damage was evaluated in 30 randomly selected 10 x 10 cm squares.