So, what's the problem
Flowering rush (Butomus umbellatus) was introduced from temperate Eurasia to North America as an ornamental aquatic plant more than 100 years ago. It escaped cultivation and spread in the wild to become a severe problem in freshwater systems of the midwestern/ western states of the USA and in western Canada with multiple impacts. Infestations can increase maintenance costs in irrigation ditches and impede recreational activities along rivers and lake shores. The plant provides habitat for the great pond snail, which hosts parasites that cause ‘swimmer’s itch’ (a skin rash caused by an allergic reaction). Dense stands of the plant may also benefit introduced non-native fish that spawn in vegetated substrate to the detriment of native fish.
No effective control techniques are currently available.
What is this project doing?
We started a biological control project at CABI in 2013. Our aim is to find specific natural enemies and assess their suitability for release as biological control agents in North America, where they could reduce the vigour and limit the spread of flowering rush.
The ideal biological control agent is very specific and inflicts serious damage to the target host plant. The specificity of a natural enemy reflects how closely its evolution has been linked to that of its host (how coevolved they are). Surveying in the area of origin of a target weed is a good way of finding coevolved natural enemies.
By reviewing the literature and carrying out field surveys in the weed’s area of origin in Europe, we identified several organisms that look sufficiently promising to warrant in-depth investigations. We are currently establishing rearing colonies and are studying their biology, host specificity and impact on flowering rush to evaluate their potential as biological control agents.
Our field surveys range from northern Germany, to the Czech and Slovak Republics, Poland, Hungary, Serbia and Georgia.
The semi-aquatic weevil Bagous nodulosus is currently our most promising candidate. Its larvae feed on the leaves and rhizomes of flowering rush. Although it was reported to be rare, we have collected it at over 20 sites so far. We have established a rearing colony at CABI’s centre in Switzerland, although we are still having problems with high larval mortality. In no-choice oviposition tests (offering females only a test species or the target weed) with 38 species, B. nodulosus has so far only accepted one other plant species, the European Baldellia ranunculoides, for egg laying besides flowering rush. This confirms that the weevil has a very narrow host range. We hope to be able to petition for B. nodulosus to be released in North America in the near future.
We also started working with a second weevil, B. validus, which appears to be even rarer and more difficult to work with. In addition, we are studying an agromyzid fly, Phytoliriomyza ornata, and a white smut fungal pathogen, Doassansia niesslii. The impact of the fly and the smut on flowering rush look very promising, but still need to be quantified. Their life cycles and host-specificity also still need evaluation.
Research Scientist, Weed Biological Control
Country Director and Head Weed Biological Control
Principal Scientist, Invasive Species Management