Listronotus elongatus, the pennywort weevil (Photo Credit CABI from New Scientist (see article link below)

Dear Readers, I cannot tell you how delighted I am to have finally had the chance to use ‘the lesser of two weevils’ as my post title. I have been waiting since 2014 when I started the blog to make a really terrible pun, and here it is. But what we have here is another example of bioremediation, where an alien organism (usually a plant) is combatted with another alien organism (in this case, a weevil) in the hope that the results won’t be worse than the initial problem.

The problem here is a water plant called floating pennywort (Hydrocotyle ranunculoides), which is native to North and South America. Unfortunately it became popular as a pond plant and promptly escaped, choking watercourses up and down the land and being a general pest. As invasive species are estimated to cost the UK along up to 1.7 billion GBP per year, it’s clearly worth thinking about creative ways of dealing with the problem, but all of us will remember the law of unintended consequences, such as the release of harlequin ladybirds, originally brought in to tackle aphids but ending up being predators of other ladybird species. Is it possible to stop this kind of thing happening again?

Floating pennywort doing it’s ‘thing’ on the River Soar in Leicester

It’s clear that our weevil has been carefully selected.  In South America, the adult insect eats the leaves, but the larvae eat the stalks from the inside out. The weevils were first deployed in the UK last winter, and everyone seems want some, from Hertfordshire to Yorkshire. Plus, the Dutch have a similar problem with the plant and are also keen to get their hands on some weevils.

Any creatures that are being considered as biological controls are assessed in the UK by CABI (the Centre for Agriculture and Bioscience International. Before the weevils are released, they were assessed on the ten or so plants that grow next to floating pennywort, to make sure that they wouldn’t eat them. They were then tested on another 70 possible target plants that are found in the UK. Amongst the tests was the ‘no-choice’ test, where the poor long-suffering weevils are only presented with a single species of plant, and will basically starve if they don’t eat it. This sounds rather like my cat being presented with a different variety of cat food – I am fairly sure that she would starve rather than eat something inferior. Fortunately, I don’t put her to the test.

Floating pennywort on a pond in Anglesey (Public Domain)

Once the weevils were considered to be very floating-pennywort-specific in their tastes, a licence was granted by the UK government, which allows for releases over the nest three years.

The next question is, will the weevils establish themselves, or will it be a case of ‘ see no weevil”? You might remember (or possibly not) that some sap-sucking bugs were released in 2010 to help to clear up Japanese Knotweed, and we can all see how effective that was (ahem).

The Japanese Knotweed bug (Aphalara itadori) (Photo from CABI)

Maybe we’ve just been singularly unlucky here in the UK – worldwide, according to New Scientist, of the 468 organisms released in 90 countries, 71 per cent established themselves, and 55 per cent caused medium or heavy damage to the target species that they were meant to be controlling. Less than ten per cent of biological control organisms target species that they weren’t meant to, and it seems that, with tests like those outlined above, we’re getting better at making sure that only the plants that are meant to get eaten get eaten.

The folk at CABI will be cheerfully breeding and rearing weevils in their hundreds of thousands now that the licence has been given. Let’s wish them (people and insects) the best of luck!

You can read the full New Scientist article here