Housed in a plastic food container and measuring just 0.5 centimetres long, the weevil doesn’t look very menacing. Yet this specimen of Listronotus elongatus is the UK’s latest hope in a new wave of biological ways to control invasive species rather than using unsustainable chemical and mechanical methods.
The South American creature’s target is floating pennywort (Hydrocotyle ranunculoides), an invasive plant that chokes rivers and lakes by creating heavy floating mats, cutting water oxygen levels and outcompeting native plants.
The weevil I saw – in a lab belonging to the , just south-west of London – will eat the plant’s leaves, but it is the beetle’s larvae that are particularly deadly, because they eat the stalks from the inside out.
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The weevils were first deployed in the wild in the UK last winter. More will be released in Hertfordshire, the Midlands, Surrey, Sussex and Yorkshire later this year. Dutch authorities are keen to get their hands on the weevil to tackle a major pennywort problem there too. “It just keeps coming back,” says at CABI.
As well as threatening biodiversity, problem invasive species also have an economic burden, . Those are two reasons why the weevil is at the forefront of a new wave of biological agents used in the UK since tiny, plant-feeding insects called psyllids were released to tackle the scourge of Japanese knotweed (Reynoutria japonica) in 2010.
It is fair to say that the sap-sucking psyllids have failed to make much headway against knotweed, with many of the insects being eaten by other bugs. However, a different weevil, Stenopelmus rufinasus, does seem to making inroads in the UK against a problem aquatic fern called Azolla filiculoides.
This mixed record leaves the CABI team still searching for a big breakthrough success against invasive species. The centre’s director, Dick Shaw, has his hopes pinned on the latest endeavour against pennywort: “It has a very high likelihood of success, because it’s a floating plant and a weevil, and the history of that is just fantastic around the world.”
Biological controls have worked well in some other parts of the world, with New Zealand taking a particularly proactive approach, unleashing mites, beetles and other agents to eliminate invasive species early, rather than when they are well established.
Nonetheless, some releases have given the approach a bad name because of unintended consequences. Infamous examples include , and .
Yet these cautionary tales may paint an unfair picture. , one study shows. The painstaking work that Djeddour and her colleagues undertake in the grounds, greenhouses and labs at CABI is designed to rule out the risk of such collateral damage.
To try to ensure that the L. elongatus weevil doesn’t attack anything else, tests were undertaken in Argentina on the 10 or so plant species that live next to floating pennywort, and 70 possible target plants were tested in the UK. Those tests vary depending on the biological agent involved, but range from seeing if the agent will feed on cut leaves in Petri dishes, then on functional plants, and then the ultimate “no-choice” test. “It’s basically a starvation test where you get them on a plant and if they don’t feed on it, they die,” says Djeddour. Four other possible agents for floating pennywort, including two flies, were dismissed.
Once a candidate agent is selected, there is then the job of carefully rearing them for deployment. It can involve a bizarre variety of techniques and tools. The job of collecting the mites that attack one of the UK’s worst aquatic weeds, swamp stonecrop (Crassula helmsii), is speeded up by using a handheld vacuum cleaner, for example. Nothing is released until a roughly year-long government licensing process is cleared, which allows releases for three years.
The first results from attacking floating pennywort and swamp stonecrop aren’t out yet. The initial hurdle will be whether the agents can establish themselves and survive UK winters in the wild. Ideally, they will become self-sustaining without too much intervention.
The next step is monitoring how effective they are at helping to eradicate or at least manage invasive plants. There are reasons to be cautiously optimistic. Of , 71 per cent established themselves there, and 55 per cent caused medium or heavy damage to the target species.
“There is good evidence of success of biological control in the past,” says at the UK Centre for Ecology & Hydrology. Preventing the arrival of damaging invasive species is the most important approach, but she thinks problem plants that have already arrived will need multiple solutions, and biological controls “can be one part of that jigsaw”.
Shaw is optimistic about achieving success with floating pennywort, but he is trying to temper hopes. “We’ve learned over time that raising expectations, if you keep failing, isn’t a good model,” he says.
