Gardening: A natural-born killer that fears no weevil

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In the war against the vine weevil, that most voracious of garden pests, an unexpected knight in shining black armour has appeared.

The Hammer House of Horror, I am sure, has nothing in its archive quite as ghoulish as the film I was watching last week: Pterostichas melanarius, blown up to the size of a dinosaur, mashing its monumental jaws round the nether regions of Otiorhynchus sulcatus, the vine weevil. It's the closest I have ever come to feeling sorry for the vine weevil, which is fast becoming public enemy number one as far as British gardeners are concerned.

I was watching the film in a laboratory at East Malling in Kent, one of Horticulture Research International's many research centres. Will the epic ever get a major release? No, probably not, though Alison Crook has spent the last three years working on the script, in fact a PhD thesis. The outcome is good news for gardeners, though not so brilliant for vine weevils.

If you've met them in your garden you will know all about the damage they do. The larvae feed underground on the roots of plants, particularly those potted up in soil-free compost, which they seem to prefer. The adults, when they hatch, feed voraciously, causing irritating and disfiguring notches on leaves and petals. Unfortunately, they are relatively undiscriminating in their tastes and will attack at least 150 different kinds of plants. I find they are particularly fond of primulas and auriculas, but they home in on strawberries, blackcurrants, conifers and young rhododendrons, too.

In the past, the only effective way to get rid of vine weevils was to drench them with a pesticide called Aldrin, but it was a potentially dangerous tool, hanging around in the soil long after it had done its work. As Dr Mike Solomon, chief entomologist at East Malling, said: "We've come a long way since that was thought to be a good characteristic." A few years ago, Aldrin was withdrawn from the market.

Alison set out to find whether the vine weevil could be controlled by ecologically kinder methods. Scientists had already discovered that a gobbling nematode could be introduced to destroy them, but she was interested in the idea of encouraging the vine weevil's natural predators. Except that nobody knew what they were.

The good guys turned out to be blackbeetles, including the mighty pterostichas that I had watched in round one of the combat arranged between it and the vine weevil. Dr Solomon, who has been supervising the project, explained that this is only one of the many helpful black ground beetles that live in gardens, especially ones that have plenty of low plants, leaf litter and other debris. "They hunt quite effectively," he said. "Mostly at night."

Their nocturnal habits made the beetles difficult to study in the field. Nobody had ever seen one attacking a vine weevil, though they are big enough and well equipped enough to do so if they chose. Alison brought some blackbeetles into the laboratory and served them vine weevils on a plate. They polished them off, which was an encouraging start, but not enough to build a thesis on. As Alison pointed out: "All it told us was that given the choice between starving and eating vine weevils, they would eat the weevils." She needed to know whether they would do so in the wild, where they would be surrounded by other equally succulent tit- bits.

But if you can't see it at work, how do you tell what a beetle has been eating? Until recently, the only way was to dissect out its gut, a massively fiddly operation, and not an entirely satisfactory one. "You might be able to identify bits of cuticle or other hard material, but if a beetle had been feeding on the liquid parts of its prey, nothing would show up," said Dr Solomon.

The answer lay in a technique pioneered in medical research, the ELISA (enzyme linked immuno-sorbent assay) test, a type of assay that uses monoclonal antibodies to recognise particular patterns of DNA. Alison set up pitfall traps for a range of beetles, ground them up in a pestle and mortar, then screened them using the ELISA technique. The test is so sensitive, it can even show whether beetles have been feeding on adult vine weevils or its burrowing white larvae. Nobody knew that ground beetles would burrow for food until Alison brought some into the laboratory and set them up in a sand-filled tank. They filled it with little tunnels. "That was very good to see," she said.

By painstakingly screening a huge variety of ground beetles, they discovered that a tiny one, Notiophilus biguttatus, not much bigger than an ant, was a key consumer of vine weevil eggs and very young larvae, hoovering them up faster than a Dustbuster. A much bigger and very beautiful beetle, Carabus violaceus, with a body that has the sheen of the most expensive satin, had an equally insatiable appetite for adult vine weevils, especially young ones just emerging from their cocoons. At that stage the carapace round their bodies is still soft. I feel the same way about French beans - they're not half so succulent once they've aged.

Oddly, the most ferocious beetle of all, Ocypus olens, a long, streamlined model commonly called the devil's coach horse, turned out to be a less useful predator than other beetles. It is so savage, it eats anything it comes across. It will take vine weevils if they are around, but it doesn't choose them in preference to other prey, and won't burrow after them, as other beetles do.

Having isolated the vine weevil's natural enemies, the next stage of the research project at East Malling will look at ways of encouraging them. The most useful beetles can't be mass reared and released where they are needed. They are so fierce that they would eat each other up in their pens. So the only way forward is to try to do everything we can to encourage the beetles to flourish in their natural habitats.

The work carried out at East Malling isn't done with gardeners particularly in mind. Its research projects are partly funded by the Government, and partly by a levy imposed on commercial growers through the Horticultural Development Council. The HDC have been funding the vine weevil project because their members - flower growers, fruit growers, nursery owners - are the ones that suffer most from it.

But ultimately, of course, gardeners benefit from this research, too. So what can we do to help the beetles in their war against vine weevils? Stop being neurotically tidy in our gardens. Beetles like cover. They like to scurry through what, to them, is a camouflaging forest of grass stems, low plants and dead leaves.

Stop covering vast surfaces with what garden designers call "hard landscaping", sterile areas of concrete, tarmac and reconstituted stone. Stop using wide-spectrum insecticides that kill everything they touch. It's no using killing the bad guys if you wipe out the potentially good ones as well. Stop using garden vacuum cleaners, to which a beetle is just another bit of rubbish.

The more you know about the enemy, the more you realise that we need every beetle we can persuade to stay with us. All vine weevils are female. They don't have to go to the trouble of finding a mate to produce 500 eggs each season. Just one of the beasts is enough to start an infestation. They are, as Dr Solomon said "extraordinarily catholic" in their diet. For all these reasons, they are dangerous opponents. So make this the year of the beetle, and give thanks that they and the entomologists of East Malling are fighting on our side.