The new green revolution

Pine weevil infected with Metarhizium

Pine weevil infected with Metarhizium

16 September 2011 by Tom Marshall

Crop pests can wreck livelihoods and endanger lives, but the chemicals farmers use to control them cause problems in their own right. Could an insect-killing fungus be the answer? Tom Marshall finds out.

The vine weevil's in deep trouble. It stopped moving hours ago, and its body is now covered in a fine layer of mould that's slowly turning from white to greyish-green.

Bad news for the hardiest bug, and not much better for the other weevils in the neighbourhood. Before long one wanders past and brushes against its dead fellow. A cluster of spores clings to its flank.

Stuck fast to the oblivious insect, the spores start germinating. Sharp peg-like appendages secrete enzymes and corkscrew through the host's tough outer cuticle to reach the soft interior.

Slender filaments infiltrate the insect's body, swelling, releasing toxins and eventually consuming its internal organs. Eventually mould spreads over its exterior, more spores forming and ripening until they're ready to infect the next generation.

It sounds like a plague from the pages of science fiction, but it's a natural fungus, and farmers are already using it against some of their worst insect foes.

Green muscardine fungus, or Metarhizium anisopliae, occurs naturally in soils worldwide. A brutally effective killer of certain insects - in the right conditions it works in days - it's harmless to beneficial creatures like bees and ladybirds.

Rhipicephalus nymph showing the green conidia of Metarhizium

Rhipicephalus nymph showing the green conidia of Metarhizium

Professor Tariq Butt, an expert in entomopathogenic (insect-killing) fungi at Swansea University, thinks this natural biological agent could transform pest control and give growers an environmentally friendly and sustainable alternative to chemical pesticides. In developing countries, it could improve the lives of both growers and local communities by overcoming the growing problem of pest resistance, as well as reducing exposure to potentially harmful pesticides.

"Many chemical pesticides are now being phased out because of their impact on wider ecosystems, so this development couldn't come at a better time," says Butt. "Each strain of Metarhizium has a different host range, providing the opportunity for growers and other end users to select specific strains or combinations of strains to target and control specific pest species prevalent in their crop or local environment."

The so-called green revolution happened in the mid-20th century. Researchers pioneered new growing methods; crop yields soared through improved plant strains and chemical fertilisers and pesticides.

It secured plentiful food for many who'd otherwise have risked starvation. But this came at a cost. Chemicals don't affect only the crops they're sprayed on. They disrupt whole ecosystems indiscriminately, affecting beneficial organisms alongside harmful ones.

They're also expensive, and many targets are becoming resistant to them. Regulators have responded by trying to reduce inputs of chemical pesticides, promote biodiversity and encourage non-chemical pest control.

A natural insecticide

Metarhizium is found in soils around the world; different strains flourish in different conditions. One strain, F52, is especially effective at controlling harmful pests like western flower thrips; wireworm, a widespread scourge of potato farmers; and black vine weevil, which costs nursery growers some £30m a year by devastating crops like strawberries.

Biotechnology firm Novozymes launched a product based on the work of Butt's team at Swansea in early 2011. Horticultural products company Fargro is selling it in the UK and Ireland as Met52. It's deadly to target species but leaves others alone, minimising the impact on insect-eating birds, mammals, amphibians and fish.

"Met52 contains the optimum F52 strain of the Metarhizium fungus," says Hugh Frost, an agronomist in Novozymes' crop protection business. "Other strains also perform well in certain niches, but this one is reliably effective in a range of situations, can be produced readily and is stable for a long time."

Black vine weevil at various stages of infection with Metarhizium

Black vine weevil at various stages of infection with Metarhizium

The fungus is part of a new generation of 'biocontrols' - living things that naturally keep undesirable organisms in check. The idea isn't new, but its time may now have come.

"People have been looking at fungal pest control since the 1980s, but it tended to be unreliable, difficult to produce and of mostly academic interest," says Dr Paul Sopp, of Met52 distributor Fargro. "But there's been a significant shift in attitudes in the last few years, and now the time is right."

In theory, Metarhizium can be sprayed over plants as they grow - Novozymes is working on such a product. At the moment, though, Met52 is mixed into their compost. It stays underground, killing pests before they get a foothold.

"We've mostly targeted the subterranean stages, as it's very hard to get at insects here normally - you need high doses of organophosphate," Butt explains. These chemicals are widely used but will be phased out in the coming years.

Beyond the green revolution

Butt believes Metarhizium could eventually help with many other problems. Strains exist which kill agricultural pests, like corn rootworm and chafers, which cause billions of pounds of damage annually to everything from root vegetables to turf and ornamental plants.

Gardeners and farmers aren't the only possible users, though. Foresters could control Christmas tree weevils with Metarhizium; wine-makers could unleash it on the sap-sucking phylloxera bugs that can quickly ruin once-productive vineyards.

Beekeepers are testing a strain that kills the honeybee parasite Varroa. Even the sports industry could benefit - leatherjackets and chafers wreak havoc on playing fields and golf courses as well as pasture.

Other susceptible pests include major vectors of plant, animal and human diseases. One transmits cotton leaf curl virus, which causes a billion dollars (£610m) of harm a year in Pakistan alone.

Metarhizium could be a powerful weapon against mosquitoes, midges and ticks, controlling the spread of pathogens from Lyme disease and bluetongue to malaria, which kills more than a million people a year according to the World Health Organization.

Alongside mosquito nets, better drugs and managing breeding habitats, the fungus could form part of a new, integrated approach to fighting malaria. This is sorely needed; more than half the world's population is already at risk, and this will increase with climate change.

Swansea University has already conducted small-scale trials to show that applying a particular strain of Metarhizium's spores to a body of water can cut its population of mosquito larvae by more than 80 per cent.

The future of biocontrol

Butt's team has also conducted trials with growers and the biocontrol industry to show Metarhizium's efficacy in various crops. For example, it proved to be far better at controlling thrips pupae than conventional chemicals.

It won't replace pesticides entirely; it needs a humid environment to spread. But the team's research shows that some of these limitations can be overcome by combining the fungus with nematode worms or low doses of chemicals. The former offers totally organic pest control, while the latter lets farmers cut pesticide use by 90 to 99 per cent while maintaining good control.

For Metarhizium to work, enough of it has to come into contact with the target insect. But it can be applied with amazing precision, if you know how. One novel approach the team has shown to work is using honeybees to take fungal spores to individual flowers in oilseed rape fields to control pests like pollen beetle.

As the bees leave their hive, they pass through a temperature-controlled structure that forces them to walk over a bed of spores, which cling to their body. When a bee reaches its target flower, it moves into position to collect pollen, which displaces the fungal spores from its body and leaves them stuck to the flower.

Pine weevil damage to conifers

Damage to conifers caused by pine weevils

Job done, the bee flies back to the hive, oblivious and unharmed. Meanwhile the next beetle that tries to attack the plant is in for a nasty surprise.

Growers are increasingly aware of Metarhizium. "Vine weevil is the major pest for the plants we grow, and at the moment we have to use a conventional insecticide incorporated into the compost to control it," explains Bill Godfrey of W Godfrey & Sons Ltd, a grower and wholesale supplier of herbaceous plants.

Some form of pest control is vital to the business - he estimates that around a quarter of the crops it grows wouldn't be viable if the weevils were given free rein. Pesticides aren't cheap.

"On top of that you have the environmental and ethical issues - where do these chemicals disappear to after we've used them here, and what effect do they have?" adds Godfrey.

"And there are major doubts about how effective these insecticides are, anyway - they provide good control in the first season, but by the second year this has pretty much worn off, so often if you haven't sold the plants by then you'll have to throw them away." Metarhizium should help address these problems.

Sharing science

Butt hopes it will also improve the lives of some of the world's poorest people. Through knowledge transfer, cultivating Metarhizium could become an important cottage industry, creating jobs and wealth in impoverished areas.

He is currently working with researchers in developing countries through initiatives like a project with Bauchi University in Nigeria, funded by the Department for International Development, which aims to show that Metarhizium can be produced sustainably. Initial findings are due to be published later this year.

In the meantime, the fungus is enjoying its first year on the market. It'll be a while before we know exactly how it's performing, but those behind it believe its future is bright.

"Without a doubt, Metarhizium will be one of the most significant biocontrols," says Sopp at Fargro. Other candidates have looked promising, but have fallen before making it to market because of practical difficulties - they don't keep well, for example, or work only in ideal conditions.

"People come along all the time with brilliant ideas of using a particular organism to kill this or that pest, but they haven't thought about how it's going to be made and distributed," he says. "If something is hard to produce in bulk or it has no shelf life, it's commercially not viable. Metarhizium has already overcome these hurdles."

Godfrey agrees that Metarhizium's future looks bright. He's talking to suppliers about getting it added to the compost his plants grow in. He hopes it will keep killing weevils for much longer than chemicals, and will save him money to boot.

With pressure growing to cut pesticide use, it's a good thing alternatives are appearing. Godfrey says Metarhizium "will give us a more sustainable and environmentally acceptable alternative to chemical insecticides - hopefully it'll help me sleep a little better at night!"