Pests cost the economy billions every year – but natural predators can help

By Charlotte Gifford

When the coronavirus spread around the world in early 2020, rumour circulated that the disease had been produced in a laboratory and wilfully exported by the Chinese government. This theory has been widely debunked. Nevertheless, the purposeful exporting of diseases is far from fiction.

Myxomatosis is a disease infecting rabbits that originates from South, Central and North America. In the 1950s, it was intentionally introduced into Australia, France and Chile to control rabbit populations. It was extraordinarily effective. The initially released virus had case fatality rates in excess of 99 percent. Given the damage rabbits can do to crops, a 2013 paper published in Australian Economic History Review estimates that the virus, along with rabbit haemorrhagic disease, saved Australia’s agricultural industries A$70bn ($51bn) over the course of 60 years.

Since the mid-twentieth century, biological control has been prevalent in the agricultural industry as a means of protecting crops from pests. The use of a disease to control a population is a relatively rare method. More commonly, biological control means introducing a natural enemy into the environment. It’s a more environmentally friendly method of pest control – but intervening in an ecosystem like this requires a careful hand.

Natural predators

Pests are more than a nuisance. They can also take a heavy toll on the economy. According to a 2017 paper released by the Royal Botanic Gardens at Kew in London, pests and pathogens could cost up to $540bn per year if left unchecked. The growth of international trade is only accelerating their spread. A 2016 study found that the US and China, due to their huge trade volumes, are the biggest sources of invasive species to the rest of the world.

The most expensive to control are those that feed on commodities. The Khapra Beetle, considered one of the most destructive pests in the world, consumes a variety of dried foods, including rice, and can damage up to 70 percent of grain stores during an infestation. Meanwhile, corn rootworms – which, as their name suggests, eat corn – cost US producers about $1bn annually in yield losses and input costs to control them.

Stefan Toepfer, Research Scientist at the Centre for Agricultural and Bioscience International, explains that corn rootworms are notoriously difficult to control, having developed resistance to some pesticides and even to some cultural methods such as crop rotation. They are particularly difficult to control in areas they aren’t native to. “In the invaded areas, such as Europe, western corn rootworm is an alien species, and is therefore lacking specific natural enemies as they have been left behind in the area of origin which is Mexico and Central America,” said Toepfer.

There are a number of methods of biological control against corn rootworms, but one of the most effective is the use of entomopathogenic nematodes. “These are tiny worms that are natural antagonists of soil insect pests,” said Toepfer. “There are other options, such as parasitoids, fungal pathogens or soil bacteria. But all those are in early research phase for this specific insect.”

Using natural predators has become an increasingly common way of keeping pests at bay. It’s been found to be highly effective; research by the University of Queenland recently found that this form of biological control is saving farmers in Asia around $20.1bn to $26.8bn per year.

There are many biological control agents out there and choosing the right one for the right pest is crucial, as Brian Spencer, President of Applied Bio-nomics, a biological pest control firm, explains. “In tomatoes, our most popular product is our non-refrigerated Encarsia formosa, a tiny whitefly parasitoid. In cannabis, it is the small black beetle, Stethorus punctillum, which is extremely climate adaptable and effective against spider mite,” he said. “Overall, the soil mite Stratiolaelaps scimitus is our most popular. It is used in almost every crop and is truly the enforcer of the soil, helping to manage fungus gnats, pupating thrips, root weevils and root aphids, over-wintering spider mite, pathogenic nematodes and many more.”

Biological warfare

One of the downsides of bio-control agents, however, is that it can take weeks before farmers see the results. “The one advantage that the synthetics still have is speed,” said Spencer. “Biological pest control requires planning and an understanding of the plant-pest dynamics.”

One only needs to look at to look at times when biological control has gone wrong understand why research and planning is so important. The cane toad in Australia, for example, was originally introduced in the hope that it would eat the cane beetles that were destroying sugar cane fields. In Australia, over A$20m ($14.6m) has been spent trying to control the spread of cane toads. Similarly, the European starling, which was initially introduced to North America, Australia and New Zealand to control insect populations, is known to decimate agricultural crops.

There are other methods of biological control that could be used. The introduction of myxomatosis to kill rabbits is not the only use of viruses as pest control. Baculoviruses have been sprayed across large areas of forest in North America to kill the larvae of the gypsy moth, a highly destructive pest that consumes over 500 species of trees, shrubs and plants.

Another method of biological control is sterilisation. This involves mass rearing a pest, sterilising it and releasing it into the population. In 1982, the US successfully eradicated the screw-worm – a parasitic fly that gets its name from its ability to burrow into skin of warm-blooded animals – using this technique. It’s estimated that the North and Central American save over $1.5bn a year as a result.

Eradicating a pest, however, is a decision that shouldn’t be taken lightly. Trying to eradicate large numbers of one animal can wreak havoc with the food chain. For example, when Mao Zedong, the founding father of the People’s Republic of China, called for the extermination of sparrows, he unwittingly caused the spread of insects – usually eaten by the sparrows – which devoured food crops around the country. It’s thought this contributed to the Great Chinese Famine, in which 20 to 45 million people died of starvation.

Protecting the environment

The agricultural industry has shifted its focus to biological control of pests partly due to increased concern over the environmental and health effects of using chemical pesticides. Over the years, the EU has banned a growing number of pesticides, which in turn has driven its trading partners to use alternative means of control.

“Now, in most cases, biological pest control is less expensive and more efficacious than the synthetic pesticides,” said Spencer. “This is partly due to the reduced availability of the extremely toxic synthetics, such as the organophosphates that were all based on nerve gas, and now, the noenicitinoids which are extremely dangerous to the bees.”

Certain insecticides can cause widespread environmental contamination, spreading through the food chain. Considering this, the introduction of a natural predator is potentially a much less serious encroachment on a given ecosystem. But it still needs to be managed carefully. For example, another problem with using natural predators is the paradox of biological control – the more effective a predator is, the more likely it is to eliminate its own food source and die out, causing a resurgence in the pest. But when applied in the right conditions, natural predators can be the perfect ally to pest-stricken farmers. As the saying goes, the enemy of my enemy is my friend.