The Essential Fly

Think before you swat a fly. They could be what's keeping your favourite foodstuff going.

When entomologist Jonathan Finch turns his dust-caked car off the highway and onto the old wartime airstrip at Manbulloo, he knows what awaits him at the other end: 65,000 blooming mango trees, an indescribably horrible smell and the unmistakable buzz of excited blowflies.

These days, the old airstrip is the access road to the vast Manbulloo mango farm — 4 square kilometers of orchards near the town of Katherine in Australia’s Northern Territory. “It’s a beautiful place — remote, peaceful and blissfully shady beneath the trees,” Finch says. “But the smell is unbelievable. You just can’t get it off you.” Although we are talking on the phone, I get the impression he’s grinning. The loathsome odor, it turns out, is one he created himself. And it’s vital to his research into the pollinating prowess of flies.

Most of us don’t much like flies. Finch, though, is a big fan. He’s part of a team investigating the role that flies play in pollinating crops and whether, like honeybees, they might be managed to improve yields. He’s traveled from Western Sydney University on the other side of the continent to test a widely held belief among mango growers: If you leave out rotting carcasses, flies will come, and more flies mean more mangoes.

Mango growers realized way back that flies are important pollinators. “Some encourage flies by hanging large barrels from their trees and putting roadkill in them,” Finch says. “Other guys bring in a ton of fish and dump it in a heap in the middle of the orchard.” The farmers are convinced that the pungent bait makes a difference, and the biology of blowflies suggests that it might. Yet there’s no scientific proof that it does.

Blowflies are drawn to the smell of rotting flesh because they mate and lay their eggs on corpses and carcasses. They also forage among flowers to fill up on energy-boosting nectar and protein-rich pollen, transporting pollen from one flower to another in the process. So it seems fair to assume that extra flies will pollinate more flowers and the trees will bear more fruit. But do they?

To find out, Finch and his colleagues have coopted the Manbulloo farmers’ bait barrels and filled them with a mix of fish and chicken. With the temperature hovering around 30ºC (85ºF), the scent of decay soon wafts through the trees and the team can put the idea to the test.

Flies generally get a bad rap. People associate them with dirt, disease and death. “No one except entomologists really likes flies,” Finch says. Yet there’s good reason why we should cherish, encourage, even nurture them: Our future food supply could depend on it. The past few years have seen growing recognition that flies make up a large proportion of wild pollinators — but also that we know little about that side of their lives. Which sorts of fly pollinate what? How effective are they at delivering pollen where it’s needed? Which flies might we harness to boost future harvests — and how to go about it? With insect populations plummeting and honeybees under pressure from multiple threats, including varroa mites and colony collapse disorder, entomologists and pollination specialists are urgently trying to get some answers.

Animals are responsible for pollinating around 76 percent of crop plants, including a large number of globally important ones. Birds, bats and other small mammals do their bit, but insects do much more — pollinating flowers of many fruits, vegetables and nuts, from almonds to avocados, mangoes and melons, cocoa and coconuts, as well as crops grown to provide seed for future vegetable harvests. In a recent analysis for the Annual Review of Entomology, Australia-based biologist Romina Rader and colleagues from Australia, New Zealand and the US calculated that the world’s 105 most widely planted food crops that benefit from insect pollination are worth some $800 billion a year.

Bees, especially honeybees, get most of the credit, but overlooked and underappreciated is a vast army of beetles, butterflies, moths, ants, flies and more. In Rader’s analysis, only a handful of crops were visited exclusively by bees; most were visited by both bees and other insects. She and her colleagues assessed the contribution of each type of insect and found that flies were the most important pollinators after bees, visiting 72 percent of the 105 crops.The realization that flies perform such a vital service has prompted a big push to learn how to make the most of these unsung heroes, by attracting them to fields and orchards and putting them to work in greenhouses and growing tunnels. As demand for food rises, growers will increasingly rely on managed pollinators reared for the job, and not just honeybees, says Rader. Flies will be crucial to ensuring future food security, she says.

Flies are amazingly diverse and near ubiquitous, living in just about every sort of habitat. Hundreds of species belonging to dozens of families have been reported visiting one or more crops, but two fly families stand out: hoverflies and blowflies. Rader’s analysis showed that hoverflies visit at least 52 percent of the crops studied and blowflies some 30 percent. Some species visit many different crops around the world: One hoverfly, the common drone fly (Eristalis tenax), has been recorded visiting 28 of Rader’s 105 crops, while the marmalade hoverfly (Episyrphus balteatus) is close behind with 24, and the bluebottle Calliphora vicina (a blowfly), visits 8.

Hoverflies and blowflies visit flowers to drink nectar, which fuels energetic activities like flying, and eat pollen to get the nutrients needed for sexual maturation. Like bees, many of these flies are hairy and trap pollen on the head and thorax as they feed. Larger flies can collect — and carry — hundreds and sometimes thousands of pollen grains as they fly from flower to flower. Unlike bees, which must forage close to their hive or nest, flies don’t have to provide for their young and can roam more widely.

They have other advantages too: Some flies forage earlier and later in the day; they tolerate a wider range of temperatures and are active when it’s too cool for bees; and they’ll be out and about even in wet and windy weather that keeps bees at home. And for those growing crops under glass or plastic, there’s potentially another plus. “Bees hate glasshouses and are inclined to sting you,” says Finch. Flies might prove more tolerant of working indoors. And crucially, says Finch: “Flies don’t sting.”

For now, honeybees still tend to do a larger share of crop pollination. With colonies trucked from crop to crop, managed bees generally far outnumber wild pollinators. Yet that’s not always the case. Flies breed faster, and when conditions are good, they can reach high densities. “Some species have fast life cycles and are very adaptable to changing conditions,” says Rader. What’s more, some of the most important hoverfly species are migratory, so huge numbers can turn up and far outnumber honeybees at crucial times of the year.

Recent radar studies tracking the migration of common European hoverflies (including the marmalade hoverfly) found that up to 4 billion fly northward into southern Britain each spring, a number not far short of all the honeybees in the whole of Britain. There have also been reports of great hoverfly migrations in the US, Nepal and Australia, suggesting that the phenomenon is widespread.

Even better, hoverflies provide valuable services besides pollination, says ecologist Karl Wotton, who heads the Genetics of Migration Lab at the University of Exeter in southwest England. Many species have predatory larvae with a voracious appetite for aphids, caterpillars and other soft-bodied pests. Wotton has calculated that the larvae of those billions of hoverflies that turn up in Britain each spring consume around 6 trillion aphids in the all-important early part of the growing season. “That’s around 6,000 tonnes of aphids or 20 percent of the population at that time of year,” he says. Other hoverflies have semiaquatic larvae that feed on waste organic material, usefully recycling nutrients. “It’s hard to think of a more beneficial group of insects,” says Wotton. “They provide great services — for free.”

But how to harness flies to maintain — and boost — food production? One way is to attract more of them to fields and orchards. Schemes that encourage farmers to plant wildflowers, keep remnant native vegetation and leave grasslands uncut can be very effective at increasing the number and diversity of insects and expanding the pool of potential pollinators. Hoverflies and blowflies need a few extras if they are to proliferate, though: carrion for blowflies, access to aphids for some hoverflies and ponds or streams containing dung, decaying vegetation or carcasses for others.

Making fields and orchards more fly-friendly won’t always be enough. With that in mind, researchers round the world are trying to identify flies that can be reared commercially and released where and when their services are needed. But where to start? The vast majority of pollination studies have focused on bees, and although many species of flies have been reported visiting crops, in most cases little is known about how good they are at transporting pollen, let alone whether their visits translate into more fruit and vegetables.

That’s beginning to change. Scattered studies have logged how often flies visit flowers, counted the pollen grains stuck to their bodies and recorded crop yields, and found that some flies give bees a run for their money — and in some cases, outdo them. Researchers studying avocados in Mexico, for instance, found that the large green blowfly Chrysomya megacephala (aka the oriental latrine fly) visited more flowers in a given time than bees and carried pollen grains on parts of the body that would contact the stigma of the next avocado flower it visited. Studies in Israel, Malaysia and India all suggest that blowflies are effective at pollinating mangoes, while trials in the US and New Zealand showed that the European blue blowfly (Calliphora vicina) produced as good a yield of leek and carrot seed as bees.

Hoverflies also show plenty of promise. In trials, a number of species have proved to be effective pollinators of seed crops, oilseed rape, sweet peppers and strawberries. Recent experiments in the UK, for instance, found that releasing a mixed bunch of hoverflies into cages of flowering strawberry plants increased the yield of fruit by more than 70 percent. What’s more, the strawberries were likely to be bigger, heavier and more perfectly formed.

Promise is one thing, practical application another. In Australia, researchers like Finch and Rader are working on a five-year, multi-institution project that, among other things, aims to match fly to crop, and then develop the best method of rearing them. At farms across the country, teams are putting candidate flies through their paces on crops as varied as mangoes and avocados, blueberries and vegetable seed.

At Manbulloo, Finch is focused on mangoes and whether the old farmers’ trick works. The stinking bait certainly attracted plenty of flies – but were they the same flies as those that growers saw visiting their mango flowers? They were. “Several large and common species seem to visit both carrion and flowers,” says Finch. Of those, one looked more promising than the others: the oriental latrine fly. “It’s big and hairy, which means it’s likely to carry and deposit a lot of pollen,” says Finch. “It’s also abundant, turns up in a lot of orchards and its larvae will eat anything that’s dead.”

After a temporary halt thanks to Covid-19, Finch plans to return to Manbulloo later this year to find out if the latrine flies live up to expectation. “They might just stick around the carrion all day, distracted by the disgusting smells,” he says. If they do venture through the orchard, he’ll monitor how many actually visit flowers and how often. The next test is whether the flies deliver pollen where it’s needed — on the stigmas of flowers that need fertilizing — a job that requires a microscope and plenty of patience. After all that, if the oriental latrine fly is still a contender, then it’s time to find out if its efforts pay off by releasing flies among trees protected from all other insects and measuring their success in mangoes.

The latrine fly might prove an effective pollinator, but that’s still not proof that the farmers’ carrion trick makes a difference. “For that, we’ll have to compare yields in orchards with carrion and without,” says Finch. If the growers are vindicated, then their cheap trick can be rolled out elsewhere. “If it turns out that they aren’t as good at depositing pollen as honeybees, then we may need to add more flies to compensate for their lower effectiveness.”

The idea of raising flies to produce food is slowly gaining traction, particularly for greenhouse crops. “Flies breed amazingly well and quickly on horrible things, which makes them cheap to use in glasshouses or release in fields,” says Finch. They are easy to transport as pupae and are expendable, unlike honeybees. Some growers are already reaping the benefits of purpose-bred flies. Tasmanian farmer Alan Wilson has been rearing his own blowflies for the past five years after discovering they improved his crop of high-value hybrid cauliflower seed. On the other side of the world in southern Spain, you can buy boxes of hoverfly pupae from Polyfly, the first company to produce hoverflies commercially for greenhouse crops.

Brilliant though flies are, they can have drawbacks. Those that attack livestock or people or are pests of other crops must be avoided at all costs. And of course there’s the yuck factor. In Spain, Polyfly has done some nifty rebranding of its hoverflies. The common drone fly — a poor choice of name for one of the world’s busiest pollinators — has been promoted to Queenfly, while its other offering, the large spotty-eyed dronefly, is sold as the Goldfly. Blowflies, linked in the public mind to death, decay and forensic examination of corpses, have a much bigger image problem. When the oriental latrine fly’s name comes up at a slick PR firm’s branding brainstorm, I’d like to be a fly on the wall.

This article originally appeared in Knowable Magazine on 03 Feb 2021. Knowable Magazine is  an independent journalistic endeavour from Annual Reviews, a nonprofit  publisher dedicated to synthesizing and integrating knowledge for the  progress of science and the benefit of society. If you've signed up for our newsletter, maybe you could sign up for theirs as well?