Written by | Stephanie Pain
Translate | Yashu
Editor-in-charge | Hui Jiaming
Entomologist Jonathan Finch drives a dusty car off the highway and onto Manbulloo's old wartime airport, knowing that what awaits him at the other end will be 65,000 blooming mango trees, an indescribably terrible smell, and the disturbing sound of flies.
The expansive Mango Farm in Manblue is a large orchard with an area of 4 square kilometres near the town of Catherine in northern Australia. "It's a beautiful place - remote, peaceful, but the smell is incredible, you just can't get rid of it." Finch said. Although we were communicating on the phone, I could feel him smiling. It turned out that this disgusting smell was his own creation, as it was crucial to his ability to study pollination in flies.
Most of us don't like flies very much. Finch is a big fan of them, though. He belongs to a team that studies the pollination of crops by flies, and his team also studies whether flies can help crops increase yields like bees. He set out from Western Sydney University on the other side of the continent to test a belief commonly held among mango growers: that rotting debris would attract flies, and that more flies meant more mangoes.
Mango growers have long recognized that flies are important pollinators. "Some people hang vats in trees and put animals in them that have been killed by the road to attract flies; while others bring a ton of fish and pile them in the middle of an orchard," Finch said. Farmers believe that irritating baits are attractive to flies, and the flies' physiological habits suggest that they might do so. However, there is no scientific evidence to prove this.
Flies are attracted by the smell of carrion because they mate and lay eggs on carcasses and cadaverics. They also forage in the flower bushes, consuming energy-rich nectar and protein-rich pollen, transporting pollen from one flower to another in the process. Therefore, we can make a reasonable assumption that additional flies pollinate more flowers so that the crop will bear more fruit. But is that really the case?
To find out, Finch and his colleagues removed a mixture of fish and chicken from a manbulus farmer's bait bucket. When the temperature hovers around 30ºC (85ºF), the smell of decay quickly drifts through the trees. In this way, the team was able to test whether the hypothesis was true.
Some Australian mango growers believe that luring flies to their fruit trees can improve pollination and increase yields. As a first step in testing the program, entomologist Jonathan Finch filled the barrel with a mixture of chicken and fish, producing a smell that the flies could not resist. Image source: AMY-MARIE GILPIN
fly
Justify the name of the fly
Flies are usually badly evaluated. People associate them with dirt, disease and death. Finch said: "No one really likes flies except entomologists. ”
Surprisingly, however, there are good reasons to cherish, encourage, and even breed flies: our future food supply may depend on them. Over the past few years, there has been a growing recognition that flies make up a large portion of wild pollinators — but we know very little about their activities in this area.
What kind of powder does each fly teach? How effective are flies in providing pollen when crops need to be pollinated? Which flies can we use to improve future harvests, and how? As insect populations plummet, bees are threatened with a variety of threats, including tile mites and colony collapse syndrome, so entomologists and pollinators are eager to get some answers.
Animals are responsible for pollinating about 76 percent of crops, including a large number of crops of global importance. Birds, bats and other small mammals do their part, but insects do more — pollinating the flowers of many fruits, vegetables and nuts, from almonds to avocados, from mangoes to melons, from cocoa to coconuts, and providing seeds for future crop vegetable harvests.
In a recent annual entomological review analysis, Australian biologist Romana Rader and his colleagues from Australia, New Zealand and the United States calculated that the harvests of the world's most widely grown crops are due to insect pollination, which generates about $800 billion a year in value.
Bees, especially bees, have received most of the praise, but beetles, butterflies, moths, ants, flies, etc. have been overlooked and underestimated. In Rader's analysis, only a handful of crops pollinated only by bees, and most crops were pollinated by both bees and other insects. In addition to this, Radar and his colleagues evaluated the contribution of each insect and found that flies are the most important pollinator after bees, with pollinators containing 72 percent of the 105 crops mentioned above.
The top ten insect taxa (excluding bees) pollinate major crops, and many insects also pollinate important food crops around the world, providing them with vital services. A study of 105 crops found that some insect taxa accounted for more than others, with aphid-eating flies and green-headed flies topped the list. Image source: R. RADER ET AL/ AR ENTOMOLOGY 2020
After perceiving that flies could provide such an important service, people vigorously learned how to make the most of these unsung heroes, draw them to fields and orchards, and let them work in greenhouses. Rader believes that as the demand for food increases, growers will increasingly rely on captive, managed pollinators, not just bees. Therefore, flies are essential to ensure future food security.
Flies are vast and almost ubiquitous, living in a variety of habitats. Hundreds of species from dozens of families are reported to pollinate one or more crops, but two fly families stand out: aphid-eating flies and lilies. Rader's analysis showed that pollinators who ate aphid flies contained at least 52 percent of the samples of the crops studied, compared with about 30 percent of the flies. Some species visit many different crops around the world: the common male bee fly, which belongs to an aphid-eating fly, which pollinates 28 of the 105 crops mentioned above in the record. The jam-eating aphid fly is close behind, pollinating 24 species of plants, while the green-headed fly is 8 species.
Aphid-eating flies and lilflies drink nectar from the flowers, which helps maintain energetic activities such as flying, and the nutrients needed for sexual maturation are obtained from the pollen. Like bees, many of these flies are furry, and they capture pollen on their heads and chests when they eat. Larger flies can collect and carry hundreds or even thousands of grains of pollen as they fly between flowers. Unlike bees, which have to forage near their hives or nests, flies do not need to feed their cubs, so they can roam a wider range of places.
In addition to this, they have other advantages: some flies forage early and late each day; they can tolerate a wider range of temperatures and become active when it is too cold for the bees; and they go out even in wet and windy weather, which keeps the bees in their nests. There may be another benefit for those who grow crops under glass or plastic film: "Bees hate greenhouses and like to sting you," but flies may be more tolerant of working indoors. Crucially, "flies don't sting people." Finch explains.
Today, bees still have a large share of crop pollination, and managed bees typically far outnumber wild pollinators. However, this is not always the case. Flies reproduce faster than bees, and under the right conditions, they can reproduce at high density. Radar argues that "some species have a very fast life cycle and are very adaptable to changing conditions," and more importantly, some of the most important aphid-eating fly species are migratory, so there are large numbers of aphid-eating flies that occur during critical periods of the year, far exceeding the number of bees.
Radar recently tracked the migration of common European aphid-eating flies, including the slender flat-eating aphid fly, and he found that as many as 4 billion fly north to the south of the UK each spring, a figure not far from the number of bees in the whole of the UK. There have also been reports of aphid-eating fly migrations in the United States, Nepal and Australia, and the phenomenon is widespread.
Aphids like this are one of the busiest crop pollinators, pollinating dozens of different crops around the world. Their larvae also provide another service – eating large numbers of aphids and other pests. Image source: JOHAN VAN BEILEN / SHUTTERSTOCK
Even more bizarre, aphid-eating flies provide valuable services in addition to pollination, says ecologist Karl Wotton, head of the University of Exeter's Migration Genetics Laboratory. Many species have predatory larvae that have a voracious appetite for aphids, caterpillars and other soft-bodied pests. According to Wharton's calculations, billions of aphid-eating larvae in the UK each spring eat about 6 trillion aphids early in the most important part of the growing season.
"That's about 6,000 tons of aphids, which make up 20 percent of the aphid population at this time of year," he said. "Other aphid-eating flies have semi-aquatic larvae that feed on discarded organic matter and effectively recycle nutrients. "It's hard to imagine what insect could be more beneficial than that, they provide great service, and it's free," Wotton said. ”
But how can flies be used to maintain and promote food production? One way to do this is to attract more flies to the fields and orchards. By encouraging farmers to plant wildflowers, retaining the remaining native vegetation, and weeding less, people can be very effective in increasing insect population and diversity and expanding the number of potential pollinators. But if aphid-eating flies and flies are to proliferate quickly, they need something extra: flies need carrion, and aphid-eating flies need more aphids. In addition, ponds or streams containing feces and decaying vegetation are also important for other flies.
It is not enough to make farmland and orchards more conducive to the survival of flies. With that in mind, researchers around the world are trying to determine which flies can be commercially reared and released at times and where their services are needed. But where to start? The vast majority of pollination studies focus on bees, and although there are reports that many species of flies pollinate crops, in most cases little is known about their ability to spread pollen, let alone whether their arrival will bring more fruits and vegetables.
This is starting to change. Scattered studies recorded how often flies visited flowers, calculated pollen grains on their bodies, recorded crop yields, and found that some flies competed with bees. In some cases, even more than bees. For example, researchers studying avocados in Mexico found that at a given time, the big-headed goldfly visited more flowers than bees and carried pollen particles to certain parts of the body that touched the stigma of the next avocado flower it visited. Studies in Israel, Malaysia and India have all shown that flies are effective in pollinating mangoes, while experiments in the United States and New Zealand have shown that the yield of leek and carrot seeds assisted by the red-headed lily fly is as high as that of bees.
Aphid-eating flies are also excellent in this regard. In the trial, many species were shown to be effective pollinators for seed crops, rapeseed, bell peppers, and strawberries. For example, a recent experiment in the UK found that putting a group of mixed aphid-eating flies into a cage of blooming strawberries increased fruit yields by more than 70%. What's more, strawberries can be bigger, heavier, and in a more perfect shape.
On paper, I finally realized that I had to do something about it. Researchers such as Rader are working on a 5-year project involving multiple institutions. One of the aims is to match flies to crops and then develop optimal feeding methods for them. On farms across the country, the team is having candidates test on a variety of crops such as mangoes, avocados, blueberries and vegetable seeds.
In Manbulloo, the big-headed golden fly is a common visitor to mango flowers. It's large, hairy, and numerous, but more research is needed to prove that it's an effective pollinator. Image source: JONATHAN FINCH
In Manblue, Finch focused on whether mango and old farmers' strategies worked. This stinking bait attracts a large number of flies, but are these flies in the same category as those that pollinate mango flowers? They are. Finch says: "Some large common species seem to be in the same time as carrion and flowers." Among them, one species looks more promising than the others: the big-headed gold fly. "They're big and hairy, which means it can carry and deposit a lot of pollen," Finch argues, "and they're also abundant, and they're found in a lot of orchards where the larvae will eat anything that dies." ”
With plans shelved due to COVID-19, Finch plans to return to Manboru later this year to see if the big-headed goldflies are as expected. "They may stay around carrion all day, distracted by the disgusting smell," he said. If they do venture out in the orchard, Finch monitors how many flies actually visit the flowers, and how often. The next test is whether the fly is getting the pollen where it's needed — on the head of the stigma that needs to be fertilized.
This work requires a microscope and enough patience. After that, if the big-headed goldfly is still on the shortlist, it's time to see if its efforts pay off by releasing it into trees free of other insect interference and measuring how successful they are in mango trees.
The big-headed goldfly may be an effective pollinator, but this still doesn't prove that the farmer's "rotten meat meter" is useful. "To do that, we have to compare the yields of orchards with and without carrion," Finch said. If growers prove right, this less costly approach can be replicated elsewhere. "If it turns out they're not as good at collecting pollen as bees, then we may need to add more flies to make up for their lower efficiency."
The idea of raising flies to produce food is slowly gaining attention, especially for greenhouse crops. "Flies reproduce well and quickly on scary things, which makes them low cost for use in greenhouses or release in the field," Finch said. Unlike bees, which, as pupae, are easy to transport and are consumable, some growers have benefited from specially bred flies.
Tasmanian farmer Alan Wilson has been raising his own flies for the past five years when he found that flies improved his high-value hybrid cauliflower seeds. On the other side of the world, in southern Spain, we can also buy boxes of aphid-eating pupae from Polyfly, the first company to commercially produce aphid-eating flies for use in greenhouse crops.
Flies, while smart, also have drawbacks. Some of them attack livestock, people, and some are pests of crops, which we must avoid at all costs. And, of course, there are some disgusting factors.
In Spain, Polyfly has done some nifty reinventions of the aphid fly. As one of the world's busiest pollinators, the long-tailed tube aphid fly is named "Queenfly," while another large, spotted tube aphid fly is named "Goldfly."
In addition, in the public mind, the fly is associated with death, decay and forensic examination of corpses, and its image problem is much greater. If the name of a big-headed gold fly appeared in a PR firm's brand brainstorming, I'd rather be a fly on the wall.
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Originally titled "The essential fly", by Stephanie Pain, published in Knowable Magazine on March 2, 2021. The link is https://knowablemagazine.org/article/food-environment/2021/the-essential-fly.