Rethinking flies as the good guys – for food, feed and ecosystem health indicators

Sensemaking / Rethinking flies as the good guys – for food, feed and ecosystem health indicators

Thanks to its feeding habits, the oriental latrine fly could just play a major role in food and environmental sustainability.

By John James Wilson / 01 Sep 2015

AsianScientist (Aug. 28, 2015) - It could be one of those rare things that all humans, and many other mammals besides, can agree upon—flies are pests. Those flies buzzing around your freshly-prepared food as you are sit down to dinner are not only a major annoyance, but, if they manage to land, could contaminate your meal with up to one hundred different pathogens, including nasty diseases such typhoid, cholera, and tuberculosis.

As members of the insect order, Diptera, mosquitoes are also ‘true’ flies and true pests. Mosquitoes’ penchant for human blood means mosquito-transmitted malaria and dengue fever are responsible for around 50,000 human deaths, and economic losses of billions of dollars, across 22 affected countries in Asia every year.

The negative impacts that flies, as a result of their feeding habits, have on human lives are well-known. Perhaps then, given their bad reputation, it may come as a surprise that flies, also thanks to their feeding habits, are poised to play a major role in positively addressing some of the major challenges facing humanity.

In particular, Chrysomya megacephala, affectionately known as the oriental latrine fly, has recently been the focus of two research projects at the University of Malaya, Kuala Lumpur, and emerged as a beneficial insect with roles in both food and environmental sustainability. The oriental latrine fly is found throughout the world, including Europe and North America, but as the common name suggests, it is most common in the Orient, namely, East and Southeast Asia.

Flies as food and feed

Food security is one of the major challenges facing humanity as the rapidly growing human population struggles to adequately distribute food resources amongst its members. The Food and Agriculture Organisation (FAO) of the United Nations recently promoted a switch to insect-based diets as one way to address this challenge, and there are good reasons for this.

Rearing insects as livestock does not require land clearing, reduces greenhouse gas emissions and produces a more efficient protein source—a higher proportion of food eaten by insects is converted into edible protein, compared to rearing traditional livestock such as cattle, sheep, and poultry. Furthermore, insects can be fed directly on organic waste, effectively recycling nutrients back to human-edible material at a faster rate.

Yes, the oriental latrine fly is on the FAO’s list of edible insects and is reported to be eaten in its grub stage by people in India. While two billion people regularly consume insects as part of their diet, for Westernized societies, including those in Asia, the thought of eating insects can be disgusting. This could be even more so when the insect in question goes by the name ‘latrine’. Addressing issues of food sustainability through a Friday night dinner of latrine flies ’n chips is undoubtedly farfetched.

So if we refuse to eat them, how can oriental latrine flies positively contribute to human food sustainability? One of the major ecological consequences of a fish supper is that the Earth’s oceans are running out of fish. This not only happens directly because of the actual fish on your dinner plate—but also because other fish are being harvested to feed the farmed fish that humans consume.

An estimated one-third of wild caught fish are ground-up into a generic protein source, referred to as fishmeal, to be fed to other fish. Due to a global shortage of fishmeal, juvenile fish in the South China Sea, those which haven’t yet contributed offspring to the population, are being actively targeted for fishmeal. This practice will inevitably lead to the collapse of fish stocks in the region.

Insects such as the mealworm beetle and the house fly have been studied as alternative protein sources to fishmeal in fish diets, with promising results. Research conducted at the University of Malaya has suggested that the oriental latrine fly makes an even better alternative to fishmeal than these other insects. The oriental latrine fly grub contains all the essential amino acids needed by juvenile fish for normal growth and an equivalent protein content to fishmeal.

Furthermore, diets comprising increased replacement of fishmeal with oriental latrine fly-meal improved the growth, feed efficiency, and survival of juvenile tilapia, one of the world’s most popular farmed fish. The total replacement diet produced the biggest fish.

What flies are eating indicates ecosystem health

While the oriental latrine fly is ostensibly a food source for humans, the reverse is certainly true—humans are a food source for the oriental latrine fly. Fans of CSI will be familiar with the way forensic scientists examine what species of insects, and at what life cycle stages, are feeding on a corpse in order to calculate the time since death—the species arrive for lunch in a characteristic time sequence.

The oriental latrine fly is one of the first to arrive, and is recognized as an important member of the group of insects used to estimate the post-mortem interval. But the oriental latrine fly is not a picky eater, and will feed on the carcasses of almost any vertebrate species. The fly will also feed on the wounded flesh of live animals and also, as its common name again suggests, feed on vertebrate excrement.

It is precisely these unfussy eating habits that have led to a role for the oriental latrine fly in environmental sustainability. A major challenge facing our planet is biodiversity loss. As expressed by prominent biologist E. O. Wilson during an interview with the United Nations Educational, Scientific, and Cultural Organization (UNESCO) in 2010, loss of biodiversity causes “loss or erosion of ecosystems due to destabilization caused by erasure of links in food webs.”

The disruption of natural ecosystem processes due to biodiversity loss, including nutrient and water cycles, will severely impact all life on earth, including humans. ‘Trophic cascades’ is the term biologists used to specifically refer to the impacts ‘cascading’ down through an ecosystem from the loss of the food web’s top links.

Considering that many of these top links are mammals, monitoring the presence or loss of mammalian species can act as an early warning system that an ecosystem is in turmoil. Tracking these top species, however, can be difficult. Even though they are relatively large, they are found at low densities and can be hard to observe in nature—think about bats compared to ants.

Rather than go searching for mammals, we can get the oriental latrine fly to do the job for us. A mammal monitoring approach using the oriental latrine fly has recently been developed by a team (including myself) at the University of Malaya. The approach involves setting traps for oriental latrine flies, then extracting and DNA sequencing the flies’ gut contents to find out what mammals they have been eating. Mammalian DNA can survive in a fly’s gut for up to 72 hours.

Using primers that detect and amplify mammal DNA but not oriental latrine fly DNA, the presence of different kinds of mammalian species DNA in the flies’ guts indicates the presence of the mammalian species in the area. The approach has been tested at Ulu Gombak Forest Reserve near Kuala Lumpur in Malaysia. DNA from the near-threatened dusky leaf monkey was detected in the flies’ guts, a new record for the area, together with DNA of numerous bats, rats and other mammals.

Unfortunate name but on balance a good guy?

On the one hand, the oriental latrine fly has been implicated in a number of cases of human myiasis (wound infestation), since the first report from Thailand in 2004. On the other hand, the oriental latrine fly has long been recognized as a friend to mango farmers in Taiwan. Given the importance of the oriental latrine fly as a pollinator of mango trees, colonies of flies have been mass-reared and released in mango fields since the early 1990s. It seems oriental latrine flies like a dessert of nectar to follow a main course of rotting animal meat and excrement.

Recent research examining the oriental latrine fly’s role as a sustainable protein source and as an environmental health indicator serves to further emphasize that this fly’s feeding habits are important for the existence of humanity in a multitude of ways. If only we can look beyond the unfortunate name…

By: John James Wilson, Institute of Biological Sciences, University of Malaya 

Source: Asian Scientist Magazine

Image credit: Bob Peterson

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