Why flesh-eating flies are our friends

2011-12-14 00:00

I GUESS we are never too old to learn new things. I recently learnt a new word — ­“debridement”. This word has absolutely nothing to do with brides, but everything to do with flies, and especially our common suburban greenbottles.

Most people have heard of bluebottle flies, but few talk about greenbottles even though most of those we encounter around our homes are green. These flies, commonly called blowflies, belong to a family called the Calliphoridae, which has over 150 South African species. Although they are attractive, brilliantly metallic insects, most people detest them because of their unsavoury habits. Adult blowflies are attracted to decaying flesh and excrement, and are commonly encountered in places where our food is being prepared. They have exceptional powers of smell and can home in on meat being prepared for a braai. When they fly into our homes and land on our food we need to consider the probability that they took off from something that had been discarded by the neighbour’s dog.

Most people disagree with me when I tell them that blowflies should be considered beneficial. They are, without doubt, superb waste-disposal agents. When an animal dies, the corpse quickly attracts both blue- and greenbottle flies. These feed on whatever decomposing matter they can find and lay their eggs on the carcass. In order to out compete other flies, some species are even able to lay freshly hatched maggots onto the source of food so that they can get a head start on other flies. The buffalo carcass in one of my pictures was photographed in the Addo Game Reserve and is covered in blowflies. Within a few days, such a carcass would be literally seething in maggots and would rapidly be reduced to skin and bone — these remains being more suitable food for other kinds of insects.

Once a maggot is fully fed, it falls to the ground and burrows into it — or, in the case of carcasses like that of the buffalo, the maggots would crawl under the dead animal – and pupate. They hide away because fat juicy maggots are very desirable food for birds and indeed many other kinds of animals. The pupal stage in blowflies is short and so the next generation of adults is generated quickly — often quickly enough to take advantage of any rotting flesh that may still be available. In this manner, even large animal carcasses are very efficiently disposed of. I often say that if it wasn’t for greenbottles we would be knee deep in all kinds of unmentionable stuff.

Our common suburban greenbottle’s scientific name is Lucilia sericata and its common name is European green bottle. My field guide tells me it is “ubiquitous in domestic and natural areas” and that summer populations can reach pest levels. So these highly successful flies are not a uniquely South African phenomenon, but are found almost everywhere.

But back to my new word — “debridement”. It has long been known that fly maggots can be useful in cleaning infected wounds. There is a medical procedure called maggot debridement therapy or MDT that involves the introduction of maggots to wounds in order to clean them. The maggots of L. sericata eat only dead tissue and so leave living tissue to heal in a relatively healthy environment. Not all bluebottles and greenbottles are useful in this way. A close relative of L. sericata is called the sheep blowfly, and although it exhibits similar behaviour, will also consume healthy flesh. It is therefore important that the correct species is used in MDT.

In our suburban setting, blowflies feed on all sorts of rotting things, including the faeces of our domestic animals, and so their numbers can be controlled by proper management of this material. Because of their beneficial services, you should think before swatting a greenbottle in your kitchen. Instead, chase it out and consider how and why it came in – and take the necessary steps to avoid a reoccurrence.

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