Pollinating Flies in the Garden

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The hirsute form of the male chocolate midge, Forcipomyia sp., which is essential for cocoa pollination. The pin obscures one of the legs.
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The wonderful bee-mimicking hover fly, Volucella bombylans, looking like a more dangerous species so as to protect itself against predators.
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“The Secret Life of Flies” by Erica McAlister, takes a closer look at the life of flies and their contributing role in nature.

The Secret Life of Flies (Firefly, 2017), by Erica McAlister, looks at the many types of flies and the roles they have in nature. Readers will get a closer look at the different jobs that flies do. After learning about the benefits of flies, readers may learn to have greater appreciation for the species. Find this excerpt in Chapter 2, “The Pollinators.”

Hate chocolate? Well, I do. I simply detest the stuff – have done for years. I dislike the texture and the way it slimes down your throat, but most of all I don’t like the smell – just thinking about it turns my stomach. Even I have to admit this is not the most normal of dislikes. It is ironic considering my love of flies. Confused? Flies, you see, are the only pollinators of chocolate, or more specifically Theobroma cacao, the cacao or cocoa tree. This plant species has a complex reproductive structure, so complex in fact that only one group of very small flies, amusingly known as No See Ums, can pollinate it. This group, from the Forcipomyia genus of the family Ceratopogonidae, are, along with the rest of the family, known as the biting midges. Biting midges are cursed across the globe for ruining many a day in the countryside, especially the infamous Highland midge swarms in Scotland. According to her diary, Queen Victoria was half-devoured by these little ladies whilst at a picnic in Sutherland woodland in 1872.

The female adults of many biting midges have very painful and sometimes fatal bites due to some of the diseases they can transmit. They can also swarm in huge densities which can cause a large amount of blood loss. But without these minute, often rage-inducing flies, many people would consider that life is no longer worth living. Cacao producers are very worried about the ongoing supply of their ‘miracle’ substance. Our (yours not mine) demand for the stuff is vast – it’s an $80 billion year industry with 3.5 million tonnes produced annually, a figure set to increase to 4.5 million tonnes by 2020. But it is a volatile time for this product – traditionally the tree has been grown on smallscale farms but these are affected by increased stochastic weather patterns, growth in the numbers of pests and diseases and by political instability in many of the countries where the cacao tree is grown. These factors are exacerbating the already naturally low pollination rate of the plant. Many of the small-scale cacao farms are now moving across to larger set ups to overcome some of the negatives and boost yields, but this has also had repercussions on production.

The cacao plant has both male and female reproductive organs on the same plant but it cannot self-fertilize and so is entirely dependent on the midge to do this. It’s one of the many little known facts about flies and the benefits they bring us. Despite the efforts of the flies, few of the flowers go on to produce fruit; it’s a tricky business. No fruit means no bars of Galaxy or Curly Wurlies. Add poor pollination rates to the cultivation of these plants and your success rates drop further. The fact is, the pollinating flies are tree lovers – they like damp and shady conditions and many of the species require aquatic, semi-aquatic or moist soil conditions for their larvae to develop in. On these cultivated farms, trees are removed to create more space for the cacao but this removes most of the shade as well. So now there is very little shade and very limited leaf litter, and so nowhere for either the adult flies or their offspring to live.

In cultivated plots the average pollination rate is shockingly low at 0.3%. By cultivating forests for the production of the plant we are ironically destroying chocolate (and the midge!). Surely for chocoholics saving this fly ranks up there with saving the giant panda, which also has reproduction challenges? Not only do flies from this family pollinate chocolate, some – again in the genus Forcipomyia – pollinate heather species within the genus Erica (I was destined to love flies). Why is this important? Well heather grows with abundance in the Scottish mountains, and the Picts – a group of people who lived in what is today eastern and northern Scotland during the Late Iron Age and Early Medieval periods – used to make heather ale as long ago as bc 325, and that they could was largely thanks to flies. The myth behind the birth of whisky was that some of this heather ale was being baled in a stone roof cottage, the steam condensed and into a cup, whisky dripped!

That story may not be totally true but there are many similar stories that reveal the close relationship between insects and flowering plants. Flowering plants evolved after insects had arrived on the planet and the success of plants is very much down to the symbiotic relationship they have with insects. Evidence of this relationship first appeared between 130 and 140 million years ago and the intervening years have seen many plants and insects, including the flies, evolve some exclusive relationships with each other. Two species of biting midge have been observed pollinating the long flowers of Erica species and are only able to do so as they have elongated mouthparts so the nectar in these plants is only accessible to insects with long proboscises. Another fly, Rhynchoheterotricha stuckenbergae, a small dark-winged fungus gnat from the family Sciaridae with a true tongue-twister of a name, is another species with a very long proboscis, about three times the length of its head! Peringueyomyina barnardi, a primitive crane fly in the family Tanyderidae, has an equally long proboscis and another crazily long name – maybe there is a correlation between the length of names and the length of the proboscis? All of these species have only been found in the Cape region of South Africa where the climate and geography have led to the development of a highly endemic flora which includes many species with nectaries found deep within the long, tubular petals of the plants. These long-named flies and long-tubed plants have become co-dependent on each other.

The pollinating role of flies is hugely important for the general health of a range of ecosystems, including agricultural ones. Of the 150 families of flies, almost half, 71, have been shown to feed from flowers and therefore in principle transmit pollen from one plant to another. It’s not just the number of species that qualifies flies as important pollinators but also their distribution. As already mentioned flies are ubiquitous – they are everywhere. My colleagues and I have caught flies in the most unlikely places, the toughest of which was at more than 4,800 m (15,750 ft) altitude, up a mountain in Peru. This is not easy as the ‘pooters’ we use to catch them rely on manual suction. I don’t know how many of you have been to those altitudes but the oxygen is so thin I could barely crawl, let alone breathe. We had to try and suck up flies into a tube, often with limited success, with many a fly just sitting there on the plants unaffected by our ineffectual pootering. The point of this story is not to highlight my own inadequacies in failing to collect flies but to point out that flies are found at such high altitudes. A whole variety of them are found in these regions, including many hover flies from the family Syrphidae.

Hover flies are exceptionally common, distributed everywhere and very species-rich, with more than 6,000 species described globally to date. They are considered to be the most important of dipteran pollinators although this may change as our knowledge of fly biology increases. As their name suggests, they hover – when I was learning Latin family names I always found this group easy to remember as I would think of them ‘surfing’ (‘Syrphing’) on the wind.

In the USA this family is also called flower flies in acknowledgement of their associations with plants and their importance as pollinators. Many species in this group are distinctive and familiar in appearance – but not as typical flies. Rather, they are clever mimics of bees, wasps and hornets. It makes sense for a species to look like a more dangerous species to protect itself against potential predators, which at a glance will ignore them for a less risky morsel. These pollinators have no venom like their dangerous doppelgangers, let alone a sting. But they do spend a lot of time out in the open, guarding their territories and trying to attract the opposite sex or feeding from plants. Most people have no idea that most of the little yellow-and-black insects zipping round their garden at speed – some can fly in short bursts at up to 25 mph (40 kph) – are not the helpful bees they think they are, but rather helpful flies. Among their many morphological adaptations to assist with pollen transfer is their covering of thick hairs which, while the fly is feeding on flowers, pick up pollen.

Hover flies are second only to solitary bees and bumblebees in their value as commercial pollinators. The economic worth of all insect pollinators of cultivated crops has been estimated at about £120 billion, which equates to 35% of global crop-based food production. Flies form a high component of that figure and they are key pollinators for many crops including mango, chilli pepper, black pepper, carrot, fennel and onion. I may not like chocolate, but I would be devastated not to have pepper in my life.


Reprinted with Permission from The Secret Life of Flies by Erica McAlister and published by Firefly Books, 2017.

Mother Earth Gardener
Mother Earth Gardener
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