Pollinating Flies in the Garden

Although sometimes pesky, flies can play an important role in the garden.


| March 2018


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.







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