Benefits of Flies (yes, flies!) to the Garden

This guest blog on the benefits of flies comes from entomologist Erica McAlister who is a curator of diptera at the Natural History Museum, London, and the author of The Secret Life of Flies (Firefly Books, available 9/5/17 wherever books are sold).

Flies... they are just a nuisance, right? Their larvae munch on the roots of your lawn or your bulbs. It is the bees that are the beneficial insects, helping to pollinate all that is good in the garden and the landscape, right? Well, no, this is largely a myth. In the UK it is now recognized that some of the top pollinators are hoverflies; globally we have no real idea yet as to how dependent we are on these creatures. In fact, the great thing about flies is that both the adult and the larval stage can be useful to the functioning of the ecosystem (unlike the lazy bee larvae, which just lie around all day waiting to be fed!).

Take the drone fly, Eristalis tenax, a common but important species of hoverfly found across the globe. This species is a marvel of machinery, with the larval stage, or maggot, adapted to living in the most putrid environments, including slurry pits, silage and, more importantly for gardeners, your decomposing compost heaps. Now, most people already associate flies with filth, but actually to live and breathe in waste, rather than just sit on it, requires some rather special physical adaptation. Most flies as both adults and larvae

breathe through spiracles or holes along their bodies. Though many maggots can breathe from their abdomens, this process frees up the mouth for eating, most larvae would not be able to breathe in these oxygen-poor environments. But these larvae have adapted their body with an elongated anal spiracle, their siphon, which enables them to feed in this rotting substance while breathing the air above.

These industrious little beasts eat away at the abundant microflora, happily feeding on the waste that few species want to get near. Aptly named, these rat-tailed maggots can reach up to 2cm (three-quarters of an inch). Their siphons have been recorded at 15cm (6 inches); the longest breathing tubes of all flies. These flies are often found in garden waste—so often, in fact, that we are forever answering questions from the public about them. Most people find it hard to believe that any good could come of living in such a fetid environment. They are amazed when they discover how useful (and attractive) the adult is.

The adults are incredibly important pollinators, not just in our gardens but also in agricultural systems. The drone fly has even been successfully trialled in greenhouses for the pollination of sweet peppers. Drone flies are similar in morphology to honeybees, and are often mistaken for same, although their flight patterns are quite different. We assume that the flies mimic bees hoping to deter bee sting-wary predators, but this could also be an example of convergent evolution. Both honeybees and hoverflies collect pollen using a similar kind of curled hairs, and so they may have developed their similar “fuzzy” appearance independently.

This is just one type of fly we are beginning to recognize the value of in terms of pollination. Multiply this by the number of fly families we know that include some pollinators (about half of the described families so far) and we start to realize that we are talking about many thousands of species of pollinators that could be crucial to crop development and horticulture in general, but which are largely ignored.

Many of these species of fly are found in areas that bees are not generally found, including the Arctic and Antarctic regions, or at altitudes of above 5000m. These species put up with a lot of crap (literally) to survive, and for that I think it is about time we start appreciating them more.

Erica McAlister is a curator of diptera at the Natural History Museum, London, and the author of The Secret Life of Flies (Firefly Books, available 9/5/17 wherever books are sold).