Flight of the hoverfly

GARDENS: They may be pests, but aphids and hoverflies are part of nature's bigger picture and we shouldn't interfere too much

ON THE SECOND weekend in July, clouds of aphids wafted over the Irish Sea near where I live. On the Saturday and Sunday evenings a gentle drizzle of live greenfly fell on the various yacht clubs in Dún Laoghaire. Sailors and their friends and families sat on the terraces after a big regatta, and picked the insects out of their hair and drinks.

The aphids were refugees from Wales, or England, or perhaps even farther afield. These tiny sap-suckers can travel hundreds of kilometres, and at heights of hundreds of metres, hitching a ride along air currents. The freshly-winged adults – which appear in colonies when overcrowding or food scarcity are imminent – are programmed to migrate. Although they take to the air in their millions, less than one per cent of them find a new host plant. Most meet disastrous ends: being snapped up by birds’ bills, crash landing on unsuitable terrain, or drowning in convivial gins and tonics.

This summer, the aphid clouds were more dramatic than usual, and they caused some disruption. A week before the Irish invasion, the insects put a stop to Rafael Nadal’s practise session at Wimbledon. Leicester city, meanwhile, was so infested that one pedestrian reported that it looked as if it were raining greenflies.

This year’s explosion is thanks, in part, to the cold winter, which killed off so many aphids. Although gardeners might have thought this to be a good thing, it meant that in spring aphid predators, including hoverflies and ladybirds, had scant supplies of food, and their populations were slow to recover after winter. The aphids, meanwhile, which have much shorter lifecycles, were reproducing unchecked. Hence the biblical swarms.

But nature is continually re-establishing equilibrium. A few weeks after the aphids floated in, hoverflies appeared (in my garden and in many others) in enormous numbers. Most of them were the dainty and well-named marmalade hoverfly (Episyrphus balteatus) and the more robust Scaeva pyrastri,which has no common name, but which bears three pairs of distinct creamy commas on its back. The larvae of both species are aphid-eaters (or "aphidophagous"), consuming around 200 of the creatures during the immature stage of their lifecycle.

When I noticed this burgeoning of hoverflies, I was delighted to see such an obvious demonstration of cause and effect in my garden: the aphid invasion had provided the food for the larvae of this abundant generation of hoverflies. As a gardener, I love finding connections. And in this case, when the dots were joined, a satisfying picture emerged.

Except, when I started to read up on “my” hoverflies, I discovered that they may have not been mine at all, but were just as likely to have been migrants, like the aphids. Some species of hoverfly, including the two above, migrate north in summer when their numbers swell. So Europe’s and Britain’s gluts of aphids produced their own bumper crops of hoverflies, some of which then ended up in Ireland. Nature does indeed even things up, but the arms of her weighing scale may be much longer than we imagine.

She also mediates in tiny ways, to retain stability in minuscule areas. Take those hoverflies, for instance. The males actually carve out individual territories for themselves in areas where females are likely to lay eggs. So, when you see a hoverfly suspended as if from an invisible string for long periods (hours, apparently), in a woody clearing or in the shade of a tree, you’ll know it’s a valiant male defending his patch.

Each female can lay up to 4,500 eggs. I don’t know what the attrition rate is for the larvae, but the current generation will find plenty of food in the visiting aphids and their progeny. Adult hoverflies mostly eat nectar and pollen. If it looks as if there won’t be enough greenfly to support their future offspring, nature has that sorted too, and has arranged that they’ll fly south, back to where their parents came from.

When I look at a complicated story like that above, which starts with some inconvenient aphids, includes millions of insects travelling hundreds of kilometres, and ends with the garden population being perfectly balanced at the end of the season, I am filled with respect for nature. I am in deep awe at the aphids and the hoverflies that, somehow, in their collective aphid and hoverfly consciousness know when to take to the air, and know how to make journeys that would kill most other animals. And when I consider how well organised and complex nature is, it seems brutish for me to muscle in with insecticides to deal with a fleeting influx of plant-eating pests. So, as much as possible, I let nature take its course without interference. Of course, I bump off the odd aphid and snail, but I avoid wholesale clearance.

And now, for those of you who have stayed with me this far on this aphid and hoverfly journey, I have to thank you for your forbearance. Normal garden-writing services will resume next week.

Diary date

Today, 2.30-4.30pm at Pobalscoil Neasain, Moyclare Road (off Warrenhouse Road), Baldoyle, the Howth and Sutton Horticultural Society autumn show and plant sale