Last week my son eagerly announced ‘Dad, I swallowed a greenfly at school today!’ 

‘That’s great,’ I retorted rather passively.  

‘Great?’ he replied, ‘there were millions of them, how are you supposed to play in that?'

‘Actually this isn’t great, it’s excellent. You, my son, are just one of the many living things that are going to benefit from the energy and protein derived from consuming this greenfly bounty!’  

‘I knew I wouldn’t get any sympathy from you.’ And off he went.  

It got me thinking: is it an experience he will remember? Not the actual consumption of the greenfly but the event of so many tiny winged creatures being blown across not just playgrounds, but gardens, greenspaces and the wider countryside. Everywhere I have been the last week or so I have marvelled at the airborne flood of tiny critters. From the rather morbid mass across my car windscreen and bonnet to the clouds that greeted me on my Friday night dog walk, and everything in between.  

Coming into contact with insect life seems to be an increasingly hard thing to do. My mind on these occasions takes me to a place where I try to imagine what my great grandparents saw in the world around them before the onslaught of pesticides and the mass reshaping and removal of nature-rich habitats from our local landscapes. 

What has brought about this deluge of greenfly though? To find that out it is worth getting to know a little bit more about our little green friends.  

The greenfly is an aphid and aphids are fascinating (a shame my son excused himself when he did!). Aphids, for the uninitiated, are small sap-sucking insects. They are members of the superfamily Aphidoidea and go by a range of names and colours. There are around 5,000 species of aphid that have been described to-date on our planet and somewhere close to 600 of these can be found in Britain. 

They are probably best known by many as plant pests; the scourge of gardeners. To refer to them in such base terms is simply doing them and the role they play in our ecology a deep disservice. Of the 5,000 or so species that inhabit this planet, more than 400 species are found on food or fibre crops. Many have evolved to feed off one species of plant, while others alternate between two species of host plant. 

The aphid feeds off a plant's sugar-rich sap. From this, honeydew - sugar-rich sticky liquid - is secreted by the aphid. A large volume of sap is required by the aphid as the essential nutrients they need to extract are present at low concentrations. When their mouthpart penetrates the phloem of the plant, the sugary, high-pressure liquid is forced out of the anus of the insects. This allows them to rapidly process the large volume of sap required. 

Other species have evolved alongside the aphid, such as the so-called dairy or dairying ants. These ants have a mutualistic relationship with aphids, tending them for their honeydew and protecting them from predators. Honeydew can form up to 90% of the ants’ diet, and they take it back to their nest to regurgitate in order to feed the queen and other ants in the nest. 

Of the predators there are many. The presence of greenfly and other aphids is a part of a balanced ecosystem. A wide range of other invertebrates and some vertebrates feed on aphids, with ladybirds and hoverflies possibly the most well known. Of the 50 or so UK species of ladybird, the majority of their larvae feed on aphids. Also feeding on aphids are the larvae of just under half of the 260 hoverfly species in Britain. Some lacewing larvae use the sucked out skins of their aphid prey to camouflage themselves by placing the skins among the bristles on their upper surface. Predatory midge larvae don’t just suck out the aphid body, once fully fed they go into the soil to pupate and emerge as tiny flies, which feed on honeydew! Then there are parasitic wasps, earwigs, predatory beetles and more. Not forgetting birds, the common garden blue tit will make a nice meal of aphids.  

A word for worried gardeners: evidence suggests that where birds are encouraged in gardens, aphid numbers are lower. With so many natural predators, does a gardener need to worry about the impact of greenfly and other aphids on garden plants?  In early spring aphid numbers can build up and by early to mid-summer aphid predators can often wipe out colonies. Tolerating the presence of some aphids and avoiding the use of pesticides can ensure a healthy population of aphid predators.  

The aphid lifecycle is quite complex in some ways and simple in others; they may reproduce both sexually and asexually, alternating between sexual and asexual generations, or, alternatively, all young may be produced by parthenogenesis (where eggs are never laid). Some species can employ both reproductive methods under different circumstances but no known aphid species reproduce solely by sexual means. Aphids can rapidly increase in number through parthogenesis, producing generations that can live up to 30 to 40 days. Some aphid species can produce up to 40 generations of females in a season, theoretically producing billions of descendants if they were all to survive.

Parthenogenesis can see the aphid have genetically identical winged and non-winged female progeny. Some aphids alternate during their life-cycles between genetic control and environmental control of production of winged or wingless forms. Winged progeny may be a response to low food quality or quantity. e.g. when a host plant is starting to senesce, with winged females able migrate to start new colonies on a new host plant. When aphids are attacked by predators, alarm pheromones can be released which induce the aphids to produce winged progeny that can leave the host plant in search of a safer feeding site. In the autumn, host-alternating aphid species will produce a winged generation that fly to different host plants for the sexual part of the life cycle. Flightless female and male sexual forms are produced and lay eggs.  

The weather of April and May has been glorious and produced the perfect conditions not just for greenfly to breed but by which winged females have been created to allow the greenfly to spread to new host plants. The need for this has potentially been two fold; overcrowding on host plants is a possibility, while the lack of rainfall has most certainly seen plants producing less sap and thus triggering the need to fly new host plants.  

I have, of course, since seized the opportunity on a car journey to explain all the above to my son (trying in vain to avoid as many greenfly as possible): how it wasn’t just a greenfly he swallowed but a female greenfly in search of pastures new to asexually reproduce, how this female would go on to have millions of grandchildren, and how these grandchildren would themselves go on to either be milked by ants or meet with a grizzly end as their insides are sucked out by predators' larvae!