They also spread plant viruses which make your courgettes develop yellow streaks and strange kinks in summer. The little dears can infest almost any soft, sappy plants from roses to lettuce and bedding plants. Even seedlings inside a propagator aren’t safe and, believe it or not, greenfly can attack carnivorous plants.
For something so tiny, defenceless and not overburdened with brains, aphids seem to be able to outsmart gardeners at every turn.
So how do they manage it? For one thing, they have a very wide diet.
Aphids feed by piercing the skin to suck the sap, which is why their numbers are concentrated at the tips of shoots and on young leaves and buds where the plant tissue is softest and thinnest.
You won’t find them on tough plants such as evergreens or cacti, nor on silver-leaved plants or those with textured leaves.
There are also a great many different species.
The peach potato aphid is the one that you commonly see on garden veg, salads, pot plants and in the greenhouse.
This type of greenfly can be light green but it also comes in pinkish, buff or pale, creamy shades.
Some aphids have narrower tastes.
Black fly are a regular sight on nasturtiums and broad beans in summer, waxy coated cabbage aphids settle in clumps in brassica crops and the woolly aphid lives in the crooks of fruit trees and related shrubs, protected by wisps of whitish fluff.
Some aphids, such as the spruce aphid which is found on certain conifers, are very specialised.
The giant lupin aphid, which reached us from the USA, looks like small lacewings lined up on the stems. It needs stamping out if you find it as it can suck plants to death.
Greenfly enjoy a population boom in late spring and early summer but they are with us all year.
Even in the middle of winter you can find greenfly lurking on greenhouse plants or in sheltered spots. Indoors, where it’s warm, they can breed full-time.
The females don’t need males and they don’t waste time with eggs or parental care. They simply pop out a stream of live, female greenfly.
The young start feeding at once and it’s not long before they are mothers themselves.
If you’re wondering why the world isn’t knee-deep in greenfly, there’s one reason – nature. Greenfly provide a very good food source for other creatures from small songbirds to spiders and ladybirds.
So if you find yourself suffering the annual invasion, don’t be tempted to reach for a bottle; it just eliminates natural predators.
Wipe off large colonies with damp cotton wool or use the hose, and encourage aphid-eating wildlife. Put out peanuts and seeds so birds start dropping in for a feed.
That and cold winters will do wonders to stop your garden becoming a greenfly sanctuary.