Aphids (greenfly and blackfly) are the most common garden plant pests. There are many different kinds, some specific to certain plant while others are less fussy. They are fascinating creatures, though I am aiming here to comment on their effect on our plants rather than their diversity. It is impossible not to mention their lives, if only briefly in order to understand why their are so many of them! They are perfectly adapted to take over the world. They feed on plant sap and once they find somewhere that provides food many of them don’t bother to move. Although, in most species and at certain times, both male and females are produced, often in autumn, so they can lay eggs to survive harsh conditions that would kill the adults, once fertile females hatch in spring they can produce live aphids repeatedly. And more impressive, their offspring are already ‘pregnant’ with more young as soon as they are born! One aphid can become a colony in days!

They are nature’s fast food. Many creatures feed on them – just as well or we would be knee deep in them. They are parasitised, eaten by birds and famously by ladybirds and hoverfly and lacewing larvae. But the aphids are always one step ahead, often helped in their cause, by ants. I am often asked if ants are a problem in the garden and, by and large, except in pots, ants are best tolerated (it is always wise to realise when you can’t win the battle let alone the war). But they do ‘look after’ aphids, at least collecting honeydew (the sticky exudate) from the aphids and often farming them, moving them from plant to plant and protecting them from predators.

So what harm do aphids do? When they are present in large numbers they suck so much sap from the plant that they can distort leaves and stems, kill or deform flower buds and reduce plant vigour. As they feed they often cause the leaves to curl, which protects them.

Some species cover themselves in waxy ‘wool’ to protect themselves too. As some aphids feed they inject toxins into the plant, causing galls or lesser distortions. And as they move from plant to plant they spread virus diseases – a particular problem with dahlias and lilies. In short, they are not good.

Here are two identical pots of impatiens*, planted at the same time. Aphids attacked one. I am not sure why one was targeted but it is likely that it dried out more – often a reason aphids attack a plant.

This is the unaffected one

And this is the one where every shoot tip is infested with aphids, causing the leaves to curl and killing the flower buds.

The good news is that aphids are killed by most insecticides. But remember that even organic pest killers can harm beneficial insects. And if you kill all the aphids there is no food for ladybirds. It is a tricky one. You can squash aphids easily enough and prune off badly affected stems as a last resort.

All I would suggest is that, if you do not cover the garden in chemicals, the predators will move in and they will hoover up the aphids and the impatiens will grow through the damage. The lesson is to keep your plants healthy and they are less likely to be attacked.

• I bought the impatiens (busy Lizzies) so do not know if they are bred for resistance to downy mildew that has stopped us growing them for the past decade. I was not devastated by the disease when it struck – we had become too dependent on them. But I confess that I am delighted to see them again.

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