The bane of rose bushes and broad beans, aphids are not welcome in many gardens but even the Royal Horticultural Society recognises their value. Apart from their remarkable life cycle which includes the miracle of virgin birth, the RHS website makes the point that aphids are the basis of many food chains and part of a healthy garden ecosystem.
Aphids, greenfly and blackfly, are the food of a wide range of other invertebrates and some vertebrates. Early in the season aphid numbers can build up but by early to mid-summer aphid predators often wipe out colonies. Tolerating the presence of some aphids and avoiding the use of pesticides can help bring things into balance.
The well loved ladybird is probably the best known of these aphid eaters. An adult ladybird can eat 50-60 aphids a day, both larvae and adults eat them and a single ladybird will consume as many as 5,000 aphids across its lifespan.
Another voracious predator of aphids is the disarmingly delicate lacewing. There are several species of lacewing but the one you are most likely to see is the green lacewing. Its scientific name Chrysoperla translates roughly to golden pearls referring to the large golden eyes.
Several ground and rove beetles will feed on aphids, as will larvae of hoverflies and solitary wasps Earwigs are another group that feed on aphids. My grandad used to trap earwigs by upending an old tin filled with grass on a stick stood among his dahlias. The nocturnal earwigs would hide in the grass during the day, and my grandad would kill them. He also had problems with blackfly on the dahlia shoots, if only he had realised he was stopping nature from helping him.
The aphid’s life style is where it makes another contribution to biodiversity. Aphids feed by sucking the sap of a plant. They can obtain energy from the sugars in the sap, but they also need protein in their diet and there is very little protein in plant sap. The aphid has to consume much more sap than it can cope with if it is to get enough protein.
The excess sugary liquid is dripped out of two small tubes called cerci on the aphid’s back, this is called honeydew, a good reason not to park your car under a street tree, especially a lime which will have several million aphids feeding on the underside of its leaves. Honeydew is another important base to food chains. Fungi, especially sooty moulds grow on the sticky residue, on leaves in the lower tree canopy or the roof of your car.
Adult hoverflies, wasps and other flies will feed on the honeydew. I stood in the park of the Knowle the other day watching dozens of various insects flying about and crawling on the leaves of the large red oak by the car park. Then along came a hunting wasp which caught and carried away one of the hoverflies, the food chain was building. Blue tits and other small birds feed directly on aphids, but they will also feed on the insects that feed on the aphids.
You can buy midge larvae and ladybirds to release into your greenhouse as a biological control of aphids but, as recommended by the RHS as well as Wildlife Trusts, out in the garden the best thing to do is to encourage a wide range of insect predators to keep things under control. If you spray to get rid of aphids, the ladybirds go too.
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