Most plants in gardens and glasshouses are attacked at some time by aphids, which suck sap and distort growth. Aphids also excrete a sweet liquid called honeydew, which makes plants sticky and allows the growth of sooty moulds. Fortunately, several predatory and parasitic insects found in gardens keep their numbers down and some of these can be purchased as biological controls for aphids on greenhouse plants. Although the adults of these insects may be familiar to most gardeners, their larvae are often less well known.
What are the aphid predators?
Ladybird larvae are up to 12mm long and are greyish-black with variable amounts of orange-red or white markings. Some species of ladybird larvae have spiny bodies. Many ladybirds prey on aphids but some specialise in eating scale insects, red spider mites or even powdery mildew spores. Adult ladybirds feed on the same types of food as their larvae.
Hoverfly larvae are legless maggots with flattened, relatively broad bodies up to 12mm long. They often have semi-transparent bodies so internal structures, such as the gut, can be seen. There are 265 hoverfly species in Britain and just under half of them have larvae that feed on aphids. Adult hoverflies feed on nectar, honeydew and pollen.
Lacewing larvae eat aphids and other small insects, which they seize with their curved jaws. They are up to 8mm long with tapered rear ends. Some lacewing larvae camouflage themselves by placing sucked-out aphid skins among the bristles on their upper surface. Adult lacewings have a similar diet to adult hoverflies.
Predatory gall midge larvae
Predatory gall midge larvae, Aphidoletes aphidimyza, are yellowish orange maggots up to 3mm long when fully grown. They insert their mouthparts into the aphid’s body and suck out the contents. When fully fed they go into the soil to pupate and later emerge as tiny flies, which feed on honeydew
Parasitic wasps (various species)
These lay single eggs in the bodies of young aphids. The egg hatches into a small grub that absorbs nutrients from within the host insect’s body. Eventually the parasite grub kills the aphid and pupates inside the dead aphid’s body. By then the parasitized aphid’s body has become whitish brown and has a characteristic inflated appearance. The adult parasite emerges through a hole cut in the dead aphid’s body.
Some of these aphid predators can be bought from mail order or some garden centres as biological controls. Biological controls are the use of natural enemies to control pests. This can be done by introducing various predatory or parasitic insects or mites, or nematodes that infect the pest with a fatal bacterial disease. This technique is mainly used in greenhouses, but some biological controls can also be used out of doors.