These sap-sucking insects first evolved around 280 million years ago. There are around 4,400 species worldwide, although only around 250 are seen as pests. Most are quite small (around 1-10mm in length), and they can be green, black, brown, pink or colourless.
Some species have one specific host plant, others have complex life cycles, involving different plants through the year. When they find the right host, they insert their specially adapted mouthparts (the styli) into the stem of the plant, like a hypodermic needle. Aphids cannot suck; they are passive feeders. The pressure of the fluid in the phloem forces the sugary sap through the aphids body. Plant sap lacks some essential amino acids, so the aphids have unique bacterial symbionts in their guts, which will provide them with the extra nutrition they need.
Ants are known to farm some species of aphid, they will actively protect them from attack, defending their ‘herds’. The ants stroke the aphids with their antennae to cause them to secrete their sugary honeydew, which the ants are extremely fond of. Some ants will even store aphid eggs in their nests through winter. In spring, when they hatch, they will move the aphids to the appropriate host plant, where they can be farmed over the summer.
The holly blue butterfly has learnt to exploit the ants ‘farming’ instinct. The butterfly lays its eggs on suitable plants and as the caterpillar develops it gives off pheromones to make it smell like an aphid. The confused ants take the caterpillar back to their nest where they care for it through the winter. In the spring it will form a cocoon and emerge from the ants’ nest as a fully-formed butterfly.
Aphids are capable of both sexual and asexual reproduction. When the eggs hatch in the spring the first generation is all female. These are parthenogenic, and will give birth to live young, without the need for fertilisation. The population builds quickly with many generations in a season, so as host plants become crowded some aphids are born with wings to spread to other plants. When the temperature drops in Autumn, winged males and females are produced by the colony. These can mate and produce eggs that will be laid on food sources, so that they can hatch there the following spring.
Aphid attacks can slow the growth of plants, causing mottled leaves and stunted or deformed growth. One of the main problems caused by aphids is the viruses they transmit, which can devastate crops although, they are not all harmful to plants and can in fact confer some advantages e.g. Aphids are the main vector for the transferral of colour break virus between tulips. This virus created the intricately patterned flowers which became so desirable to collectors. Ithas also been found that plants infected with cucumber mosaic virus were more attractive to pollinating insects.
Aphids have very few defences to protect themselves. Some species will produce soldiers which can protect the colony. Some species will produce a woolly or waxy coating to hide themselves from predators (like the woolly apple aphids in the picture above). Some cabbage aphids can even secrete potent mustard oil to repel predators.
They need all the protection they can get as they will often end up as food for another animal, such as ladybirds and their larvae, hover fly larvae, wasps, spiders, lacewings and even some birds.
Before we reach for the spray bottle we should consider their importance. Whilst they seem insignificant compared to larger, more enigmatic species, they are one of the cornerstones of the intricate food webs that exist in our local ecosystems, and as such are fundamental to the survival of many other species. Without the foot soldiers like aphids and ants to support the next layer of the ‘food web’, we wouldn’t have foxes, badgers or hen harriers. It is only through the abundance of all of the elements that constitute a healthy ecosystem that we are able to fully enjoy a truly stable and sustainable environment.