Most of the 600 or so aphid species in Britain feed on a single genus or species of plant, where they suck phloem sap, and are not pests.
Aphids evolved feeding on trees but diversified to colonise herbaceous plants, mosses and ferns. Diversity is highest on trees, where more feeding niches are available. For example, in Britain silver birch (Betula pendula) supports 13 species, each feeding on a different part of the tree. Host alternation from trees or shrubs to herbaceous plants has evolved in many aphids. This allows them to take advantage of different periods of nutritious growth.
Three factors enable aphids to multiply quickly:
- Parthenogenesis, where the development and growth of embryos occurs without fertilisation.
- Viviparity, where embryos develop inside the mother and are born live.
- Telescoping of generations, where a young aphid inside its mother’s body already contains a developing embryo.
Aphid colonies on wild plants are usually small and short-lived. This may be due to competition and natural enemies, but aphids are also ‘self-regulating’, so when their population outstrips resources, reproduction slows and winged morphs are produced that migrate to find more suitable hosts.
Many predators, pathogens and parasitoids (parasites that kill their host) target aphids. Ladybirds, hoverfly larvae and lacewing larvae are the dominant predators. Primary parasitoids lay eggs inside aphids and the larva eats the aphid from the inside, before pupating inside its mummified body. The primary larva may then be targeted by secondary parasitoids.
Sugary ‘sap’ excreted by aphids is honeydew, and is an important energy source for many organisms. It also drives the mutualism between some ant and aphid species, whereby aphids supply honeydew to ants in return for their protection from predators and parasites.
During one week in June 2012, focusing on the woodland at Dundreggan, I found 35 aphid species and over 20 parasitoid species, involved in 64 trophic associations. Two parasitoids were new to Britain and an aphid (Cinara smolandiae) feeding on juniper (Juniperus communis) is also new to Britain. The survey was especially useful in identifying the key tree feeding aphids, including those involved in mutualism with wood ants (Formica lugubris).