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How to find aphids
by hand-searchingSearching by hand - find aphids in-situ To find many aphids - look for ants Look for effects on the plants
As Heie (2009) says, although most people probably believe that all aphids are common, the truth is that they normally are rather difficult to find. Finding the 20 to 30 UK pest-species of aphids is fairly easy, although even some of them may be elusive given the heavy insecticide use on famers crops. So forgive us if, here, we concentrate on finding the other 570 'indigenous' species that may be lurking in the UK undergrowth...
Searching by hand - to find aphids in-situ
Anyone who has tried to survey aphids outside the crop environment knows you can commonly search hundreds (sometimes thousands) of plants to find one infested specimen. (It's easier to find needles in a haystack.) Nevertheless the best way to understand aphids is to try to find them in situ by hand. That way you get information not only about which plant species the aphid is feeding on, but also the precise feeding site, the size and appearance of the colony (if one is formed), and the presence of any predators, parasitoids, fungal pathogens and ectoparasites (mites).
The majority of aphid species are either plant-genus specific, or plant-species specific. They usually exist at overall low densities, but with highly aggregated spatial distributions. Most aphid colonies are small, well hidden or well disguised (such as root aphids, of which the UK has more than 50 species). They can be extremely time-consuming to find, but a good understanding of their biology and keen eyesight help enormously.
An especially interesting group of aphids is the bark aphids, Pterocomma spp.. Whilst most aphid species usually exist as single-species colonies, these commonly occur as mixed-species colonies. The picture below shows a mixed colony of black willow bark aphids (Pterocomma salicis) and hairy willow bark aphids (Pterocomma pilosum). The black willow bark aphid is dramatically coloured in orange, black and white. This is most likely aposematic colouration since the aphid is thought to sequester phenolic glycosides from the willow. The colour warns predators that the aphid is distasteful.
The hairy willow bark aphid on the other hand normally relies on cryptic colouration to protect it from predators, with its mottled green colouration matching well with moss and lichen growth on the bark. Whether they live with black willow bark aphids because they benefit from a particularly favourable habit, a nutrient sink or their fellows putting off the predators, is debatable.
This is an invasive species which may become more common as global warming intensifies. It is called (not surprisingly) the onion aphid (Neotoxoptera formosana). It is an Asian species not normally resident in Britain, although it has been detected before in aerial suction traps and imported onions.
The common sycamore aphid is sometimes abundant in spring and early summer, and supports a fascinating diversity of predators and parasitoids.
Thecabius affinis on the flower head and stem of a lesser spearwort.
In the summer Thecabius affinis is normally found on the roots of Ranunculus species. But lesser spearwort grows in very swampy conditions, and the roots of this plant were flooded. As a result, most of the colony had moved to the flower stem, leaving behind a few on the roots.
The solitary birch aphid Monaphis antennata has the reputation of being extremely rare (Hopkins et al., 1998), but we have found it several times just by searching both sides of an awful lot of birch leaves. Adults live on the underside, nymphs on the upper side. Whether it really is rare, or just has a low availability to the 'sampling method' is unclear.
Once you think you are getting good at finding aphids, you can also try looking for the eggs of aphids in late summer or early spring.
To find many aphids- look for ants
About a quarter of aphid species in Britain are attended by ants.
The large pine aphid is sometimes attended by ants and sometimes not. But some aphid species are nearly always attended, and may have their distributions determined by the ant species which attend them. In those species, the interaction can be remarkably strong. An example is the shiny birch aphid Symydobius oblongus which is nearly always attended by whatever ant species happens to be in the area. The picture below shows how even small immature aphids can attract large numbers of attending wood ants.
Such aphids species survive poorly without their attendant ants, and may need a particular ant species to prosper. Whilst standard texts might suggest ant-aphid interaction is simple and invariant, intensity of attendance obviously varies according to ant activity and abundance, and the availability of alternative food. Formica lugubris often gets much of its food from tending Symydobius oblongus, but if the Symydobius population fails, these ants can switch to other aphid species. It seems likely that changing levels of attendance must also occur seasonally in 'normal' years as different aphid populations peak and decline in numbers.
Ants may deter small birds and mammals, and when tending some aphid species, may attack vigorously. When it comes to invertebrate predators and especially parasitoids, the situation is more complicated. Some ant species will predate coccinellid and syrphid larvae, but certain parasitoids routinely evade the ants protection by mimicking the ants' appearance and behaviour.
In addition to the close relationships between some aphids and ants, there are many less close relationships, about which we know rather less. Some aphids such as the giant willow aphid Tuberolachnus salignus and the mealy rowan aphid (Dysaphis sorbi) are attended by species other than ants. The picture below shows a wasp and several ants feeding on honeydew which has dripped from a colony of Dysaphis sorbi on rowan.
Although not normally regarded as a mutualistic relationship, there seems little doubt that these wasp swarms around aphid colonies may well dissuade bird and small mammal predators from predating the aphids. The aphids in turn provide the Diptera and Hymenoptera an important source of carbohydrates - especially in late summer. Moreover, one of the oft-quoted benefits of ant attendance to aphids is that honeydew is cleaned up around the colony - and vespids and various Diptera perform this role just as effectively as ants.
Sometimes of course the attraction of other insects has nothing to do with the honeydew, but lots to do with the aphids themselves. Although ants tend some aphid species for honeydew, they are voracious predators of other species. We have seen much less predation than ant attendance, but wood ants are known to love eating the sycamore aphid Drepanosiphum platanoidis. Similarly many species of hover flies lay their eggs near aphid colonies, and their larvae feed on the aphids.
Look for effects on the plants
The second picture above shows the developing elm-currant aphids covered in wax - the winged aphids will leave the gall and migrate to currants.
The other tree species affected by aphid galls is black poplar (including Lombardy poplar). This probably has more species of gall-forming aphids than any other plant. The shape of the gall will give clues to the identity of the aphid.
The picture above shows the reddish gall formed by a poplar leaf blade being thickened and folded down on one or both sides of the midrib caused by the poplar-buttercup gall aphid Thecabius affinis. We have already met this aphid on its summer host, Ranunculus.
This pouch gall on one side of the leaf petiole (see picture above) is caused by the poplar-lettuce aphid (Pemphigus bursarius) - which we meet again below on its summer host.
We finish with probably the most understudied group of aphids, the root aphids. They do not belong to one taxonomic group, but come from a number of different genera. There are about 50 such species in Britain and many more worldwide, as well as doubtless many undiscovered species.
The first picture below shows a close up of a poplar-lettuce aphid (Pemphigus bursarius) on the roots of its summer host - lettuce.
The second picture above shows the extent of the colony of Pemphigus bursarius on lettuce roots.