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"It has long been an axiom of mine that the little things are infinitely the most important" (Sherlock Holmes)

 

 

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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).

   

  • Sometimes much the easiest way to find aphids is to look for their predators, such as ladybirds (coccinellids), their larvae, or hoverfly (syrphid) larvae - as above.

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.

  • When searching for aphids it is a good idea to target unusual and marginal microhabitats. These include wood boundaries, roadsides, around car parks, river valleys, and old or fallen trees. Most aphid species seem to prefer host plant under some stress, or in an atypical environment.

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.

  • When you find a colony, don't assume only one species is present - always check amongst the colony and on other parts of the infested plant for other species.

  • Rapidly growing plant organs have lots of nutrients available to aphids - and are often good places to check.
An unusual example of this is shown in the picture below. The aphids are living on the growing tip of an onion purchased at a supermarket in Scotland (sample courtesy of Alan Watson Featherstone Trees for Life )

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.

  • Apart from the growing tip, flower stems are also good places to look for aphids as the plant is channelling nutrients to the developing seeds.
Uroleucon aphids are mainly found on the flowers stems of the plant families Asteraceae and Campanulaceae. The dark red aphids below are Uroleucon campanulae on a harebell.

  • Another time when lots of nutrients are available to aphids is in the autumn when leaves are senescing. It is also the time of year when egg-laying aphids (oviparae) appear on the scene to lay the overwintering eggs.
The picture below shows an ovipara of the common sycamore aphid Drepanosiphum platanoidis  on a senescent sycamore leaf. Note its long ovipositor at its back end.

The common sycamore aphid is sometimes abundant in spring and early summer, and supports a fascinating diversity of predators and parasitoids.

  • Feeding site is not always selected primarily with nutritional quality in mind. Some aphid species select their precise feeding position with defensive considerations uppermost.
The light green pine needle aphid Eulachnus brevipilosus  likes to feed between pairs of pine needles, as shown below.

  • Host-site preferences are not invariable. In unusual conditions aphids may take up different sites.
The picture below shows a colony of the wax-covered poplar-buttercup gall aphid 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.

  • Some aphids such as the bracken aphid Macrosiphum ptericolens  are wonderfully cryptic and do not form dense colonies. As such, the only way to find them on the plant is to spend many hours searching.
The bracken aphid below was found after several hours searching at Burton Pond Nature Reserve in West Sussex.

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.

  • Finding aphid eggs is easiest for tree-dwelling aphids which lay their eggs on the twigs, branches or trunk, or in the case of conifers on the needles.
The first picture below shows a freshly laid yellow brown of the large pine aphid Cinara pinea  whilst the second shows the shiny black mature egg of the same species.

 

 

To find many aphids- look for ants

About a quarter of aphid species in Britain are attended by ants.

  • Ant attendance makes aphids much easier to find.
Whilst commonly described as 'farming' aphids, this is a bit of an oversimplification. Contrary to popular mythology, ants do not 'milk' aphids (unless your idea of milk is very peculiar). Aphid honeydew is excreted via the anus as a waste product. Ants do however encourage the aphids to excrete more honeydew by 'antennation' - this involves the ant rhythmically tapping or stroking the aphid with its antennae. The picture below shows a wood ant antennating a winged large pine aphid Cinara pinea. 

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.

  • We have several times had our attention drawn to large colonies of the giant willow aphid Tuberolachnus salignus  (see picture below) by the swarms of wasps and flies around a tree trunk.

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.

  • Some honeydew spatter (for example by the common lime aphid, Eucallipterus tiliae ) seems rather less attractive to other insects and routinely coats cars parked under infested trees. The sticky honeydew spatter, and fungal growth on honeydew-coated surfaces (such as sooty mould), enable you to spot those species.

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

  • Several species of aphids cause a gall on the plant, with the aphids feeding and developing inside the gall. These galls tend to be fairly easy to spot. Note: some authorities use 'gall' to describe any deformity caused to the plant, even wilting and stunting. (Remember to check wilted or stunted plants.)
The first picture below shows galls of the redcurrant aphid Cryptomyzus ribis  on redcurrant bushes in a plant nursery in Sussex. Each of the red blisters has many aphids on the underside of the leaf shown in the second picture.

 

  • Perhaps the biggest difficulty in finding aphid galls is finding the right tree species since the two most affected trees are now quite rare in UK.
The first is the elm tree which is now mainly found as regrowth in hedgerows. The yellowish or whitish-green gall (first picture below) is caused by the elm-currant aphid Eriosoma ulmi

 

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.

  • There are two main clues to the presence of root aphids.

    Firstly they are often attended by ants which may be evident on the soil surface.

    Secondly infested plants grow poorly and look as if they are suffering from lack of water - which of course they probably are.

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.

  • Although hand searching gives the maximum amount of information about host associations and interactions with other organisms, it has a number of drawbacks:

    Firstly the area covered, both in terms of land surface area and plant surface area, is very limited.

    Secondly hand searching is not quantitative and does not enable one to assess population sizes, nor even which species is most abundant, other than at the grossest level.