You may be surprised to learn that, far from being common, many aphid species are rare or extremely rare.
Yet, of the 59 insect species listed by wikipedia as endangered and threatened in the British Isles, none was an aphid - but 36 were butterflies.
Conservation resources being limited, they should be used to the best effect. So, before we can consider what value rare aphids may have, we must consider what makes an aphid rare - and whether, simply by being uncommon, all are of equal value.
NBN Gateway maps
Deciding what is 'rare' is not quite as simple as you might imagine.
For example Lorn Natural History Group described the Giant Willow Aphid (Tuberolachnus salignus) as "Normally a rare beast, with only four dots on the NBN map of Britain".
Indeed, according to the NBN maps of Britain (shown below), Macrosiphum rosae (the ubiquitous rose aphid, so detested by gardeners) is far rarer - having been recorded in just four 1km-gridsquares between 1600 and May 2015.
Aphis fabae, arguably the most common ubiquitous abundant polyphagous aphid pest in UK, was recorded in more locations - but few people would accept these maps bear much resemblance to reality!
(c) Crown copyright and database rights 2011 Ordnance Survey 
Comparing the NBN map for all species of aphids combined recorded since the year 1600, with their records of Garrulus glandarius (the Jay - a retiring persecuted seldom-seen but widespread bird) the problem becomes explicit:
Aphids are ludicrously under-recorded.
NBN maps provide neither a worthwhile index of aphid rarity - nor their UK distribution.
Suction-trap monitoring records
Although spatial records are sparse there are some long-term but local data. Scientists at Rothamsted Research Station have used suction traps in selected locations to routinely monitor flying aphids for some years. Unfortunately, even were their raw data publicly available, there are two crucial shortcomings:
Their records exclude species which are not available to the sampling device (either because they produce few or no winged-forms, or only indulge in a few low down local flights), and any species which cannot be separated by conventional identification methods (this applies to many Aphis,Dysaphis,Pemphigus and Uroleucon species).
Simply because a winged aphid is caught, we cannot be sure it occurs on host plants in Britain.
This second point merits some explanation:
Of perhaps 4400 aphid species worldwide between 600 and 700 species have been recorded in Britain, either because they have been found on host plants or caught in suction traps. Including the latter group as 'British aphids' without records on host plants is controversial because various studies indicate that aphids are transported considerable distances by wind. Presumably Britain has received such arrivals for millennia, yet the species composition of American and European aphids remains relatively distinct. In America, some species are commonly described as 'migratory' aphids. These tend to be those species that are now distributed worldwide such as Macrosiphum avenae,Rhopalosiphum maidis and Rhopalosiphum padi. These species are known to be wind-transported for long distances (Parry, 2013). However, unlike migratory birds, no aphid returns from a country in which it does not breed.
The situation with Schizaphis graminum is interesting. Schizaphis graminum have definitely arrived in UK - two alatae trapped in suction towers had identical mitochondrial and nuclear DNA sequence to the sorghum-adapted form in the USA. Yet, although Schizaphis graminum is of Palaearctic origin and now widely distributed, there are no records from northern European field crops (Blackman, AWP). "Records from grasses in western Europe are now thought to apply to other closely-related species or subspecies, all of which are very difficult to tell apart."
Hence, whilst Schizaphis graminum appears on the Rothamsted list of "British aphids", this major grain-pest is not on the FERA 19/5/2015 Risk Register.
Why Schizaphis graminum has failed to establish itself is another question. As Loxdale, et al., (1993) noted, whilst successful long-distance movement undoubtedly occurs from time to time, aphids "probably die without feeding and reproducing". Or, then again, this aphid's niche may already be occupied.
In the last few hundred years most, if not all, aphid species new to Britain seem to have been carried on their host by man (albeit unintentionally), rather than through migration.
Other measures of rarity
Allowing for those problems, you might assume limited or local surveys of aphid species abundance must be available...
In practice, whilst some monitoring has been performed, we have been unable to find any systematic quantitative survey. Surveys such as Ed Baker's and ours at Dundreggan tend to list the species present, or to locate species of especial interest, rather than quantify their relative abundance. There are excellent practical reasons for this: sampling and identifying aphids is labour intensive and often impossible (for example, no keys exist for aphid nymphs). Moreover the two obvious measures of abundance, number of individuals, or number of colonies, are horribly problematic to quantify.
As a result, the best available data are qualitative: Estimates of abundance (and distribution) from publications such as Blackman (2010) and Stroyan (1977), or from field-researchers such as ourselves.
Accepting that records on aphid abundance are problematic, it might help if we considered a simple measure of rarity for a rather better-studied group.
Below is a 'rank abundance diagram' of 187 bird species observed in 2544 roadside bird counts Within approximately 1 km of the Great Lakes shoreline during 2002 & 2003 (Howe, et al., 2006). Number of individuals is the total number of birds reported in all point counts combined. Representative species are shown with arrows pointing toward the corresponding bar.
The (approximately lognormal) shape of this plot is fairly typical. Accepting the Bald eagle was not a very common species when the survey was performed, roughly 38% of the bird species were seen more rarely. (This is almost certainly an underestimate since rare species are most likely to escape notice.)
To get some idea of how this compares to aphids, at the time of writing our InfluentialPoints website describes about 240 species of aphids, of which we have classed just under 70 (30%) as common British species. On this page we select just 29 (12%) of them as examples of "rare" species. However, because we try to use our own photographs (and our contributors') when describing aphids, our 230 species pages exclude most of the rarest species (plus a few 'common' species): Implying that around half of British aphids are rare - or very rare.
That said, since a Bald eagle is more easily noticed than an Eastern phoebe, and the same is true for aphids, these estimates of abundance are biased accordingly.
There are several reasons why a species is seldom observed:
They may be secretive, camouflaged, or otherwise fail to arouse human interest.
Thus whilst aphids as a whole tend to get ignored, unless they are pests, many species are under-reported, including some common ones.
Some species are highly localized, sometimes because of their host plant, but not always so.
Some species are genuinely uncommon, or very uncommon, or only very rarely achieve appreciable numbers.
In addition, regarding suction tower results, some species produce few winged forms, or they may be weak fliers, or it may be impossible to identify their winged forms.
Which aphid species merit conservation?
The popular answer is "none of them. Ever."
Since at least 90% of aphid species are harmless, let us attempt a more objective view.
Why some aphids species are rare is often very obvious - their host plants are very rare or highly localized.
Winter host is Sorbus torminalis (wild service tree) which is uncommon & local. Summer hosts are Plantago (plantain) species which are very common.
On Sorbus live in a pseudogall on the leaves. On Plantago live under leaves between the veins.
Wingless adults on their winter host are greyish-green or pinkish-yellow, with brownish to reddish areas at siphuncular bases. Old ones are heavily wax powdered. Winged adults from their winter host have ochreous to greenish-yellow abdomen with a black patch & cross bands, those from their winter host are reddish-ochreous with a blackish pattern.
Often ant attended.
Blackman (2010) describes it as rare with only 3 records in England & 1 in Wales. We have not found it.
In addition, some aphid species are surprisingly particular about their niche (such as dying branches) or they may require another species 'in the mix' - such as a particular species of ant to attend them.
That said, there are a number of aphids which have abundant and widespread host-plants - yet seem to be genuinely very rare. This may be because they are on the edge of their distribution for climatic reasons:
Preferred hostSalix caprea, Salix cinerea, Salix aurita (sallows) which are very common. Aphids along veins on underside of leaves.
Wingless adults are black with a pale spinal stripe in spring, but in summer are light yellowish-green with reddish-brown or greyish-black dorsal markings. Winged adults are dark with dark cross-bands.
Rare according to Stroyan (1977) - who thinks the identification may be confused. We find it rare, but don't think its identification is the problem.
The winter hostSorbus aucuparia (rowan ), is very common in northern Britain, less so in the south. The summer hostsCampanula (bellflowers) is locally common. On rowan the aphids crumple the leaves creating leaf nests, later moving to the berry stems.
Wingless adults are reddish-brown to dark-green, with cylindrical slender pale-yellowish siphunculi.
Winged adults from the winter host have an ochreous to reddish-yellow abdomen, with a dark patch & dark slender cylindrical siphunculi.
Does not always move to summer host.
Wingless adult viviparae are variably grey or dull reddish-brown to dark-brown with powdery wax spots, yellowish slightly to markedly swollen siphunculi, and antennae half the body length.
Winged adult viviparae have variably developed abdominal cross-bands.
Shown here: Wingless adults and nymphs on Ribes rubrum (Redcurrant).
There are a few aphid species that are endemic, in other words they are seldom if ever found anywhere else. If they disappear, that is the end of them, for ever. But it is unlikely anyone will either know or care. The species at most risk may not be hugely rare, but are host-plant specific, highly localized, and occupy habitats under threat - such as saltmarsh or shingle vegetation.
Preferred host common but local: Cochlearia officinalis (common scurvygrass) flower heads, or rosettes of young plants growing within or just above the intertidal zone.
Dull olive-green with brown patches.
Not ant attended.
According to Blackman (2010) not known outside Britain. Habitat local & threatened.
Image copyright Roger Blackman, AWP all rights reserved.
Some aphids are rare because they have only recently arrived, and have yet to achieve a foothold. Conventional wisdom is that most such arrivals simply die out, which makes sense given we have but a small proportion of those species which have arrived on imported vegetables and plantstock. That said it is hard to be certain whether species only recorded once are examples of that, or are simple misidentifications. Why some species succeed where others fail may be anything but obvious, and is partly a matter of chance - too few may have arrived, the weather happened to be unsuitable, too few found good mates, their one host-plant was cut down, or predators happened to wipe out their only colony.
Wingless adults are elongated, have very short antennae, grey-green thorax, pale lime-green abdomen with variably-faint brown spots.
Not ant attended.
As of 2015, a new species to UK, only known from 2 UK sites. We found the second on Pinus montezumae. No evidence of spread so far, but it is cryptic. According to a FERA PRA likelihood of establishment is high. All Pinus species can act as hosts, and the UK climate is mostly suitable for establishment and spread.
Shown here: wingless adult on Montezuma pine needle.
Preferred hostsQuercus ilex (holm oak) which is uncommon but widespread. Aphids are on leaf undersides.
Winged adults are elongate & pale yellow-green. Their abdomen has dark spinal markings with pale centres.
Immatures are broader and flatter, & have four longitudinal rows of brown pale-centred hairy spots.
Not ant attended.
A recent invasive species: only known from 2 sites, but uncommon everywhere in its range.
Shown here: Nymph & winged adult on Holm oak leaf underside.
Preferred hostsAllium (onion) species. Often found on leaves or bulbs in store.
Wingless adults are shining magenta-red to dark reddish-brown or almost black. Winged adults are very dark-red to black, with heavily but evenly black-bordered wing veins.
Not ant attended.
In October 1999 it was only found in one UK site (RHS Wisley in Surrey, England). Eradication failed. It is invasive & established but, as of 2015, very rare. We have found it once in a supermarket in Scotland.
Shown here: Colony on growing-tip of stored onion (Allium cepa).
Many exotic pest species fall into the above category. But, if their pest potential is recognised beforehand, their rarity partly depends upon control / extermination efforts. It also depends upon how popular their host plant happens to be. Food and garden fashions change, and some years turn out to be good for aphids but bad for aphid natural enemies. So what is a rare aphid one year may be very different thereafter. In 1932 one aphid (Myzus ornatus) was a previously unknown rare endemic species found infesting violets in an English (Devon) nursery. It is now a worldwide pest.
No known primary host. Extremely polyphagous, Feeding on members of Violaceae (violets) and many other families, often in mixed-species colonies.
Small. Wingless adults are pale yellow or green, with conspicuous dark paired abdominal bars.
In 1932 it's worldwide distribution was restricted to 1 UK site: in other words this species was endemic and highly local. As of 2015 this serious pest is extremely common & its distribution is cosmopolitan.
There are (at least) four important reasons why aphids really ought to be of conservation interest:
Many aphid species provide the main source of nutrition - honeydew - for ants. This is best documented for wood ants - which are recognised as a keystone species in the forest ecosystem.
Many more aphid species support a wide variety of insect and bird species, either as hosts of parasitoids, or as prey for predators. They are known to be the main source of food for young birds such as blue tits.
Also many birds that normally eat seeds use aphids and caterpillars to feed growing young as these are high in protein, and often among the most readily available food sources early in the year.
Some species, no-one knows how many, may benefit their host plants by providing sugars (via their honeydew) to crucial root mycorrhizae.
The presence of uncommon species is a useful indicator of an ecosystem's biodiversity. Not all 'ancient' woodlands are the same - many only have common plants and insects - genuinely ancient woodlands are now extremely rare and fragmented!
Accepting that rarity per-se is a poor indicator of conservation value, setting up reserves to protect any rare species seems destined to fail if we ignore the ecosystem of which that species is a component. Or, to put it another way, only protecting the very rare is 'penny wise, pound foolish'.
Admittedly it is hard to get support for conserving what most people regarded as pests.
Nevertheless, if we only conserve "cuddly, cute, or magnificent" species, whilst the rest become extinct, there won't be an ecosystem to support those cuddly species - or us.
Our especial thanks to Kent Loeffler (USDA), whose image we have reproduced above.
We also thank Alan Outen (founder of the Bedfordshire Invertebrate Group, and instigator of the Neglected Insects in Beds initiative) for his enthusiasm and support, without which this page might not exist.
Whilst we make every effort to ensure that identifications are correct, we cannot absolutely warranty their accuracy. We have mostly made identifications from high resolution photos of living specimens, along with host plant identity. In the great majority of cases, identifications have been confirmed by microscopic examination of preserved specimens. We have used the keys and species accounts of Blackman & Eastop (1994) and Blackman & Eastop (2006) supplemented with Blackman (1974),Stroyan (1977),Stroyan (1984),Blackman & Eastop (1984),Heie (1980-1995),Dixon & Thieme (2007) and Blackman (2010). We fully acknowledge these authors as the source for the (summarized) taxonomic information we have presented. Any errors in identification or information are ours alone, and we would be very grateful for any corrections. For assistance on the terms used for aphid morphology we suggest the figure provided by Blackman & Eastop (2006).
Dixon, A.F.G. & Thieme, T. (2007). Aphids on deciduous trees. Naturalist's Handbooks 29. Richmond
Howe, R.W., Hanowski, J.M., N., Gerald J., Smith, C. (2006) Final Report: Development and Assessment of Environmental Indicators Based on Birds and Amphibians in the Great Lakes Basin United States Environmental Protection AgencyAbstract
Loxdale, et al. (1993)Biol. Revs.68 291-311. Abstract
Parry, H.R. (2013) Cereal aphid movement: general principles and simulation modelling. Movement Ecology1 14. doi:10.1186/2051-3933-1-14
Stroyan, H.L.G. (1977).Homoptera: Aphidoidea (Part) - Chaitophoridae and Callaphidae. Handbooks for the identification of British insects 2(4a). Royal Entomological Society, London. Full text
Stroyan, H.L.G. (1984).Aphids - Pterocommatinae and Aphidinae (Aphidini). Handbooks for the identification of British insects 2(6). Royal Entomological Society, London.