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

 

 

  <May 2015

Foreword by Alan Outen 

Bedfordshire Natural History Society has a long tradition of recording many groups of organisms, with many well known and eminent naturalists having been involved. Recognising that some groups receive little attention, in 2010 the Bedfordshire Invertebrate Group,  launched the Neglected Insects initiative.

One such group is the aphids. Contrary to popular supposition, very few are significant as pests! From just a handful of aphid records pre-2010 we now have over 100 species recorded, including several little-known species.

I hope you will find this page as fascinating and as useful as I do. It has really encouraged me to get out and search for some of these! I find it much easier targetting specific host plants for distinctive things than finding things that do all look very similar on a range of plants that do indeed then all turn out to be Aphis fabae!


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".

Closer inspection reveals Tuberolachnus salignus has rather more than four 1km-gridsquares on its NBN map,  but judging from the submissions to iSpot, Tuberolachnus salignus is one of the most commonly seen aphids  in 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 [100017955]

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:

  1. Aphids are ludicrously under-recorded.
  2. 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:

  1. 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).
  2. 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.

    Schizaphis graminum 

    Preferred hosts  Grasses & cereals (abundant).

    Adults are small elongate-oval & green with a darker-green stripe down their spine.

    Not ant attended.

    If present, very rare in UK. Originally Palaearctic, now widely distributed.

    Shown right: Wingless adults on oat leaf.

    Image source: Kent Loeffler, USDA 

  • 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 shape of this plot is fairly typical. Accepting the Bald eagle is rather rare, roughly 38% of those bird species were seen more rarely.

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:

  1. 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.
  2. Some species are highly localized, sometimes because of their host plant, but not always so.
  3. Some species are genuinely uncommon, or very uncommon, or only very rarely achieve appreciable numbers.
  4. 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 plant is very rare, or it is highly localized.
     
  • 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:
     
  • Or they may be rare for no obvious reason - this group includes a lot of uncommon, but probably not endangered, species.
     
  • There are a few aphid species that are endemic to Britain, 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.
     
  • Some aphids are rare because they have only recently arrived in Britain, 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.
     
  • 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.

    Show examples? 

 

So why are aphids of conservation interest?

There are (at least) four important reasons why aphids really ought to be of conservation interest:

  1. 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.

  2. 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.

  3. Some species, no-one knows how many, may benefit their host plants by providing sugars (via their honeydew) to crucial root mycorrhizae.

  4. 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.

Acknowledgements

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.

We have mostly made identifications from high resolution photos of living specimens, along with host plant identity. Microscopic examination of preserved specimens was used for confirmation if necessary. We have used the keys and species accounts of Blackman & Eastop (1994)  and Blackman & Eastop (2006)  supplemented with Stroyan (1977),  Stroyan (1984),  Heie (1980-1995),  and Dixon & Thieme (2007).  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).

Useful weblinks 

References

  • Baker, E.A. (2012). Aphids and their parasitoids at the Dundreggan estate. Trees for Life Dundreggan estate biodiversity survey report. June 2012.

  • Baker, E.A. & Blackman, R.L. (2014). Cinara (Cupressobium) smolandiae (Aphidoidea: Aphididae), a juniper-feeding aphid new to Britain. British Journal of Entomology and Natural History 27(1). Abstract 

  • Blackman, R.L. (2010). Aphids - Aphidinae (Macrosiphini). Handbooks for the identification of British insects 2(7). Royal Entomological Society, London.

  • Dransfield, R.D. & Brightwell, R. (2013). Aphids and their natural enemies and mutualists at Dundreggan, Scotland. Trees for Life Dundreggan estate biodiversity survey report. 105 pp. Full text 

  • Carter, C.R. & Maslen, N.R. (1982). Conifer Lachnids. Forestry Commission Bulletin No. 58, 75pp.

  • 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 Agency Full text 

  • Loxdale, et al. (1993) Biol. Revs. 68 291-311. Abstract 

  • Parry, H.R. (2013) Cereal aphid movement: general principles and simulation modelling. Movement Ecology 1 14. doi:10.1186/2051-3933-1-14  Full text 

  • 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.