How to identify aphid genera

Assuming you have mastered basic aphid identification, and have established that you are looking at an adult apterous vivipara, you now need to determine what genus it belongs to. There are hundreds of genera worldwide, but far fewer in any particular geographical region. Our aphid genera page should be useful here, used in conjunction with Blackman and Eastop's host lists and keys Some or all of the following features will be important. If you have good eyesight they can be seen using a good hand-lens, or better, a high-resolution photo of a live aphid.
 

Are the aphids attended by ants?

Whether or not an aphid species is usually attended by ants is often noted in standard texts. This can be a useful clue for identification but one should not take the presence or absence of ants with aphids in the field as definite proof of the presence or absence of ant attendance (myrmecophily). Ant-aphid interaction is not invariant - the presence and intensity of attendance can depend upon the availability of alternative food as well as on the presence of very small numbers or another aphid species.

What is the aphid's size, colour and shape?

Most aphids are described as being spindle-shaped (wide in the middle and tapering at both ends), elongate-oval (undefined, a sort of stretched out egg shape!) or oval (elliptical or egg-shaped). The 'common nettle aphid' Microlophium ca'rnosum (below first) and the 'common rose aphid Macrosiphum rosae (below second) are both commonly described as spindle shaped whilst Periphyllus aphids (below third) are described as elongate-oval or pear shaped. (Note, both nettles and roses host a number of aphid species.)

All of the aphids below are usually described as 'oval' (from first to third) Aphis grossulariae, Tuberolachnus salignus and Crypturaphis grassi, although the latter looks rather different as it is dorsoventrally flattened.

The pictures above give some idea of the variation of colour in aphids. Most are cryptically coloured to a greater or lesser degree, although a few such as Tuberolachnus (above) have aposematic colouration. Note that the colour of aphids usually disappears in alcohol, so a photo should always be taken of the live insect. In fact colour may well be more useful for identification than it is currently considered to be so, as photography of the live insect continues to improve. Some aphid species (unless they have recently moulted) are covered with wax, for example Phyllaphis fagi) which renders examination of features impossible. Isopropyl alcohol will remove most of the wax to reveal (and kill, and preserve) the aphid underneath.

As for size, there are a few 'giant' aphids (such as Tuberolachnus salignus which measures up to 6 mm long) and some very small species (such as Callipterinella minutissima which lives inside birch catkins, and is scarcely more than 1 mm long). However, most aphid species are of intermediate size. Overall aphid size provides some guide to species but, because it can vary with nutrition, size by itself is not a reliable measure.

Aphid size measurements require a microscope with a graticule. Aphid identification keys usually give the overall aphid length (minus the cauda) but the length of second hind tarsal segment is commonly used to standardize for size variation due to nutrition. Most keys assume you are dealing with a dead, preserved specimen (known as a clarified slide mount), so they use the relative sizes of parts such as antennal segments, legs and hairs, which don't shrivel when preserved.

Relative to the body, how long are the antennae?

The length of the antennae relative to the body length can be readily assessed and is a useful character at the genus level. Genera such as Microlophium have antennae markedly longer than the total body length - see picture of the common nettle aphid Microlophium carnosum first below. Others, such as the large walnut aphid Panaphis juglandicola (second below), have unusually short antennae - only about only about 0.3-0.4 times the body length. Another useful measurement that can often be carried out on photos of live aphids is the length of the terminal process of antennal segment 6 relative to the length of the base of that segment.

What shape are the siphunculi and cauda?

The siphunculi (also called cornicles) are a pair of tubes or pores on abdominal tergites 5 or 6. (The tergites are the plates or sclerites that cover each body segment). The siphunculi serve to exude a rapidly-solidifying waxy secretion which impedes the movement of an attacking predator as well as containing an alarm pheromone which alerts other aphids to the danger. The abdomen has a maximum of 9 visible tergites, of which the ninth is more or less produced into a cauda. In some species of aphids the hairs on the cauda are used to hold honeydew droplets for ants; in other species a longer cauda is used is to flick away droplets. The shape and length of both the cauda and siphunculi are very important for identifying aphids.

The first picture below shows the siphunculi and cauda of an Acyrthosiphon species (the pea aphid Acyrthosiphon pisum). Its siphunculi are long and tapered, as is its cauda. The second picture below shows the siphunculi and cauda of an Amphorophora species, the large blackberry aphid, Amphorophora rubi. Its siphunculi are slightly swollen on the apical half, and its cauda is tongue shaped. Beware! You need good lighting as in this picture to see only slight swelling of the siphunculi. Neither of these aphids is attended by ants. The third picture below shows the siphunculi and cauda of a typical Aphis species, the green apple aphid, Aphis pomi. Its siphunculi are somewhat tapered and the cauda is finger-shaped.

Some other aphid genera have much reduced siphunculi and cauda. The siphunculi of the Anoecia aphid in the first picture below Anoecia corni are reduced to small pores, rendered visible in the picture below by the waxy exudate produced from one of them. The cauda is even less apparent, but is hairy to trap honeydew droplets for the attendant ants. The second picture below shows the barely visible stump-shaped siphunculi of a Chaitophorus aphid: Chaitophorus tremulae along with the rounded cauda. The third picture below shows the siphunculi and cauda of a Cinara aphid: Cinara pini. Its siphunculi are pore-like and located on broad, pigmented, hairy cones. The cauda is again broader than long, and rounded.

How are the wings patterned?

The wings of aphids are used relatively little for identification possibly because the winged form is rather rare in many species. In addition there seem to be very few useful taxonomic characters on the wings to distinguish species. They can provide useful generic characters. A good example is the wings of Lachnus species where more than half the area of the forewings is covered with dark pigment. (See first picture below of Lachnus roboris). Callipterinella species have the wing venation strongly marked (see second picture below of Callipterinella tuberculata).

Is the abdominal dorsum sclerotized?

The extent of sclerotization (hardening) of the thoracic and abdominal dorsum is heavily used in aphid keys. The dorsum ranges from being membranous to nearly wholly sclerotic. The sclerotic areas may be solid, or broken into various segmental bands. Such areas may be more-or-less pigmented with melanin or they may remain, at most, smoky-yellowish which makes them difficult to see in live aphids. Do not assume a black aphid is necessarily sclerotic, or that a green aphid is unsclerotized.

For example the dorsum of many Aphis species (such as Aphis fabae, first picture below) is largely unsclerotized, whilst most Myzus species (such Myzus persicae, second picture below) have the dorsum uniformly sclerotic. If unpigmented, the sclerotic areas are best recognised by their shiny nature or by their sculptural patterns.

In a few species the sclerotized area of the dorsum is very obvious. For example, the Corsican pine shoot aphid Cinara brauni (third picture above) has black shiny sclerotized patch at the end of the abdomen which contrasts with the whitish wax powder covering the rest of the body.

Are there antennal or median tubercles?

The frons (the upper front portion of the head) may or may not have lateral tubercles (protuberances) on which the antennae are sited, and a median tubercle in between. The prominence of these various tubercles and the angle of the inner sides (whether converging or diverging) is commonly used in aphid keys. This can be difficult to spot in live specimens, but a photo taken from directly above the insect will usually be effective. The first picture below shows the frons of a Pterocomma aphid, the rufous willow-bark aphid, Pterocomma rufipes. Its frons is almost straight, with very low antennal tubercles and a slight median tubercle. The second picture below shows the frons of a Rhopalosiphoninus species, the marsh-marigold aphid, Rhopalosiphoninus calthae. Its antennal tubercles are divergent, and the median tubercle prominent. The third picture below shows a Drepanosiphum species the sapling sycamore aphid, Drepanosiphum acerinum. Its antennal tubercles are again divergent, with a small median tubercle.

Is there a supracaudal process?

The supracaudal process is a second tail-like process located directly above the cauda. Hence the aphid appears two-tailed. Only a few genera have a such a process, including Cavariella and Diuraphis. The relative length of the supracaudal process and the cauda can be helpful in distinguishing species of Cavariella. We show two species here. First below is the Willow--carrot aphid, Cavariella aegopodi, which has a supracaudal process longer than the cauda and swollen siphunculi. Second below is the willow--parsnip_aphid, Cavariella theobaldi, which has a very short supracaudal process (only about half the length of the cauda) and cylindrical siphunculi.

Are there capitate hairs?

Some genera have very distinctive capitate hairs (hairs enlarged and globular at its tip). (such as Capitophorus, Cryptomyzus and Corylobium). These show up remarkably clearly in photos, providing one has a highly contrasting background. See below left for capitate hairs on the large hazel aphid Corylobium avellanae and below right on the small downy birch aphid Betulaphis quadrituberculata.

Identifying an aphid to species

Definitive aphid identification to species usually involves microscopic examination of specially prepared aphid specimens. Aphids are usually preserved in alcohol - specifically 90% isopropyl alcohol (IPA, 'rubbing alcohol') or ethyl alcohol (industrial methylated spirits, IMS). Lactic acid may also be added to keep specimens soft. Note that neither vodka, household meths, surgical spirit, formalin, nor drying, are recommended when preserving aphids for laboratory examination.

The classical method is as follows:

• Preserved specimens are 'macerated' by a procedure which involves gentle boiling in potassium hydroxide in alcohol, then, usually, they are 'clarified' using some rather less pleasant chemicals. (Or see Blackman and Eastop's Methods.) Applied to a well-preserved aphid, those procedures remove the lipids, waxes, internal organs and suchlike, leaving only the outer skin.

  The resulting semitransparent ghost-like cuticle is carefully arranged on a glass slide, under a dissecting microscope, and permanently mounted (sometimes after staining) for subsequent examination and keying-out. This very tedious process is only undertaken by (increasingly fewer) specialists.

  Note: Permanently-mounted clarified specimens on microscope slides are variously known as clarified permanent mounts, clarified slide mounts, clarified mounts, or permanent mounts.

The micrograph below is of a stained, permanent mount of the asparagus aphid (Brachycorynella asparagi). You are looking at the insect's ventral, or underside. Dorsal features, such as the siphunculi and dorsal plates, are visible through the clarified specimen.

Image copyright Caroline Harding, MAF. Free for use under the Creative Commons Attribution 3.0 Australia License.

Fortunately it is often possible to key to species by examining (unmacerated unclarified) specimens mounted wet in isopropyl alcohol on a cavity slide under an ordinary microscope. Features that can be assessed include the length of parts of the rostrum, the number of sense organs on the antennae and presence or absence of abdominal marginal tubercles and polygonal reticulation on siphunculi.

Given the correct ancillary information was recorded (appearance in life, host plant etc.), a reliable identification can be made, and indeed the major work on agricultural pests assume that this approach is used (see Blackman and Eastop, 2000). We will concentrate on this approach in a later aphid blog.

Note: Since aphid taxonomy is still based upon morphology, not DNA analysis, no one knows to what degree current taxonomic keys reflect relatedness. Therefore it is uncertain to what extent differences in aphid morphology reflect differences in their host plants, rather than genetic differences between aphids, especially when there are only very slight differences between aphids with very closely-related hosts. Favret et al. (2010) raise this issue in relation to their aphid survey in the Great Smoky Mountains National Park, USA. They estimated that there were more species of the genus Uroleucon in the park area than in any other genus, but suspected that this apparent diversity was simply a result of historical reliance on host plant data to describe species. They noted that preliminary study suggests that a number of these 'species' are synonyms.

References  Blackman, R. L. & Eastop, V. (2000). Aphids on the World's Crops. 2nd Edn. Wiley, Chichester, 466 pp.  Favret, C. et al. (2010). Actual and inferred checklist of the aphids (Hemiptera: Aphididae) of the Great Smoky Mountains National Park, with attendant ant and host plant associations. Proceedings of the Entomological Society of Washington 112(3), 381-403