HOST LISTS AND KEYS
Below there are links to plant genera arranged in alphabetical order, with lists of the aphids recorded from the plant species within each genus, followed where necessary with identification keys to the aphids. Before clicking on these links and using these lists and keys please read the introductory information about the Host Lists and Keys section provided below.
Please keep in mind that the keys are for guidance only. They are not intended to provide definitive identifications on their own, but to indicate a possible name for an aphid that has been found colonising a particular plant. This name should be regarded as a hypothesis requiring further confirmation, initially by consulting the Aphids section of this website to check the appearance in life, recorded hosts and geographical distribution. If your aphid does not conform to all the information given, or if doubt remains for any reason, then a specialist in aphid taxonomy should be consulted, and the identification checked against Museum specimens and published descriptions. In view of certain recent publications it seems necessary to stress that it is unwise and scientifically unsound to publish records of species new to a country or geographical region solely on the basis of the information provided on this website.
Any person using a dichotomous identification key should also be fully aware that the features presented as the alternatives in each couplet are those specifically chosen by the author of the key in order to allow the user to follow one of two separate pathways, taking into account the ultimate goals of each of these two pathways, which usually will have additional branches. The features used early in a key therefore become less and less appropriate as more and more target species are eliminated. It follows that the information presented in any of the keys on this website should never be used or cited out of the context of the key as a whole, except where this information refers to a single target species.
Here are links to names of plant genera beginning:
N.B. The pages in the Host Lists and Keys section are best viewed at a magnification of 125% or 150%.
NOTES ON THE USE OF THE HOST LISTS AND KEYS SECTION
The host-aphid lists and keys in this section demonstrate, and in fact owe their feasibility to, the fact that most aphids are relatively host specific, and that this specificity is most evident at the level of the host genus. The number of aphid species recorded from any one plant genus varies greatly, from one to more than 260 (on Artemisia), and the proportion of these that are monophagous, oligophagous or polyphagous also shows considerable variation. The reasons for these differences are presumably part physiological and part phylogeographic. We hope that the lists will serve the supplementary purpose of providing a useful database for anyone studying the origins and evolution of the present-day associations between aphids and their host plants.
For those who prefer to work with hard copy, all text files can be printed in their entirety, or individual host lists and keys can be copied and pasted into Word or an equivalent word-processing programme.
The Host Plant-Aphid Lists
Aphid/host plant records are extracted from a wide variety of literature sources and will inevitably include a percentage of misidentifications, both of aphid and host plant. Many of the new host records for palaearctic aphids added since our 1994 and 2006 books are from the major work by Holman (2009), which includes all sources and is therefore a very valuable source of additional information. Many new host records for aphids in the western USA have been added in 2016, thanks to Andy Jensen and his website https://aphidtrek.org/.
As the aim is to list only true host plants we have omitted any records that are clearly spurious, e.g. tree-dwelling aphids such as Drepanosiphum platanoides and Eucallipterus tiliae that will often be found on vegetation below their respective host trees, and other aphids that were obviously vagrant individuals. When an aphid-host plant association is unusual or doubtful, the aphid species is placed in square brackets. We have used square-brackets in all cases where an aphid is listed but not included in the key, not only for records that we consider doubtful, but also for unseen and little-known species where the description does not provide sufficient information to discriminate it from other related ones occurring on the same plant genus. Further information on most of these species can be found in the Aphids section, referring to the index to aphid species names if necessary. In general we have tended to adopt a liberal approach, including species in a key even when we think that their normal hosts are in other genera.
Plant nomenclature has been extensively revised, and to a large extent now follows The Plant List. Authorities for plant species names are omitted except where there is ambiguity. Plant names that were misspelt in the original records have been corrected where we could be reasonably certain of the intended species. Plant names that could not be identified by reference to any available database, or where the application of the name is uncertain, have been included, but are followed by “(?)”.
The Keys
A key is provided to the aphid species on each plant genus in all cases except where only one species is recorded from that genus, or where all the species are polyphagous. Sometimes the aphids on related plant genera are combined in a single key. In particular, we found it most convenient to key all grass-feeding aphids (even although some are monophagous or genus-specific) together under Digitaria, and a similar procedure was adopted for aphids on ferns (under Polypodium), mosses (under Polytrichum) and orchids (under Cymbidium). There are cross-references to these keys under the host lists of all the relevant plant genera. The very large aphid faunas of some genera – Artemisia, Quercus and Salix for example – are divided up in a preliminary ‘master’ key, to avoid long and cumbersome keys with 100 or more couplets. If numerous aphid species in one genus are involved – Cinara on pines or Eriosoma on elms for example – then we have taken advantage of any apparent specificity shown by the aphids to limit the number of species that have to be discriminated in any one key.
It is very important to note that the keys are intended specifically for aphids found feeding on or colonising a named plant species, and those for aphids on herbaceous plants are based almost exclusively on the apterous viviparous morphs (apt.) found in mid- to late spring and summer. The stem mother or fundatrix (fund.) developing from the overwintering egg usually has a distinctive morphology, so samples collected early in the season (particularly when consisting of adult apt. with a few progeny) must be treated warily. There are inevitable exceptions, particularly in the case of tree-dwelling aphids, where there are no apt. to key, either because the viviparous females are all alate (al.) or, in the case of some heteroecious aphids on their primary host plants, because all the progeny of the fund. are al. spring migrants. In all such cases the morph(s) to which the key can be applied are, we think, clearly indicated.
Polyphagous aphids occur on most common plant genera, and to avoid a great deal of repetition many keys at some point lead the user to a key to polyphagous aphids, or to some part of it. Links in the keys to tree-dwelling aphids take the user to a separate, shorter key to the polyphagous aphids recorded from trees, although many of the species are the same as those in the more comprehensive key that is provided to polyphagous aphids on herbaceous plants and shrubs. Many of the polyphagous aphid species on herbs and shrubs are in any case likely to be found on any plant along with aphids with more specific tastes, so it makes sense to transfer the user to the polyphagous aphids key at an appropriate point, even when only rather few polyphagous species have actually been recorded from the plant genus in question. In fact the first question for anyone setting out to identify an aphid from any herbaceous plant or shrub should be “Is it one of the common polyphagous species?” (See also the introductory comments to the polyphagous aphids keys.)
Whereas the keys in Aphids on the World’s Crops (Blackman & Eastop 2000) are relatively simple and can be used for unmounted specimens viewed under a binocular microscope, the user of the keys on this website will need to make microscope slide whole mounts of the aphids to be identified. We recommend that Canada balsam mounts are prepared as these are of proven permanence, and can withstand a range of temperatures and humidities. A simple procedure for preparing balsam mounts is that of Martin (1983); for details of this and other advice on mounting, labelling and storage of aphid specimens see the Techniques section of this website. An important point to emphasise is that the exposure to and removal of potassium hydroxide need to be carefully carried out, as over-potashing will cause bleaching, and the extent and distribution of cuticular pigmentation is often used as a key character.
The degree of confidence one can have in any identification made with a key in this book will depend on a range of factors. We have tried to make the keys as comprehensive as possible. This meant including species that we have not been able to examine ourselves on the basis of their published descriptions. Unseen species are indicated by an asterisk (*). Also included are little-known species of which only a few specimens have been described or examined (in some cases only a single specimen), and which potentially have a much greater range of character variation. Where there are discriminants between two nominal species we have used them, even when we suspect that further work may prove them to be unreliable. If we suspect that two species are synonymous then this is indicated in the text for one or other of them in the Aphids section.
So as a general rule, specimens that run in a key to a common species are more likely to be reliably identified than those that run to an asterisked name, or to a species that transpires to be little-known or locally distributed when the name is looked up in the Aphids Section. You need to proceed with caution, for example, if the name that you arrive at is that of a species new to your region, and be extremely sceptical if, for example, you are in Patagonia and you have identified your aphid as a species that is only known from a single sample collected many years ago in Mongolia! If in doubt, always consult an experienced aphid taxonomist.
Also included in the lists and keys are a rather large number of undescribed species from the BMNH collection, providing details of their host, country of origin and collector. The formal naming of new species is, of course, fundamentally important, but describing species properly is very time-consuming. Descriptions should normally include both apterous and alate morphs, and if at all possible they should be based on several samples collected at different times of year and in more than one locality. Most of the undescribed material in the BMNH collection does not fulfil these criteria. Also, descriptions of new species are best included in revisions of the groups concerned, so that they can be adequately compared to existing species. Nevertheless, we believe that by including undescribed species in the host lists and keys we can at least make known their existence, so that this material can be borrowed and included in future taxonomic studies.
Aphid morphology and key characters
Fig. 1 illustrates the characters and morphometric parameters of a typical aptera of the tribe Aphidini which are in common use in the keys, and the abbreviations. Fig. 2 shows the principal features of two aphids of different genera within tribe Macrosiphini. Fig. 3 illustrates features of the terminal segments of the rostrum (R IV+V); the number of accessory hairs is frequently of taxonomic importance. The morphology of a generalised alate aphid is illustrated in fig. 8. For more detailed information on aphid morphology consult Miyazaki (1987), or Blackman & Eastop (2000). Many of the discriminants used in the keys are morphometric, and require measurement of parts of the aphid such as antennal segments with a micrometer eyepiece or a digital measuring system. Some of the commonest measurements are shown in more detail in fig. 9. Correct and accurate calibration of the measuring device is obviously extremely important. The parameters measured are mostly as in Ilharco & van Harten (1987), except that it is important to note that in the keys on this website body length (BL) is always measured to the posterior end of the anal plate and does not include any projecting cauda. Please also note that the measurement of the basal width of the cauda in our fig. 9 also differs from that shown in Fig.1.3.13 of Ilharco & van Harten (1987).
Key couplets may offer a choice between two ranges of measurements or ratios. Sometimes when species are particularly difficult to separate these ranges are contiguous, or even overlap. For reliable identifications one should therefore ideally examine a series of 10 or more apterous adult aphids, so that if necessary the range of variation in the sample can be assessed and compared with the range given in the key.
Keys will not work if applied to immature aphids, and in several groups, especially when the adult cauda is broad and rounded as in Lachninae, Greenideinae and many Eriosomatinae, it can be difficult to distinguish between late instar immatures and apterous adults. If the form of the adult cauda is not distinctive, then the presence of rudimentary gonopophyses, and of fully-formed and clearly defined anal and genital plates, can provide recognition features for adults, but even these are not wholly reliable. To be confident about the recognition of adults in difficult cases it may be necessary to examine a number of specimens of different sizes from the same colony, in order to establish what are the particular features of the adult insect in that species. In any case collection and examination of a large sample should be the general rule, because identifications should always be based on an examination of the fullest possible range of variation.