1 Taxonomic Issues ROGER L. BLACKMAN1* AND THE LATE VICTOR F. EASTOP 1Department of Life Sciences, The Natural History Museum, London, UK Introduction Auchenorrhyncha plus Heteroptera (von Dohlen There are more than 5000 species of Aphididae and Moran, 1995). in the world (Remaudière and Remaudière, 1997; One major feature, their well-known cyclical Favret, 2014). Of these, about 450 species have been parthenogenesis, sets aphids apart from other recorded from crop plants (Blackman and Eastop, Hemiptera, and has influenced every aspect of their 2000), but only about 100 have exploited the agricul- biology. The system of alternating one bisexual tural environment successfully to the extent that they generation with a succession of parthenogenetic, are of significant economic importance (Table 1.1). all-female generations evolved in the ancestral line The agriculturally important species are mostly in of all Aphidoidea, probably as far back as the the subfamily Aphidinae, not only because this is the Triassic. At first, the parthenogenetic females must largest subfamily but also because it contains a very have laid eggs like their sexual counterparts, as do high proportion of the aphids that feed on herba- the parthenogenetic females of present-day adel- ceous plants (Blackman and Eastop, 2006). Some gids and phylloxerids. Then, in the line leading to quite large aphid subfamilies – the Calaphidinae all modern Aphididae, the parthenogenetic gener- and Lachninae, for example – are associated almost ations developed the further refinement of vivipar- exclusively with woody plants, as are most of the ity, which effectively ‘telescoped’ generations into smaller ones. one another, thereby greatly reducing the gener- The Aphididae is one of three families of Aphidoidea, ation time and enabling far more efficient exploit- the other two being the Adelgidae, or conifer woolly ation of periods of rapid plant growth. aphids, and the Phylloxeridae, which are also nearly Aphids (Aphididae) can be recognized by a num- all associated with trees but include the notorious ber of shared morphological characteristics that Daktulosphaira (= Viteus) vitifoliae (grape phylloxera) developed before the divergence into present-day (Table 1.1). In the context of the order Hemiptera subfamilies: e.g. siphunculi (secretory organs, but as a whole, the superfamily Aphidoidea is placed in with their precise function still strangely enigmatic); the suborder Sternorrhyncha, along with Coccoidea five- or six-segmented antennae composed of two (scale insects and mealybugs), Aleyrodoidea (white- basal segments and a segmented flagellum with a flies) and Psylloidea (psyllids or jumping plant lice). terminal process; two-segmented tarsi, with the All these insects are phytophagous, and most of first segment much shorter than the second; and a them are sap sucking. Historically, the Sternorrhyncha cauda, or tail, that is often used for flicking away have been grouped with the Auchenorrhyncha (leaf- droplets of honeydew from the anus. These features hoppers, cicadas, etc.) as Homoptera, but molecular have been modified, reduced, or secondarily lost in work has provided strong support for the long-standing some species, but are evident in most aphids that view, based on morphological and embryological are pests of crop plants. evidence, that Sternorrhyncha and Auchenorrhyncha Of the present-day subfamilies of Aphididae, do not share a common ancestor, so this grouping is one in particular was successful at exploiting the not phylogenetically sound. The general consensus rapid expansion of numbers and diversity of her- now is that the primary division of the Hemiptera is baceous flowering plants in the Tertiary period. into Sternorrhyncha and a sister group comprising This subfamily, the Aphidinae, with 2900+ extant *Corresponding author. E-mail: [email protected] © CAB International 2017. Aphids as Crop Pests, 2nd edition (eds H.F. van Emden and R. Harrington) 1 2 Table 1.1. Biological features and distribution of agriculturally important species among the major groups of aphids. Geographical Number of agriculturally Taxon Characteristic host plants distribution Host alternation? important spp./total spp. Representative genera Adelgidae Coniferae Holarctic Most 0/60 Adelges, Pineus Phylloxeridae Fagaceae, Juglandaceae Holarctic (mostly Some 1/70 Phylloxera, Daktulosphaira Nearctic) Aphididae: Eriosomatinae Ulmaceae, Salicaceae, Holarctic, Oriental Many or most 9/365 Eriosoma, Pemphigus, Anacardiaceae, Prociphilus roots of gymno- and angiosperms Hormaphidinae Hamamelidaceae, Mostly Oriental Many or most 5/222 Astegopteryx, Cerataphis, Styraceae, palms, Ceratovacuna bamboos Anoeciinae Cornus, grass roots Holarctic Most 2/30 Anoecia Calaphidinae Deciduous trees Holarctic None 6/351 Chromaphis, Myzocallis, Therioaphis Saltusaphidinae Sedges Holarctic None 0/59 Iziphya, Subsaltusaphis, Thripsaphis Chaitophorinae Salicaceae Holarctic None 2/173 Atheroides, Sipha Greenideinae Dicotyledonus trees Eastern Palaearctic, None 2/180 Anomalosiphon, Oriental Greenidea Aphidinae: Aphidini Many families, Holarctic, a few Some 20/808 Aphis, Rhopalosiphum, including Rosaceae, southern Schizaphis R.L. Blackman and V.F. Eastop V.F. and Blackman R.L. Poaceae, hemisphere Acyrthosiphon, Brevicoryne, Macrosiphini Asteraceae Some 60/2116 Macrosiphum, Myzus Lachninae Trees (dicotyledonous Holarctic None 4/400 Cinara, Lachnus, Trama and coniferous) 15 other Mostly dicotyledonous Some are Oriental None 0/185 subfamilies trees and southern hemisphere species, is predominantly a northern temperate and requiring specialist identification, it is clearly a group, with life cycles closely tied to temperate sea- distinct species. There are plenty of other cases, sonality and the phenologies of temperate plants. however, including other close relatives of A. gos- Originally on woody plants, they evolved a system sypii, where the question of identity is not so easily of host alternation, migrating to completely unre- answered. Several of these are discussed in the next lated herbaceous plants for the summer months, section of this chapter. Sometimes, taxonomic dif- where their parthenogenetic generations could ficulties arise as a result of founder effects and, continue to utilize stages of rapid plant growth. even in the case of the soybean aphid, it may be However, today, only about 15% of Aphidinae necessary to bear in mind that the introduced host alternate. Some of the other 85% live only on population has been through a recent ‘bottleneck’ woody plants, but most of them, including some of and can be expected to have less allelic diversity the largest and most successful genera, have lost or than East Asian populations. given up the ancestral woody (primary) host and Taxonomy and identification are a matter of now live all year round on herbaceous plants. Host interpreting observed variation. The first problem alternation has, in fact, evolved independently in to be overcome is the effect of the environment on several other aphid subfamilies (von Dohlen and the phenotype, especially if the only available data Moran, 2000), but the Aphidinae are the only sub- are morphological. Most species are distinguished family to exploit numerous families and genera of and described originally, often from one small flowering plants. sample only, using morphological criteria, and In the rest of this chapter, we will look at aphids most identifications are based on keys that use from a taxonomist’s viewpoint. We will highlight morphological discriminants. As a group, aphids some of the problems of interpreting the observed are renowned for the considerable extent to which variation within and between species, and discuss the phenotype is influenced by environmental fac- what the names we give to aphids really mean. tors. Within any aphid species, there are a number Then, as examples, we will discuss 15 taxa that of different forms (morphs) with discrete morpho- probably head the list of economically important logical differences, which may be triggered by aphid species. specific environmental stimuli such as day length or crowding. It is well known, for example, that in most aphids, the parthenogenetic females can be Interpreting Variation in Aphids winged or wingless, the differences being not just In 2000, soybean crops in the USA and Australia in the presence or absence of wings but involving were attacked for the first time by large numbers of every part of the body. Separate identification keys an aphid closely resembling the well-known poly- are therefore needed for each morph, but the phagous species, Aphis gossypii (cotton or melon structural distinction between the winged and aphid), but obviously with a far greater affinity for wingless morphs is not always as clear as might be soybean. Taxonomists identified the species as expected, as wingless individuals may occur with Aphis glycines (soybean aphid), previously known some tendency towards the characters of the only from the Far East. It had been introduced winged morph. Likewise, intermediates between probably a year or two earlier, building numbers other morphs can occur; for example, between and spreading until field entomologists and grow- viviparous parthenogenetic females and oviparous ers realized that they had something new on their sexual females. hands (see review by Tilmon et al., 2011). Aphis The range of continuous morphological variation glycines is biologically quite different from A. gos- is also wider than in many other insect