Adult apterae of Protaphis middletonii are pale green, bluish green, grey-green or olive-green, and more (see first picture below) or less (see second picture below) dusted with greyish wax. The apterae have 0-12 secondary rhinaria on antennal segment III. The antennal terminal process is 1.4-2.1 times as long as the base of antennal segment VI. The abdomen has dark marginal spots and also commonly some transverse black bands, but not an extensive solid black patch. Their legs are more or less dusky. The siphunculi are black, short, slightly thicker at base, and less than one tenth the body length. The cauda is also black and bluntly triangular. The body length of adult apterae is 1.5-2.5 mm.
Note: It is unclear to what extent Protaphis middletonii are waxed in life. Oestlund (1887) describes them as "leaden gray with young specimens all more or less pulverulent". The first image below shows a heavily waxed adult; in other images the aphids are only lightly dusted or unwaxed. There is some evidence that the degree of waxing depends on whether or not the aphids are being ant-attended. The high level of variation between specimens suggests several species are involved, but most of the proposed ones are insufficiently distinct. Until better information is available on the number of Protaphis species present in North America, Jensen advocates grouping most of them as Protaphis middletonii.
Alatae (see first picture below) have less apparent dark dorsal cross bands than the apterae. They have secondary rhinaria distributed 4-22 on segment III, 0-9 on segment IV, and 0-6 on segment V. Immature Protaphis middletonii (see second picture below) are a somewhat paler green than the adults.
Protaphis middletoni occur on the roots of plants in numerous families including the Asteraceae, Brassicaceae, Poaceae, Lamiaceae and Apiaceae. Oestlund (1887) found it very plentiful on the roots of horseweed (Erigeron canadensis), and more rarely on giant ragweed (Ambrosia trifida). Jensen noted that Aphis middletonii is highly variable in habits and appearance, feeding both underground and low down on the stem above ground. Colonies are usually ant-attended. Sexual forms may develop in autumn, but anholocyclic overwintering is probably also common. Protaphis middletoni is found throughout western and central North America. Aphids of this group are also recorded from Asteraceae in Brazil, and from prickly pear (Opuntia) roots in Australia and South Africa.
Infestation of maize with corn root aphid results in stunting of the plants especially during dry years. Feeding by the aphid causes a characteristic yellowish to reddish tinge in the leaves when the plant is young (see Radliffe's IPM World Textbook).
We are grateful to Claude Pilon for the picture of Protaphis middletoni. We are also grateful to Andrew Jensen and James Bailey for making their images available under a creative commons licence.
Identification of specimens photographed by Claude Pilon was confirmed by Eric Maw by microscopic examination and DNA analysis of preserved specimens. For taxonomic details we have used the accounts of Oestlund (1887) (as Aphis middletoni) and Palmer (1952) (as Aphis armoraciae), together with information from Roger Blackman & Victor Eastop in Aphids on Worlds Plants. We fully acknowledge these authors and those listed in the reference sections 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).
Oestlund, O.W. (1887). Synopsis of the Aphididae of Minnesota. Minnesota Geological Survey. Bulletin No. 4.Full text
Palmer, M.A. (1952). Aphids of the Rocky Mountain Region: including primarily Colorado and Utah, but also bordering area composed of southern Wyoming, southeastern Idaho and northern New Mexico.Full text