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Macrosiphum walkeriOceanspray-fern aphidOn this page: Identification & Distribution Other aphids on the same hostIdentification & DistributionAdult apterae of Macrosiphum walkeri may be coloured yellow, green, pink or red (see two pictures below). The antennae are mostly pale, but dusky on the antennal joints, on the distal half of antennal segment V and all of segment VI. The tips of the tibiae, tarsi, and siphunculi are also dusky. The antennae are 6-segmented, with the terminal process about 7 times the base of antennal segment VI. There are 1-7 secondary rhinaria on antennal segment III. The rostrum reaches to the hind coxae. The apical rostral segment (RIV+V) is about 1.35 times as long as the second hind tarsal segment, with 8-12 accessory hairs. RIV+V is usually less than 0.14 mm (cf. Macrosiphum rhamni, Macrosiphum clydesmithi & Macrosiphum osmaroniae, which usually have RIV+V more than 0.14 mm). Spinal or marginal tubercles on the head or abdomen are rarely present. The siphunculi are cylindrical, and have a reticulated area at the apex. The cauda usually has 3 lateral pairs of hairs and 1 dorsoapical hair (cf. Macrosiphum dryopteridis, which usually has 4 lateral pairs of hairs). The body length of adult Macrosiphum walkeri apterae is 1.7-3.3 mm.
Images above copyright Andrew Jensen, under a creative common licence. Alatae of Macrosiphum walkeri are green, with dusky marginal sclerites, and mostly dusky siphunculi. There are 19-37 large and round secondary rhinaria, not quite in a straight line on antennal segment III, and 0-9 on antennal segment IV. The rostrum reaches to the middle coxae. Over much of its range, Macrosiphum walkeri host alternates from its (recently discovered) primary host, cloudberry (Holodiscus) (see first picture above), to numerous genera and species of ferns (see second picture above). The host alternation was only confirmed in 2019 when Andrew Jensen found a dense infestation of Macrosiphum walkeri on a tall Holodiscus discolor shrub adjacent to the Trampas Creek Trail in the Carson National Forest of New Mexico. The leaves of this plant were coated in aphids (alatae, oviparae, and males). This find prompted a vigorous search for aphids on all ferns living nearby, aiming to confirm that aphids were emigrating from one of them. A single apterous vivipara, three males, and numerous nymphs were found which microscopic inspection showed were conspecific with the samples from Holodiscus. Macrosiphum walkeri does not always host alternate. In the warm, wet forests of California and western Oregon there are anholocyclic populations on the secondary host, overwintering on the ferns Polypodium and Polystichum. The species is restricted to western North America. Other aphids on the same hostPrimary hostMacrosiphum walkeri has been recorded on 1 Holodiscus species (Holodiscus discolor).
Secondary hostsMacrosiphum walkeri has been recorded on many genera & species of ferns (e.g. Adiantum, Asplenium, Athyrium, Cystopteris, Davallia, Dicksonia, Dryopteris, Matteuccia, Nephrolepis, Osmunda, Pellaea, Polypodium, Polystichum, Woodwardia). Macrosiphum walkeri has been reported on 1 Asplenium species (Asplenium bulbiferum).
Macrosiphum walkeri has been reported on 2 Athyrium species (Athyrium alpestre, Athyrium filix-femina).
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