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Buckthorn-fern aphidOn this page: Identification & Distribution Other aphids on the same host
Identification & Distribution
Adult apterous viviparae of Macrosiphum rhamni are yellow green. The oviparae (see picture below), additionally have a large round patch of reddish internal pigment in the centre of the dorsum (this may also apply to other morphs, but see note below). The antennae are pale with the joints between segments III-IV and IV-V (& segment VI in the ovipara) contrastingly black (cf. Macrosiphum clydesmithi & Macrosiphum osmaroniae, which have those joints dusky). The antennal tubercles are well-developed, and divergent. The antennae are longer than the body, with the terminal process more than 6 times as long as the base of antennal segment VI. Antennal segment III has (0-)1-2 secondary rhinaria near the base (cf. Macrosiphum pteridis and Macrosiphum ptericolens, which have more than 3-35 rhinaria on that segment). The apical rostral segment (RIV+V) is 1.3-2.1 times as long as the second hind tarsal segment (HTII), and has 5-8 accessory hairs. RIV+V is 0.15-0.17 mm in length (cf. Macrosiphum dryopteridis, which has RIV+V < 0.14 mm.). The abdominal dorsum is entirely sclerotic, pale and generally smooth. The siphunculi and cauda are pale. The siphunculi are 3-4 times as long as the short cauda, with only 1-3 rows of hexagonal reticulations at its apex. The rather short bluntly triangular cauda has 6-9 hairs. The body length of Macrosiphum rhamni apterae on buckthorn is 1.8-2.4 mm, on fern they are 1.8-2.6 mm.
Note: The literature is rather contradictory on which morphs display a reddish spot on their dorsum. Blackman, quoting Essig (1917), reports it as present on apterous viviparae on Rhamnus in spring. Wilson (1912) reported the presence of "an orange spot in the centre of the body just back of the thorax" on some emigrant alatae on the primary host in June. Jensen & Holman (2000) report the red spot is present on summer and autumn specimens on Rhamnus, but in Aphidtrek (undated) Jensen states explicitly that "the red internal pigment is typical for oviparae and no other morph". The red spot is certainly not present on apterae on the secondary host - at least we have found no record of such.
The alate Macrosiphum rhamni (not pictured) is lemon-yellow, with orange head and thorax; the antennae have segments I, II and base of III pale, and the remainder dusky to black. Antennal segment III bears about 25 nearly circular secondary rhinaria of variable sizes and irregularly placed. The basal halves of the siphunculi are pale, the distal halves dusky to black. The siphunculi in alatae have reticulations covering the distal 0.5.
Macrosiphum rhamni is dioecious, host alternating from the primary host buckthorn, specifically California coffeeberry (Rhamnus californica) and cascara (Rhamnus purshiana), to the secondary hosts, ferns (Pteridium). In western Oregon the eggs hatch from mid-February to March, with adult fundatrices present from mid-March to late April. There is usually a second generation of apterous viviparae, followed by a third generation comprised of emigrant alatae which migrate to Pteridium. In some areas a small number remain on buckthorn all summer. Gynoparae and males return to Rhamnus in autumn, and oviparae can be found abundantly from mid-October to late November. Jensen & Holman (2000) note that males and alate viviparae often drop readily when disturbed, but apterae less so. They also note that they are frequently ant-attended, as is the case for another member of the Macrosiphini, Macrosiphum weberi, in Europe.
Other aphids on the same host
Macrosiphum rhamni has been reported on 2 buckthorn species (Rhamnus californica, Rhamnus purshiana).
Macrosiphum rhamni has been reported on one Pteridium species (Pteridium aquilinum).