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Frosted rabbitbrush aphidOn this page: Identification & Distribution Other aphids on the same host
Identification & Distribution
In summer the adult apterae of Pleotrichophorus oestlundi (see two pictures below) are bluish green to apple green, with apparent white pruinosity (=frosting) resulting from a dense covering of capitate to fan-shaped hairs. In autumn the aphids acquire a slight reddish cast, later turning reddish to reddish brown. The antennae are pale, with antennal joints between segments III & IV, the tips of segment V, and all of VI dark. Antennal segment III bears 1-4 secondary rhinaria, with the longest hairs on that segment less than 0.33 times the basal diameter of that segment. The antennal terminal process is 4.75-7.5 times the base of segment VI. The apical rostral segment (RIV+V) is rather slender, tapering to an acute point, and with posterior margins slightly convex; it is 0.75-1.0 times as long as second hind tarsal segment (HTII). The tergum is slightly sclerotic, densely imbricate, spiculate between hair bases and densely covered with uniformly fan-shaped hairs. The legs have 3 hairs on each of the first tarsal segments. The siphunculi are only moderately long, 1.0 to 1.8 times as long as cauda, and pale throughout (cf. Pleotrichophorus stroudi on Ericameria, which has long, thin siphunculi which are dark over at least the distal half; and cf. Pleotrichophorus pycnorhysus, which has dark-tipped siphunculi). The cauda has an acute but rounded apex, with slight basal constriction; it is spiculate, and bears 3 pairs of pointed lateral hairs, and 4-10 flattened or funnel-shaped hairs dorsally (cf. Pleotrichophorus wasatchii, which has 2 pairs of lateral hairs, and a single non-capitate dorsal hair on the cauda).
Note: In the text above, we have compared Pleotrichophorus oestlundi to only a few of the 21 Pleotrichophorus species found on Ericameria nauseosa. Fortunately Pleotrichophorus oestlundi can be distinguished from all other Pleotrichophorus species by the presence of several capitate hairs on the cauda.
The alate Pleotrichophorus oestlundi (see first picture below) is bluish green. The antennae are pale except for the joints between antennal segments III and IV, IV and V, the apices of V, and all of VI, which are dark. Antennal segment III bears 8-14 secondary rhinaria. The wings have dark prominent wing veins. The abdomen is like that of the aptera, but with 3 pairs of rows of sclerites; the spinal ones are small and oval, the pleurals are larger and transverse, and the marginals oval, larger than spinals.
First image above copyright Andrew Jensen, Second image copyright Matt Lavin,
Pleotrichophorus oestlundi is monoecious holocyclic on rabbitbrush (Chrysothamnus spp. and Ericameria spp.) (see second picture above). Sexual forms (red to brownish red oviparae and alate males) occur in October. Knowlton (1927) describes how the colonies of this aphid are usually found on the younger leaves towards the tips of the branches. Here the larger forms are usually lined up, end to end, in the V-shaped depression on the upper side of the leaves with heads pointed toward the petiole. The immatures crowd in around the larger, arranged in all positions. When the infestation is heavy, the flowerheads and their stems are attacked; later all of the leaves may be quite highly infested on both sides.
The apterous viviparous and oviparous forms of Pleotrichophorus oestlundi usually lie quietly with legs drawn up and antennae lying along the back. When moving around, the antennae are waved about although not as actively as in the case of the males. It takes considerable disturbance to excite the apterous females, but when noticeably disturbed they often drop from the plant to the ground. Very little disturbance, however, will usually cause the males and winged viviparous forms to take flight. The feeding of even large numbers of this aphid, seems to do little damage to the plant. The frosted rabbitbrush aphid is one of the commoner species of Pleotrichophorus, and is widely distributed in western USA.
Other aphids on the same host
Blackman & Eastop list 21 species of aphid as feeding on grey rabbitbrush (Ericameria nauseosa worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists 0 as occurring in Britain (Show British list).