Aphis pentstemonicola

Adult apterae of Aphis pentstemonicola are green (see first picture below), greenish yellow, or light yellow (see second picture below), with a dark green patch in the mid-dorsum (the dark green colour is in the internal tissue, so is lost in clarified mounts) (cf. Aphis asclepiadis, which does not have a dark green mid-dorsal patch). The head and thorax are dusky, and there are traces of mid-dorsal bands on tergites VI, VII & VIII. There is a large dusky patch on the dorsum on the inside of each siphunculus (cf. Aphis asclepiadis, which does not have dusky patches, but may have a dark narrow sclerite by each siphunculus). The antennae are dusky or dark, apart from the basal part of segment III which is pale. The number of secondary rhinaria on antennal segment III is extremely variable, varying from 0-18, leading to suggestions that more than one species might be involved here (see note below). There are marginal tubercles on the prothorax and abdominal tergites I and VII. The siphunculi, cauda, anal plate and genital plates are dark, and the legs are pale to dusky, with the tarsi and tips of tibiae blackish. The siphunculi are cylindrical, coarsely imbricated and with a flange. The anal plate is slightly produced on the ventral margin. The cauda is bluntly triangular with only a slight tendency to constriction near the base, and bearing three hairs on each side. The body length of adult Aphis pentstemonicola apterae is 1.8-2.0 mm

Note: There is another species recorded on Penstemon, namely Aphis sierra (described by Essig, 1947), that is very similar to Aphis pentstemonicola (described by Gillette & Palmer, 1929). The only difference between the two species is in the number of rhinaria on their antennae. The apterous Aphis pentstemonicola (usually) has numerous rhinaria on segments III-V, whilst Aphis sierra (usually) has none. There are also possible differences in the number of secondary rhinaria on the alatae. Blackman in Aphids on Worlds Plants casts doubt on whether the differences between them warrants separate species status.

First image above copyright Andrew Jensen under a cc by-nc-sa licence.
Second image above, copyright Jesse Rorabaugh no rights reserved.

The alate Aphis pentstemonicola (not pictured) has a black head and thorax, but is otherwise coloured as the apterous vivivipara. They have 20-43 secondary rhinaria on antennal segment III, 12-20 on segment IV, and 5-9 on segment V.

The first image below shows a clarified mount of an apterous Aphis pentstemonicola vivipara.