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Identification & Distribution

Adult apterae of Rhopalosiphum enigmae are broadly oval and coloured dark reddish brown to greenish brown or dark green (see first picture below). They usually have a quite marked red-brown patch between and anterior to the siphunculi (best seen in second picture below) similar to that found in Rhopalosiphum padi. Hottes and Frison (1931) noted that alate viviparaous Rhopalosiphum enigmae have a pair of small wax glands - and the second picture below shows that wax may be present on the legs, antennae, cauda and dorsum of the head. The terminal process is 4.0-6.3 times as long as the base of antennal segment VI (cf. Rhopalopsiphum maidis, which has the terminal process 1.7-2.8 times the base of segment VI). The longest hairs on antennal segment III are 0.8-2.0 times the basal diameter of that segment. The dorsal abdominal cuticle has a pattern of blunt spicules arranged in polygons. Small marginal tubercles are usually present on some or all of abdominal tergites II-VI as well as I and VII (cf. Rhopalosiphum padi, which has no marginal tubercles on tergites II-VI). Their femora are dark brown, paler at the base. The siphunculi appear more or less cylindrical or slightly swollen, narrowing sharply just before a large flange (cf. Rhopalosiphum nymphaeae, which has slightly swollen siphunculi with a narrower section on the basal half; and cf. Rhopalosiphum maidis, which has siphunculi with no subapical constriction and a small flange). The siphunculi are 0.12-0.17 times the body length, and more than 2.1 times the length of the cauda (cf. Rhopalosiphum padi, which has siphunculi less than 2.2 times the length of the cauda). The body length of adult Rhopalosiphum enigmae apterae is 1.7-2.3 mm.

Note: Skvarla et al. (2018) synonomized Rhopalosiphum laconae Taber with Rhopalosiphum enigmae.

Both images above by permission, copyright Claude Pilon, all rights reserved.

Rhopalosiphum enigmae alatae (not pictured) have 7-15 secondary rhinaria an antennal segment III, and 0-3 on segment IV. Young immatures (see first picture below) are light greenish yellow to umber (greyish-brown to dark reddish-brown).

Images above by permission, copyright Claude Pilon, all rights reserved.

Rhopalosiphum enigmae feed on cattails (Typha spp.), especially Typha latifolia, forming colonies concealed within leaf sheaths (see second picture above). The species is also recorded from a bur reed (Sparganium sp.). There is no host alternation and sexual forms develop in autumn (Hottes and Frison, 1931). The American cattail aphid is widely distributed in North America.  

Biology & Ecology

Ant attendance

Until recently, there were no records of ant attendance of Rhopalosiphum enigmae. But Skvarla et al. (2018) found that when ants had access to American cattail aphids (e.g. when the cattails were in dry patches or when connected to dry soil via bent plants), ants often attended the aphids. The presence of ants or ant activity, such as dirt and detritus around a cattail stem, proved an excellent indicator for the presence of an aphid colony. Eleven ant species are now known to attend Rhopalosiphum enigmae, including the rarely collected wetland specialist ant Crematogaster pilosa, not previously known to tend aphids.  

Natural enemies

Skvarla et al. (2018) found the coccinellid Diomus terminatus on three occasions under cattail leaf sheaths in association with American cattail aphids. Diomus terminatus is a generalist aphid predator known to feed on a wide variety of aphids, including other Rhopalosiphum species.

In addition, the hymenopteran parasitoid Aphelinus (Aphelinidae), and hyperparasitoids Alloxysta (Figitidae), Dendrocerus (Megaspilidae), Asaphes and Pachyneuron (Pteromalidae) were reared from parasitized American cattail aphids. Two aphidiine braconids were also reared, but not identified beyond subfamily.

 

Other aphids on the same host

Rhopalosiphum enigmae has been recorded from 2 Typha species (Typha latifolia, Typha orientalis).

Acknowledgements

We are especially grateful to Claude Pilon for pictures of Pterocallis alnifoliae (for more of her excellent pictures see).

We have made provisional identifications from high resolution photos of living specimens, along with host plant identity. In the great majority of cases, identifications have been confirmed by microscopic examination of preserved specimens. We have used the keys and species accounts of Hottes & Frison (1931), Palmer (1952) & Skvarla et al. (2018) as well as Blackman & Eastop (1994) and Blackman & Eastop (2006). We fully acknowledge these authors 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).

Useful weblinks

References

  • Hottes, F.C. & Frison, T.H. (1931). The Plant Lice, or Aphiidae, of Illinois. Illinois Natural History Survey Bulletin 19(3), 123-447. 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

  • Skvarla, M.J. et al. (2018). Taxonomy and natural history of cattail aphids, Rhopalosiphum enigmae (Hemiptera: Aphidomorpha:Aphididae), including a new synonymy and notes on ant and parasitoid associates of Rhopalosiphum. Insect Systematics and Diversity, 2 (2), 114. Full text