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Identification & Distribution

Adult apterae of Forda marginata on their secondary host (grass roots) vary in colour from green to greenish yellow to brownish yellow (see two pictures below).

The body is highly domed dorsally and there are no siphunculi and no wax. Adult Forda marginata apterae have short antennae (0.5-0.64 mm) (see first picture below) which are only 0.19-0.29 times the body length (cf. Forda formicaria  which has markedly longer antennae). The rhinarium on the base of the fifth antennal segment is almost circular (see second picture below) and less than 2.5 times larger than that on the fourth antennal segment (cf. Forda formicaria which has that rhinarium very large and extending around the segment). The cuticle has thorn-like sculpturing which is especially well developed on the head and thorax.

In southern Europe, the Middle East and north west India the crusty pistachio-grass root aphid host alternates from Pistacia, where they live in yellowish or green galls formed by upward rolling of the leaf margins, to grass roots (Poa, Bromus, Dactylis etc.). In northern Europe, North America and parts of Asia Forda marginata exist as parthenogenetically reproducing populations on grass roots year round.

 

Biology & Ecology

Life cycle

The primary host, Pistacia, does not occur naturally in Britain, so not surprisingly we have only found the crusty pistachio-grass root aphid on its secondary host, grass roots. Most of the colonies we have found have been on the roots of annual meadow grass (Poa annua) growing as a weed usually on the edge of concrete or tarmac.

The presence of Forda marginata is sometimes indicated by earth tenting carried out by the attendant ants, although this may be for other species of grass root aphid found on the root collar.

Forda aphids can be separated from other genera on grass roots by their characteristic highly domed body which can best be seen in side view (see picture above).

Ant attendance

Paul (1977)  considered Forda marginata, an obligate myrmecophilous species, being almost always found with Lasius flavus.

However, if Lasius flavus is not present, Forda marginata seems equally at home with Lasius niger (see picture above and below).

 

Other aphids on same host:

Paul (1977)  found at least 16 other aphid species recorded on grass roots in Britain: Anoecia corni,  Anoecia furcata (= A. nemoralis), Anoecia major, Anoecia (Paranoecia) pskovica Mordvilko, Anoecia vagans (= Anoecia willcocksi), Anoecia zirnitsi, Aploneura lentisci,  Baizongia pistaceae (=Pemphigus cornicularius), Forda formicaria,  Forda marginata, Geoica setulosa, Geoica utricularia, Paracletus cimiciformis, Rhopalosiphum insertum,  Smynthurodes betae  and Tetraneura ulmi. 

It can be especially difficult to identify root aphids in the field because they often occur in mixed species colonies, such as shown in the picture below.

The pale green aphids are Forda marginata, but the heavily sclerotized grey-black aptera is an Anoecia species with its pinkish yellow immatures.

Acknowledgements

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 Blackman & Eastop (1994)  and Blackman & Eastop (2006)  supplemented with Blackman (1974) , Stroyan (1977) , Stroyan (1984) , Blackman & Eastop (1984) , Heie (1980-1995) , Dixon & Thieme (2007)  and Blackman (2010) . 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

  • Paul, R.G. (1977). Aspects of the biology and taxonomy of British myrmecophilous root aphids. PhD thesis. Imperial College, London. Full text