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Identification & Distribution:

The fundatrix (not pictured in this section) is green or bluish green, covered in wax and lacks siphunculi. It inhabits a small gall of its own on poplar formed by folding the edge of a leaf. The winged viviparous female of Thecabius affinis (see first picture below) that develops from the offspring of the fundatrix is greenish, covered with wax spicules and has siphuncular pores. The antennae are short, about half the body length. The terminal process is 0.25 times the length of the base of the last antennal segment. The body length of winged Thecabius affinis females is 2.2-3.1 mm.

The winged females develop on Populus (poplar) species in May-June within galls which are formed by folding of young leaves in half along the mid-ribs (see second picture below). The outer, upper side of the leaf becomes blistered and yellowish or reddish.

 

The viviparous alates migrate to their secondary host - the stem bases and on the runners of buttercups (Ranunculus) - where their offspring develop to adult apterae (see first picture below) which are densely covered in wax spicules. The second picture below is a micrograph showing a dorso-lateral view of an alatiform fourth instar nymph of Thecabius affinis in alcohol.

 

The pictures below are micrographs showing a dorsal view of an aptera of Thecabius affinis in alcohol, and an alate in alcohol. Note the absence of siphunculi, although siphuncular pores are present.

 

Thecabius affinis usually host alternates from its primary host of poplar (Populus) species to buttercup (Ranunculus spp.), although some populations remain all year on buttercup roots. On poplar the fundatrix inhabits a small gall of its own on poplar (Populus), but its offspring leave this gall and stimulate development of a larger gall where the outer, upper side of the leaf becomes blistered and reddish. The winged viviparous females that develop in the gall on poplar leave in late June-July to found waxy colonies at the stem bases and on the runners of buttercups. Alatae migrate back to the primary host in autumn.

 

Biology & Ecology:

Life cycle

The overwintering eggs of Thecabius affinis laid the previous autumn by the ovipara hatch in spring to give the fundatrices. The fundatrix inhabits a small gall of its own on poplar (Populus) formed by folding the edge of a leaf (see first picture below). As with the later gall, the fundatrix gall is yellowish or reddish. The fundatrix itself (see second picture below) is green.

 

Its offspring leave this gall in May-June and move to feeding on the midrib of a young leaf. This induces the leaf lamina to fold along the midrib towards the underside, folding young leaves in half along the mid-ribs (see picture below).

The offspring of the fundatrix develop into winged viviparous females (see picture below) that leave in late June-July.

The winged females migrate to the runners, roots and stem base of buttercup (Ranunculus spp.), resulting in one or more generations of apterae. The picture below shows yellowish green Thecabius affinis immatures on a basal leaf of spearwort.

The apterae of Thecabius affinis on Ranunculus are dirty yellowish white or yellowish green with shiny wax wool (see picture below).

In most populations viviparous females produce alate sexuparae in the autumn while feeding on the summer host. These migrate to the winter host and give birth to sexual forms, males and oviparae. After mating, a single diapausing egg is deposited by each ovipara in a crevice in the bark of the tree.

Anholocyclic populations

The life cycle is modified in arctic and subarctic regions where the sexual generation on the primary host is lost, and the species only lives on Ranunculus (Dolgova, 1971;  Stekolshchikov & Buga, 2009 ). In northern Europe some of the populations are anholocyclic. The overwintering viviparae differ in many respects from the summer form, especially in the accumulation of large lipid reserves. Sutherland (1968)  showed that, whilst the summer forms contain small amounts of several neutral lipids, quiescent individuals contain large quantities of a very few lipids. Offspring of a summer form become quiescent if deposited and reared at a temperature of 8° C, regardless of photoperiod.

It is very likely that the population of Thecabius affinis that we found in the far north of Scotland (at Dundreggan) was anholocyclic on lesser spearwort (Ranunculus flammula), since there are few if any of the primary host that far north in Scotland, and we found the same patch of lesser spearwort with aphids on in both 2013 and 2015.

Instead of colonizing the roots, the aphids colonized the flower stalk (see picture below). This was presumably because lesser spearwort grows in swampy locations with the roots often underwater.

 

Other aphids on same host:

Blackman & Eastop list about 120 species of aphids  as feeding on poplars worldwide, and provides formal identification keys for aphids on Populus (poplar and aspen). They do not list Thecabius affinis as feeding on either White poplar (Populus alba), Grey poplar (Populus canescens), or European aspen (Populus tremula).

Of the 40 aphid species Blackman & Eastop list as feeding on Black, or Lombardy poplar (Populus nigra) Baker (2015)  lists 17 as occurring in Britain: Aulacorthum solani,  Chaitophorus leucomelas,  Chaitophorus populeti,  Chaitophorus populialbae,  Pemphigus bursarius,  Pemphigus gairi,  Pemphigus phenax, Pemphigus populi, Pemphigus populinigrae, Pemphigus protospirae,  Phloeomyzus passerinii, Pterocomma populeum,  Pterocomma tremulae,  Stomaphis longirostris, Thecabius affinis, Thecabius lysimachiae, and Tuberolachnus salignus. 

 

Damage and control

The leaf galls cause little or no harm to adult trees, but they can adversely affect the growth of young trees in nurseries.

Acknowledgements

We especially thank Trees for Life  for their kind assistance, and permission to sample.

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

  •  Dolgova, L.P. (1971). Aphid of the genus Thecabius (Homoptera, Aphidoidea)in the Altai District. Zoologicheskii Zhurnal 50(8), 1205-1213.

  •  Stekolshchikov, A.V. & Buga, S.V. (2009). Aphid fauna of the arctic and subarctic regions. Redia 92, 101-104.  Full text 

  •  Sutherland, O.R.W. (1968). Dormancy and lipid storage in the pemphigine aphid Thecabius affinis. Entomologia Experimentalis et Applicata 11, 348-354. Abstract