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Small willow aphid, Sallow aphidOn this page: Identification & Distribution Biology & Ecology: Life cycle Aphid size Herbivore induced responses Ant attendance Natural enemies Other aphids on the same host Damage & Control
Identification & Distribution:
Aphis farinosa apterae are quite small, and are green mottled with yellow-orange. The antennal terminal process is 1.6-2.3 times as long as the base of antennal segment VI. The abdominal dorsum in apterae is usually entirely pale and membranous with fairly numerous protuberant, hemispherical or teat-shape marginal tubercles on the tergites. The siphunculi are long and pale with a slightly dusky tip and the cauda is distinctly darker. The siphunculi of the aptera are 1.74-2.95 times as long as the cauda. The body length of Aphis farinosa apterae is 1.6-2.5 mm.
Their oviparae are dull green, but the young nymphs and the apterous males are reddish orange. The alates (see second picture above) are dark green, and their siphunculi are more or less dusky. Immature alatae (mainly fourth instar) often have white wax spots on the dorsum. The images below show a viviparous aptera and a male Aphis farinosa.
The small willow aphid is fairly common, forming dense colonies on the young shoots of willows (Salix spp.) especially sallow (Salix caprea) in spring and early summer. Aphis farinosa do not host alternate and are attended by ants. They have a sexual stage in the life cycle, with oviparae and males appearing from July onwards, although occasionally populations of viviparae persist until August or September. Aphis farinosa occurs throughout northern temperate parts of the world (North America, Europe & Asia) and in South America (Argentina).
Biology & Ecology:
The eggs of Aphis farinosa laid the previous summer on willow hatch in spring, and aphids colonize the young shoots.
Alatae then disperse to other young shoots to establish new colonies.
Most of the Aphis farinosa we have found have been feeding on Salix caprea or Salix cinerea in southern England. But in the north of Scotland, at Dundreggan, we found Aphis farinosa feeding on a different host, eared willow (Salix aurita). The aphids appeared to be markedly smaller than those we had found in southern England. Measurements of body length on 29 apterae, preserved in alcohol, gave mean body length of only 1.30 mm - with a range of 0.8 - 1.5 mm. The distribution is shown below (note: this not a histogram).
The normal size-range for (what we assume are) clarified slide mounted specimens according to Stroyan (1984) is 1.6-2.5 mm. Given our measurements are somewhat left-skewed, their median length (1.33 mm) would be a more appropriate summary measure of length. Also, an aphid body is an asymmetric three-dimensional object, whereas a clarified mount is two dimensional. Hence we estimated that our measurements were biased downwards by, at most, a factor of 0.83. Correcting our measurements accordingly, their range is 0.96 - 1.8 mm. This is still markedly smaller than Stroyan's estimate of 1.6 - 2.5 mm. We cannot tell whether this difference is related to climatic factors or to their diet of Salix aurita.
Herbivore induced responses
Although Aphis farinosa is distributed through most of in the northern hemisphere, there has been relatively little work done on its ecology. The exception is in Japan where the effects of herbivore-induced plant responses on Aphis farinosa have been investigated. Nakamura et al. (2003) showed that colonization rates by this aphid were four times higher on Salix eriocarpa shoots that had been galled by the gall midge Rhapdophaga rigidae than on ungalled shoots. The gall on Salix is shown here:
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The gall midge stimulated regrowth responses in Salix which, in turn, positively affected the aphid and other phytophagous insects. Ohgushi (2007) noted that the increased densities of aphids and leaf beetles were not only due to increased numbers of newly emerged shoots and leaves but also to improved leaf and stem quality for the herbivorous insects. Nitrogen and water contents were significantly increased but toughness was decreased in apical stems and upper leaves of galled shoots. Furthermore, they frequently observed that increased ant tending in aphid colonies for honeydew resulted in the removal of leaf beetles from adjacent shoots.
The small willow aphid is often (nearly always) attended by ants. Stroyan (1984) describes it as being 'eagerly visited by ants'. If there are any wood ants in the vicinity, they will often be found attending Aphis farinosa.
Southern wood ants (Formica rufa) will fiercely defend Aphis farinosa if they are threatened, often by covering the aphids with their bodies.
The level of ant attendance may vary seasonally because of competition for ants with other aphid species. Addicott (1978) described how large numbers of Formica integroides attend Aphis farinosa on willow trees in Canada during the early part of summer, but that these numbers declined to zero by mid-summer. This change apparently arose partly because of a decline in the number of Aphis farinosa, and partly because of an increase in the number of Aphis varians on fireweed adjacent to the willow.
A variety of other ant species also attend Aphis farinosa. In southern England we have found Formica fusca attending it (see first picture below). Lindsey (2007) found it being tended by the, related, Slavemaker Ant (Formica sanguineae).
Myrmica species may also be found tending Aphis farinosa (see picture below).
Rakhshani et al. (2007) reported that Aphis farinosa was an important reservoir of this parasitoid in cotton, citrus and vegetable agroecosystems. Our own observations suggested that attending ants do not seem to attack or in any way inhibit the activities of Lysiphlebus. This may well be another case where parasitoids mimic the ants, as with Lysiphlebus cardui which parasitizes aphids on thistles. There the parasitoids are the main benefactor of ant attendance, since ants create an enemy-free space for the parasitoid larvae within the mummified aphids (Völkl, 1992.)
Despite the presence of ants, predators can also often be found amongst Sallow Aphid colonies. The first picture shows a syrphid egg in the midst of the same aphid colony as shown above attended by Southern Wood Ants. The second picture shows a syrphid larva, possibly a Scaeva species, predating sallow aphids in the same colony. Several coccinellid species have also been recorded predating this aphid, including Two-spot Ladybirds (Adalia bipunctata) and Cream-spot Ladybirds (Calvia quattordecimguttata).
Other aphids on same host:
Blackman & Eastop list 33 species of aphid as feeding on sallow (=Goat Willow, Salix caprea) worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists 23 as occurring in Britain (Show British list).
Damage and control
So do Aphis farinosa infestations cause any significant economic damage?
Salix caprea is not a good source of timber, although there are several ornamental varieties and the species may be grown for biofuel in energy forestry systems.
No studies have been carried out to assess the direct effects of populations of this species on biomass yield. However, Kondor et al. (2007) reported viral transmission by Aphis farinosa causing leaf deformations and forking. Plants were said to have overcome this damage in a few weeks time.