Biology, images, analysis, design...
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Identification & Distribution:
Aphis farinosa apterae are quite small and are green mottled with yellow-orange. The siphunculi are pale with a slightly dusky tip and the cauda is distinctly darker. The body length of Aphis farinosa apterae is 1.6-2.5 mm. Their oviparae are dull green and the young nymphs and the apterous males are reddish orange. The alates (see second picture below) are dark green, and their siphunculi are more or less dusky.
The small willow aphid is fairly common in 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. The species occurs throughout northern temperate parts of the world (North America, Europe & Asia) and in South America (Argentina).
Biology & Ecology:
Despite the near worldwide distribution of Aphis farinosa 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 shoots that has been galled by a gall midge than on ungalled shoots. The gall midge stimulated regrowth responses in S. eriocarpa which, in turn, positively affected the aphid and other phytophagous insects.
This is a dense colony of Aphis farinosa on Sallow (also known as Goat Willow) in mid-June. 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 Sallow Aphid is often, but not always, attended by ants. We have usually found it attended by the Southern Wood Ant (Formica rufa) as shown in here. Lindsey (2007) found it being tended by the related Slavemaker Ant (Formica sanguineae).
Despite ant attendance, Aphis farinosa is often heavily parasitized by braconid parasitoids, especially Lysiphlebus confusus. 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 of 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).
Damage and control