Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Giant willow aphidIdentification & Distribution Biology & Ecology Damage & Control
Identification & Distribution:
Tuberolachnus salignus are very large aphids with a body length of 5.0-5.8 mm. Apterae are mid-brown to dark brown with several rows of black sclerotic patches. The body is covered with numerous fine hairs, which give a greyish-golden sheen to the abdomen. There is a large dark brown tubercle in the centre of the back, just in front of the siphunculi which are on large dark cones. The antennae are less than half the body length. Alates have the forewing membrane unpigmented but the pterostigma and costal margin are dark brown.
The giant willow aphid lives on the stems and branches of numerous willows & sallows (Salix spp.) and is also very occasionally recorded from poplar (Populus). Its distribution is almost cosmopolitan wherever willows are grown.
The first image shows several apterae in the midst of a large colony on a young branch of white willow (Salix alba). Note the back of each aphid bears a large dark brown tubercle, the function of which is unknown (Heie, 2009 ). The second image shows an alate Tuberolachnus salignus - it also has the characteristic dorsal tubercle. Unlike Lachnus aphids (to which it is closely related), the forewing membrane is unpigmented.
Biology & Ecology:
Tuberolachnus salignus is anholocyclic and no males have ever been found, so all reproduction is parthenogenetic. The females produce live young which are all genetically identical (clones). The species is strongly aggregative and can build up to very large colonies in late summer. Density is the main factor affecting whether a nymph develops into an aptera or alate, although temperature may also have an effect.
Hargreaves & Llewellyn (1978) showed that the degree of crowding also affects the number of young produced. The number of young nymphs produced reaches a maximum at moderate degrees of crowding, but declined in both uncrowded and densely crowded situations. Collins (2001) notes the colonies can cover much of the 1-3 year old stem surface of a tree. Much like Lachnus species, they wave their hind legs in the air if disturbed by a predator. This behaviour may be synchronised over large areas of the colony. Large Tuberolachnus salignus colonies attract numerous honeydew-feeding insects.
Colonies of nymphs with the founding alate(s) are usually ant attended. The two images above show jet black ants (Lasius fuliginosus) attending such colonies on white willow, the first record as far as we know of this ant species attending Tuberolachnus.
Southern wood ants (Formica rufa) also attend (and vigorously defend) colonies of nymphs with the founding alate. These pictures show ant-attended Tuberolachnus salignus colonies on sallow (Salix cinerea) rather than Willow. The alate has an unusually long postreproductive life (Collins & Leather, 2001 ) and remains with her nymphs long after they have been born. Dixon & Wratten (1971) have suggested that aggregative aphids benefit their clonal siblings by improving the nutritional qualities of the host through the 'sink effect' which may explain the long post-reproductive life of Tuberolachnus.
The image below shows several southern wood ants tending a large colony of giant willow aphid on Sallow in October.
Despite the presence of large quantities of honeydew, colonies are not always ant attended. Collins (2011) reports that the aphids may actively repel ants. Many years ago, Buckton (1876) recorded the behaviour of aphids if there were no aphids to remove honeydew droplets. He reports that "if [the droplet] is not quickly withdrawn by an attendant ant ... it is projected by a peculiar jerk to a considerable distance". This ejected honeydew then attracts a different group of honeydew feeders.
These two images show social wasps gleaning (collecting) the aphid honeydew from leaves beneath a Tuberolachnus salignus colony. Unlike ants they do not collect the honeydew directly from the aphids. However, there is some evidence that they deter predators (Letourneau & Choe, 1978 ), which would make this a mutually beneficial relationship, much as that between ants and aphids. Certainly the swarm of wasps around some colonies would dissuade many mammals from approaching.
No parasitoid of the species has been recorded in Europe. In Japan, Tuberolachnus salignus is parasitized by the braconid Aphidius salignae and by a specific hyperparasitoid Pauesia salignae, which might indicate that this is where it is endemic. Predators of this species are rarely recorded, not suprisingly because they appear to have aposematic colouration, perhaps to indicate they are distasteful. Hence our interest in the photograph below. Our thanks to Dr Wagner for submitting this.
Guest image, Copyright Dr Wagner, all rights reserved
This is not the first report of Harlequin ladybirds predating giant willow aphids (see also Keith Edkins ) so it seems likely that this predator at least finds this species palatable.
Then there is the 'Giant Willow Aphid Mystery'. Tuberolachnus aphids have only been found on willow trees from July through to early March. Numbers tend to peak in October, but by February few are left on the trees and none has been seen from April to July. They are said to be very active in January and February when they often walk around off the host. (Natural History Museum, 2011 ). Perhaps they go into diapause in spring, living in bark crevices before re-emerging in July to feed on willow.
Damage and controlCollins et al. (2001a) demonstrated that Tuberolachnus salignus markedly reduced the above and below ground growth of willow trees both during and after infestation. Infestations also reduced the survival of infested trees. This is serious problem where willow is being grown for fuel, and also for ornamental willow varieties.
The most viable control strategy is to select willow varieties for pest resistance. Collins et al. (2001b) showed that reproduction rates of T. salignus differ between willow varieties and species. Aradottir (2009) has since demonstrated host preferences by olfactometry studies. These results were supported by subsequent field studies.