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Giant aphids of challenged trees
Are these rare giants due a comeback?On this page: Cinara kochiana Clethrobius comes Why are these aphids so big?
Natural forest always have a goodly collection of broken &/or overshadowed branches, plus senescing &/or fallen trees. These often persist for many years, in various states of health, dyeback, and untidyness - much to the horror of modern foresters, who expunge them periodically along with much of their specialized flora and fauna. We suggest that some (consequently) rather rare aphids may specialize therein, along with their particular predators and parasitoids. Whilst ecologists have studied the inhabitants of dead trees in great depth, little attention has been paid to 'challenged' trees and treelimbs...
Giant larch aphid (Cinara kochiana) - exploiting damaged trees
Wandering around an East Sussex woodland in July we noticed unusual wood ant activity on the trunk and downward-pointing branches of a fairly young larch tree blown over in a storm several years ago. As often happens, because its fall had been partly arrested by surrounding trees, this larch had not completely fallen, but was resting upon its lower branches. As a result some roots were still in the ground, and the tree was still alive - albeit somewhat challenged - particularly those lower, shaded, supporting branches.
Fighting off rather aggressive wood ants, we investigated further - and found some very large aphids on the lower, older, most challenged branches.
After four years of looking, we had finally found the giant larch aphid (Cinara kochiana)!
The wingless adult female of Cinara kochiana (shown above) is much larger than most aphids, with a body length of up to 6.1 mm. It is greyish-brown to lead-grey or greyish-green, which provides excellent camouflage against the bark. Cinara kochiana has a long rostrum which, when not being used to feed, is held curving under the body.
We also found winged adults (see second picture above). These are destined to disperse from this colony to found new colonies elsewhere. After the storms of last December there are still, presumabaly, some challenged larch trees awaiting a chainsaw.
There have been few recent observations on the giant larch aphid, and we know no detailed studies on its ecology. Past reports state it occurs in bark crevices on the lower part of the trunk or at the bases of older branches, or in midsummer on exposed roots. All the colonies we found were at the bases of the older (lower) branches of a rather young fallen tree in places where the bark (and hence the aphids) were shaded by other vegetation. We found none of these aphids on the main trunk or roots.
Ant attendance has been reported before for the giant larch aphid, and the colonies we found were attended by large numbers of southern wood ants (Formica rufa).
Despite the presence of large numbers of (protective ?) ants, the parasites and predators were making depredations on these colonies. The first picture below shows a colony with a single mummified aphid in the centre, probably caused by a Pauesia braconid wasp. Other workers have also reported parasitism of Cinara kochiana, notably Pontin (1960) who found a heavily parasitized population on larch roots.
Predators were also present, in particular the unusual-looking syrphid larva shown in the second picture above. Since we did not recognise this species, we asked some other entomologists for ideas.
Graham Rotheray (18 Aug 2014) commented: "This is an early stage larva that hasn't developed all the features I need to be certain of its identity. Only three genera have the upright projections along the body shown in the image, Didea, Eriozona and Megasyrphus. But which of these genera the larva belongs to I can't really tell. ..... Simply on the grounds of abundance, it is most likely to be a Didea".
Hairy birch aphid (Clethrobius comes) - another rare aphid on a challenged tree
A very large aphid you are unlikely to come across very often is Clethrobius comes (the hairy birch aphid). The body length of its winged adult is up to 5.6 mm.
Again the only times we have encountered Clethrobius comes, they were on damaged or fallen trees. We found the specimens pictured below on a broken branch of birch in the New Forest in 2011. The winged female is a very light-brown because she has only just moulted to her adult form.
The more usual appearance of these winged adults is shown in the picture below.
This is again a species about which very little has been written. Clethrobius comes is recorded as being rare in the UK, forming clusters on branches and twigs of Birch (Betula spp.), often where the new growth is dying back, or on twigs of Alder (Alnus spp.) overhanging streams. Osiadacz & Wieczorek (2007) noted that this species specifically likes birch and alder trees growing in marshy areas, where incidentally one often finds fallen birch trees.
This colony was not ant attended (although they sometimes are) and they were certainly attracting their fair share of predators. There were numerous syrphid eggs along the twigs bearing these aphid colonies. The syrphid eggs bore numerous minute papillae, the function of which remains a mystery (possibly respiratory).
Why are these aphids so big ?
When one looks at these aphids the immediate feature that one notices is that they are big - for aphids very big! The giant larch aphid, Cinara kochiana, can be more than 6 mm from head to tail. This is not quite as long as Stomaphis quercus, an exceedingly rare UK aphid, which can make 7 mm. Nevertheless Cinara kochiana may well be heavier of the two. The largest species within any taxonomic group tend to be the so-called 'K - strategists' (forget about "all aphids being r-strategists" - they are if you compare them with mammals, but not if you compare different genera of aphids).
So are these challenged-limb specialists the K-strategists of the aphid world ?
Note we are assuming at present that the actual age of the tree is not the most important factor, since that fallen larch was no older than any of the surrounding trees (we would guess perhaps 20-30 years old). But would expect that, left to itself, its 'challenged' lower branches would progressively die back - albeit taking many years to completely do so.
For challenged-limb specialists to be K-strategists, those plant limbs under stress must offer a stable favourable environment. In commercial forestry fallen trees and damaged branches are usually removed - which if we are correct, eliminates most of the aphid K-strategists. Perhaps some support for the idea that Cinara kochiana likes challenged trees comes from Rozhkov & Mikhailova (1993) . They report that Cinara kochiana has been recorded as one of the aphid species damaging larch trees in coniferous forests polluted with fluorides in Eastern Siberia.