Galls on Aspen - A first look
Countless species from many disparate classes of organisms use living or dead trees for food or shelter. Some have taken the process a step further and have evolved means of inducing their hosts to co-operate by providing enclosed safe havens, which also double up as food supplies.
These extra growths on leaf, twig or stem are known as plant galls, and are of such a standard pattern, according to the host tree and inducing organism, that they can frequently be used to identify the organism living inside them. I have to use the term organism because these gall causers include everything from simple viruses to insects with complex life histories. Cecidology is the study of these growths and their occupants.
Just how these growths are initiated and produced is poorly understood. There is almost certainly no standard mechanism for all gall forms. With some, a simple wound may trigger the host plant's response. More generally, it is believed, an active material is introduced into the host, which stimulates the production of extra tissue, whilst at the same time regulating its ultimate form.
Galls appear in many contrasting forms, as the following few examples indicate. Merely recognising them in the field is fascinating in itself. Taking the next step and recording gall types and locations adds to the sum total of our knowledge, especially as so little of the UK has been systematically studied.
Wherever birches grow, some of the trees are likely to contain witches' brooms, which are dense bunches of short shoots with small leaves. These are often assumed to be rather untidy, derelict birds' nests, but the direct causer is now known to be a micro-fungus, Taphrina betulina. Because the fungus feeds on the extra growth put on by the tree, witches' brooms are classified as galls, and they do not seriously harm the tree. Galls which are induced by micro-fungi are rarely this obvious, and considerable experience is necessary to find many of them.
The best known plant gall is possibly the hard, spherical growth on oak buds, which is known as the marble gall. This is induced by the gall-wasp Andricus kollari. The rightful occupant is a single grub but other tiny wasps may utilise the extra tissue by laying their eggs in its protective walls. The offspring which hatch from these eggs are known as inquilines, or lodgers. Occupant and inquilines are subject to parasitisation by another tier of wasps. The result is that an old gall may exhibit numerous exit holes where the mature adults have emerged. My record count is 36 exit holes!
Galls such as these are easily recognised, but others are more inconspicuous. Oak trees bear the greatest diversity of different galls, and adding the fact that many of the gall-wasps responsible for these have extraordinary life cycles, the reason so many plant gallers spend most of their time looking at oaks is clear. Yet while the oak may be king, other trees provide exciting challenges and bear galls induced by species which are currently little understood.
Aspen is a prime example. Well-established stands of this tree are hard to find in the south of the UK, which is where most studies of plant galls have been carried out. We can reasonably assume that fewer and sparser hosts means there are far fewer gall inducers than where aspen is more common, such as in the Highlands of Scotland.
Gall mites (members of the Eriophyidae) are frequent gall causers. One of them, Aceria populi, is responsible for probably the most obvious gall on aspen in Scotland. It is a lumpy, warty growth about 10 mm. across, starting from a bud and going on to envelope the twig. It begins as a soft, downy mass, and goes through shades of yellow or red until finally becoming dark-coloured and woody.
Gall-causing mites are quite unlike the free moving pests abhorred by gardeners. They are tiny creatures, merely fractions of a millimetre long. Even the largest species needs a considerable degree of magnification to be viewed. Some species roll the edges of leaves, while some induce felty patches on the upper or lower leaf surface, Others live inside raised pustules arising from the leaf.
The felty patch, or erineum (to use its technical name), found on aspen leaves is induced by the mite Phyllocoptes populi. The gall can be found by looking for raised yellow bulges on the upper leaf surface. The mites live amongst the multitude of pale hairs which make up the felty patch on the underside of the leaf. Later in the season the hairs turn reddish and then brown.
A beetle, Saperda populnea, creates a large gall in both willow (Salix spp.) and poplar twigs, including aspen. This is an elongated swelling of about 20 mm. The single beetle larva pupates inside the gall and emerges the following spring, leaving a distinctive exit hole.
Many galls on aspen are induced by midges. Some of these consist of globular growths on the leaf blade. The genus Harmandiola (previously known as Harmandia) is responsible for many spherical growths growing under or through the leaf. The differences between the various galls are not always easy to see, and experience is necessary to provide watertight identifications of the species responsible for each type of gall.
The latest published plant gall key describes five such gall inducers belonging to the genus Harmandiola, which are associated with aspen trees in the UK. A similar thick-walled gall some 3 to 5 mm. across, is induced by the gall-midge Lasioptera populnea. The structure is slightly conical above the leaf blade and rounded below. The resident larva is orange.
Rounded 1 - 4 mm. swellings at the base of an aspen leaf where it joins the petiole may be mistaken for one of these galls. However, in this case the gall is induced by the gall-mite Eriophyes diversipunctatus.
If the swellings are larger (up to 8 mm. in size) pear-shaped and actually on the petiole, the causer is probably the midge Contarinia petioli (previously known as Syndiplosis petioli). Occasionally a number of eggs are laid close together in the midrib or the upper end of the petiole, and the resultant galls coalesce. Familiarity with the basic pear-shaped structure aids in the identification of these multiple growths.
Since the founding of the British Plant Gall Society in 1985, Cecidology has made enormous strides. However, we are constantly having to remind ourselves that the more we learn, the more we discover how little we really know. The galls on aspen are a case in point. As is illustrated above, identification is frequently far from certain and the actual distribution of the causative agents little understood. More eyes in the field must surely bring in much information and every bit of help enables us to fill some of the gaps!
Rex Hancy
Rex Hancy has been East Anglian Regional Co-ordinator of the British
Plant Gall Society since its founding in 1985. His guide, The Study of Plant Galls in Norfolk, was published
in 2001 by the Norfolk and Norwich Naturalists' Society.
Related pages
