Galls

Gall on willow caused by the sawfly Pontania proxima.
Byfield Pocket Park. 23 July, 2013

I like galls - so if you are prepared to be bored, read on.

Galls come in a variety of forms. A swelling on the leaf is perhaps the commonest type with some trees such as oak and willow being afflicted with a great variety, both in form and in the type of organism responsible. Sawflies are may be involved, as shown on the first picture. In fact sawflies are a very tricky group to identify and it is often easier to name them through the galls they form rather than trying to dissect the actual insect.

  

Gall on leaf of Lime tree formed by the mite
Eriophyes exilis.  Byfield playing fields.
20.July, 2013

Among the commonest of gall-forming organisms are mites.  These are a challenging  group of organisms and specialists (acarologists) are few and far between. For the layman noting the position of the gall is often crucial. The next photograph shows the gall of a mite, Eriophyes exilis, on a lime tree; in this instance the fact that the gall is located at the junction of veins is key to the identification. 

A related mite is responsible for the example shown next. It affects leaves of the blackthorn and again the position of the galls is diagnostic. In this case the the gall's positioning on the leaf margin allows it to be identified as Eriophyes similis. 

Eriophyes similis galling blackthorn.
Byfield Pocket Park. 26 July, 2013

Peach Leaf Curl. An example from the gardens at
Northcourt, Isle of Wight. 24 June, 2013 

Many gardeners will be familiar with peach leaf curl. Here the problem is caused by a fungus, Taphrina deformans and, as my picture shows, the distorted, thickened leaves can be quite grotesque. Few plants seem to avoid attacks by fungi, although the consequence is not necessarily as dramatic as in this example, photographed on the Isle of Wight.

Mites, sawflies, fungi - many organisms may cause galls, but among the commonest are diptera. i.e. two-winged flies. The galls may be simple swellings on leaves or stems, but may be more complex and even not obviously gall-like at all. Bunched leaves at the tips of hawthorn branches indicate that Dasineura crataegi has been at work. The result is known as the Hawthorn Button-top Gall. Dasineura species are members of a family of flies called the Cecidomyiidae (Greek: kekis - a gall, and myia - a fly) and attack a wide range of plants. 

Hawthorn Button-top Gall. Byfield Pocket Park
26 July, 2013

The study of galls, known as cecidology, has become increasingly popular in recent years, not only because the curious form of galls gives them interest but many of them such as Gooseberry "big bud" and Peach Leaf Curl do considerable damage to commercially important plants.