jimmymac2's picture

Galls

What is the definition of a gall?
I have started to see many new species of gall in my garden like Hedgehog gall, Common Spangle Gall and Silk Button Spangle Gall, but what are they?
When I posted an observation of a Hedgehog Gall (that I spotted doing an OPAL bugs count), beneath it in 'species with which Hedgehog Gall interacts' I saw that it ate Common Oaks and Downy Oaks (among other Oaks), but how does it eat?

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RHoman's picture

Galls

It isn't the gall that does the eating; instead it is the larva of the insect (Andricus lucidus) inside the gall that feeds on material from the oak tree. If you are interested in galls then I recommend that you get hold of a copy of "British Plant Galls" by M Redfern and P Shirley, published by FSC. Therein you will find the clearest definition of a gall - in essence an abnormal growth on a plant induced by the presence of another organism, such as an insect, a mite, a fungus or another plant.

Robert Homan

Joe Botting's picture

gall communities

They are a fascinating subject and well worth looking into more. The ecosystem associated with galls can actually get quite complex - for example, there's a species of flower bug (Anthocoris gallarum-ulmi) that feeds almost exclusively on a particular species of gall-forming aphid on Elm. The gall is a leaf-roll, which lets the bug in, and keeps it hidden from predatory eyes. In other cases, large, untidy galls on trees (e.g. witches' brooms) can offer shelter to a whole menagerie of things that are more vulnerable when out on the open branches.

Have a good look for minute wasps (a couple of millimetres long) around your galls, and you might be able to see the adults of the little grubs that are inside them. They really do repay some patience and a very close look...

jimmymac2's picture

galls

Some galls such as the Silk Button Spangle Gall (which is very common in our garden), are tiny, so wouldn't that not provide very much shelter?
Also, how would an adult insect get the gall onto the leaf, because the galls are usually much bigger than the insect itself?

James
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Joe Botting's picture

Little fleas have lesser fleas...

It all depends how tiny the beasties are in comparison; there are whole communities of microscopic mites, for example, which can potentially thrive in very small areas.

It's important to realise that 'gall' is a very general term, and includes a lot of different forms. All they have in common is that they are induced in the plant by the gall-causer. The insect/fungus/whatever doesn't stick a gall to the leaf - it is the behaviour of the creature that causes the gall in situ. It's rather like scar tissue, or a benign tumor - the gall is a reaction to the presence of the gall-causer. In some cases there is a single larva inside a sealed gall, which continues to grow as the insect does and hence provides a constant supply of food. In other cases, an insect causes the leaves to distort, often rolling in along the edges, or twisting. These leaf-roll galls are not sealed, but still provide good shelter. Does that make more sense?

jimmymac2's picture

Shieldbug

I noticed a Hawthorn Shieldbug really close to a Common Spangle Gall, would a Shieldbug want to eat the insect that's inside the gall, or does the gall have any additional protein the Oak it's on doesn't?

James
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Joe Botting's picture

Coincidence, probably...

That species feeds only on sap (although other shieldbugs are predatory), so it's unlikely to be interested in a gall, I think... although it is certainly possible that there would be a more concentrated supply of some nutrient or other.

Not sure on the answer to that one, but it's worth seeing if you can find it actually feeding on the gall itself. We should probably assume it's coincidence unless you get evidence to the contrary.

jimmymac2's picture

Microscope

I looked at a hedgehog gall (Andricus ludicus)under a microscope (it was really clear, from the natural history museum) where I thought its mouth would be and I saw a small hole that contained tiny, teeth like hairs along the edge that are invisible to the naked eye. Would that be where the insect would feed?

James
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petershirley's picture

Galls

Interesting thread folks and some good comments.

This (cecidology) is a big subject which crosses disciplines (entomology, botany, mycology, ecology, pest control, social history etc.)

We have a society dedicated to the subject - the British Plant Gall Society, http://www.british-galls.org.uk/ New members welcome. The website has details of publications including guides, keys and a comprehensive New Naturalist volume called 'Plant Galls' by Margaret Redfern.

Peter Shirley