March 2015

The Gall of Nature

We have found a few of these odd looking rocks on our property over the years; they are about the size of a golf ball. They are very hard but quite light what are they?

Answer

Your curious items are actually galls and are the work of an insect in association with a plant. Galls are growths that form on a plant usually in response to injury or attack. Often the cause of the attack or injury is an insect. This particular gall is called a Bush Coconut or Bloodwood Apple and is the protective case of a female scale insect called a coccid Cystococcus pomiformis. The Bush Coconut forms on the branches and stems of the Dessert Bloodwood Corymbia terminalis an Australian native tree and can vary in size from golf ball size to tennis ball size. Usually these galls form in clusters but may end up on the ground if the trees have been felled or due to animal activity. If you were to crack open the gall, inside you would find a white flaky lining protecting the female scale insect coccid. The white flesh is said to taste like coconut. Both the white flesh and the creature inside have been a source of food for Aboriginal people.

The female scale insect coccid injects a chemical into the host plant that causes a gall to grow around her. She will spend her entire life secreted away, sucking on the sap of the tree, growing and moulting.  The only contact she has with the outside world is when a male coccid pokes his abdomen through a tiny air hole in the gall to mate with her. The female will produce male offspring first and as the males reach maturity the mother will produce the females. These minute immature females will cling onto the mature adult males and be carried from the gall through the air hole when the males fly off to find a mate. The immature females find a spot on a suitable tree and start the cycle again. The female scale insect (Cystococcus pomiformis) resembles a grub, as she has no legs or wings or antennae.

Galls can be started by wasps, beetles and flies. In most cases gall types relate to the type of animal that caused them. In Australia our native trees are host to many different types of insect galls. 

Follow the links for more information on these curious insects:

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