A tree urchin
Yesterday I discovered red, sea urchin-like galls in a propeller tree (Dipterocarpus tuberculatus, Dipterocarpaceae) at Dokmai Garden, Chiang Mai, Thailand. I asked Ketsanee’s family if they had any names or use for such galls, but they did not seem to know anything about them.
I googled pictures of ‘Dipterocarpus’ and ‘gall’ but got no hits, which is quite remarkable since Dipterocarpaceae is an important timber family in Asia. When browsing the entire internet for any information I came across an Indian/Japanese publication from 1992 (pdf):
Raman A & Takagi S (1992) Galls induced on Hopea ponga (Dipterocarpaceae) in southern India and the gall-maker belonging to the Beesoniidae (Homoptera:Coccoidea). Insecta matsumurana 47: 1-32.
The authors described a new genus (Mangalorea hopeae) of scale insects causing ‘durian-like galls’ on Hopea. Apart from the different substrate, these galls were 2 cm and brown, not large and red. However, the authors also cited Green (1928) who found structures similar to mine on Dipterocarpus tuberculatus trees, caused by the scale insect Beesonia dipterocarpi (Beesoniidae). I have failed to obtain pictures of Green’s galls, but the informative website Scalenet listed more references. Pictures of the insect can be seen at the USDA Systematic Entomology Laboratory. In the latter website it was stated that the English name for the insects of the family Beesoniidae would be ‘beesoniids’.
Most monsoon gardeners would observe the fruit-like tumours or galls and they would need an English name for them. The English name ‘urchin’ would refer to a hedgehog, and so ‘tree urchins’ would be a suitable name for beesoniid galls, also alluding to sea urchins which they resemble.
An important question is whether the galls are dangerous to the tree? I could see many old galls so it seems this is frequently occurring. From the ground it may look like the galls grow out from the wood, but a closer look reveals they affect the leaf buds. That means they do not seem to cause immediate danger to the wood, although all scale insects may transfer virus. Here at Dokmai Garden we are not going to do anything about them, but show them to keen nature-loving visitors.
Below are the pictures I took this morning:
Female beesoniid nymphs will suck sap from a leaf bud wich will alter the bud’s morphology. The female(s) will be protected inside crevices. This gall is hard and resembles a custard apple (Annona squamosa).
A recently sectioned tree urchin gall. At this early stage there are only young female (nymphs?) of beesoniids. In this picture you can see two brown females to the lower left near the stalk. Unfortunately I have no camera on my dissecting microscope but the females look like brown balloons stuck in their crevices. The rear end is exposed to the outside and that part is pointed and sclerotinized (armoured) to resist a hungry spider. They do exudate sugars like their aphid relatives, and so the gall attracted red weaver ants (Oecophylla smaragdina). It is quite amazing with an insect which has its own fortress, a plate mail and a private cohort of guards.
In this section of the mature red gall you can see the waxy coating of male beesoniids. The whole structure is soft and spongy, now allowing the offspring to leave. Open doors also allowed other arthropods an entrance. Studying the interior of this red tree urchin under the dissecting microscope was like diving with a submarine; all sorts of weird life forms in a strange alien landscape. What also amazes me is the perfection of the gall structure. There is a solid roof held up by twisted soft pillars which also act as food for the male nymphs. This could not be the result of any insect sucking. Maybe the beesoniids exudate special plant hormones? The biochemical/genetical mechanism for this morphogenesis would be so wonderful to understand.
Text and photo: Eric Danell