Sunday, 12 October 2008

Wattle Apple-gall Wasp

.....Trichilogaster sp.

Apple-galls in the inflorescences of Acacias are caused by wasps which lay there eggs in the young flower buds. These wasps belong to the genus Trichilogaster of the family Pteromalidae. Some species are known to lay up to 400 eggs. When they do so, they also inject chemicals which cause the woody growth known as a gall. The wasp grubs then feed on this plant material from within. Eventually they will pupate and then eat their way out. The adult females are short lived surviving just 2 - 3 days.

As you may imagine, there is potential here for a lot of stress and damage to the tree as well as a reduction in it's ability to set seed. For this reason one species, T. acaciaelongifoliae, has been introduced to South Africa as a biological control agent for Acacia longifolia. So, you may wonder why our own Acacias are not being wiped out. It's because we have something the South Africans don't want. There are a range of parasitic wasps that keep the gall wasp population in check. These lay there eggs in the galls and there grubs feed on the gall wasp grubs. OK so what stops the parasites from wiping out the gall wasps? Simple - we have a range of hyperparasites which keep the parasitic wasps in check. In fact, in one study, "twenty-one insect species were reared from galls formed by Trichilogaster acaciaelongifoliae on Acacia secies in Tasmania".

There are at least 10 Trichilogaster species in Australia. From what I've read, T. trilineata is the most likely species to seek out Acacia dealbata an so it is the most likely candidate for the galls below which I photographed at Latrobe the other day. You'll notice in the photos that the plant is making a pretty good effort at flowering despite the galls but they will not set seed.


(Click on Photos to Enlarge)

Wattle Apple-galls on Acacia dealbata


Wattle Apple-galls on Acacia dealbata



References:
  • Bashford, R. 2004, The insects associated with galls formed by Trichilogaster acaciaelongifoliae (Froggatt) (Hymenoptera: Pteromalidae) on Acacia species in Tasmania. - Australian Entomologist, 2004 (Vol. 31) (No. 1) 5-12 - Entomological Society of Qld


6 comments:

  1. Hi Mosura

    Very interesting post. I have never seen galls formed in the flowers (or never noticed them). I did once split open a gall in a leaf of an acacia, and got a very nice photo of a sumewhat surprised grub.
    From what you say, it stood a good chance of being parasitised anyway.
    Amazing statisitics on the number of parasitic species.
    Denis

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  2. New one on me Alan, great pictures.

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  3. Thanks Denis and Duncan - Fascinating things aren't they but at the same time it's so easy to dismiss such things without realising the complex interconnections that are going on around us.

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  4. You are truely an artist

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  5. Interesting post mosura. Just goes to show what goes on around us without us even knowing.

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  6. Nice one Alan. Very informative, and some new words for me, ... 'hyperparasite', etc. Great post.
    Gouldiae

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