Thrips and Australian Acacia species
Laurence Mound, CSIRO Ecosystems Sciences, Canberra
Introduction
In Australia, the plant genus Acacia includes about 1000 species of shrubs and trees <http://www.worldwidewattle.com/>.
In most of these, the leaf-like foliage of mature plants is actually modified leaf petioles, termed phyllodes, and the normal bipinnate leaves are progressively lost. Thus, among phyllodinous Acacia species, bi-pinnate foliage occurs only on very young plants.
Phyllode-bearing species are classified into three major Sections of the genus Acacia: Phyllodineae with almost 400 species, Juliflorae with about 255 species, and Plurinerves with about 215 species (Maslin, 2001).
- Acacia pubifolia broad phyllodes.jpg
Broad phyllodes of Acacia pubifolia
- Acacia tetragonophylla spiny phyllodes.jpg
Spiny phyllodes of Acacia tetragonophylla
- Acacia clelandi fleshy phyllodes.jpg
Thick fleshy phyllodes of Acacia clelandi
- Acacia doratoxylon slender phyllodes.jpg
Slender leaf-like phyllodes of Acacia doratoxylon
More than 250 thrips species in the Thysanoptera subfamily Phlaeothripinae are now known from the phyllodinous Acacia species of Australia. These apparently all feed on green tissues, but they are restricted largely to the two Sections Juliflorae and Plurinerves. These thrips species, also the genera to which they are assigned, are not known from any other plants, and this is considered to represent a unique host-plant restricted lineage reflecting a single invasion of the genus Acacia (Morris et al. 2002; Crespi et al. 2004).
Four suites of species are recognised among these Acacia Phlaeothripinae of Australia, each exhibiting very different behaviours.
1. The gall-inducers comprise more than 25 species of the single genus Kladothrips. These induce galls on phyllodes of particular Acacia species, usually with a high degree of host specificity.
2. The domicile builders are species of at least seven genera that create their own domiciles (or nests) by fixing two or more pairs of phyllodes together with a secretion from the anus, or even by weaving a tent-like structure from this secretion.
3. The exploiters are species comprising six further genera that behave as kleptoparasites, usurping either a thrips gall or a domicile on Acacia phyllodes.
4. About 50% of the Phlaeothripinae species found on Acacia in Australia represent a diverse suite of opportunistic species that invade and breed in any available space that some other insect has created but abandoned on an Acacia.
Despite their behavioural diversity, all of these Acacia Phlaeothripinae species are phytophagous – the kleptoparasites are not predators. Radiation among the four suites of species appears to have been driven primarily by competition for small enclosed spaces that provide protection from sunlight and dehydration in the extremely arid environment of Australia. Moreover, arid Australia is notable for the abundance of ant and spider species, although the significance of these predators in driving thrips diversity remains unexplored.
