Ecological strategy of the pea Virgilia w.r.t. wood-eating insects
(writing in progress)
Here are some thoughts about the ecological strategy of Virgilia oroboides.
The main idea here is that Virgilia is more than just an exceptionally soft-wooded, short-lived forest-edge pioneer. It is an example of what trees in general would be like if they did not allocate so much seemingly gratuitous energy to their wood.
Virgilia oroboides can reach 15 m high, so it is certainly a tree. However, it produces wood of such limited density and durability that the plant collapses and rots rapidly, after a short life of only about 15 years (during which it can grow at about 1 m of height per year).
This is extreme in the range of life history strategies, i.e. Virgilia seems designed to grow fast and collapse and rot fast, by virtue of extremely soft wood that is poorly defended chemically. Why are more trees not like this? The question is particularly puzzling considering that the forest at the edge of which Virgilia grows generally has such hard wood (including species of ebony, e.g. Diospyros whyteana). And considering that many pioneering acacias, in both Africa and Australia, are nitrogen-fixing, fast-growing and short-lived but have remarkably hard wood (introduced Acacia saligna on the Cape Flats being a good example and indigenous Acacia karroo likewise).
Please see the thesis on Virgilia by Machingambi, which contains much useful information.
http://scholar.sun.ac.za/handle/10019.1/79902?show=full
https://www.researchgate.net/publication/269578288_Death_of_endemic_Virgilia_oroboides_trees_in_South_Africa_caused_by_Diaporthe_virgiliae_sp_nov
https://bsppjournals.onlinelibrary.wiley.com/doi/full/10.1111/ppa.12341
https://pubmed.ncbi.nlm.nih.gov/24863476/
https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.925.3860&rep=rep1&type=pdf
There is a remarkably diverse list of insects known to consume the wood of Virgilia: a whole suite of ambrosia or bark beetles (weevils belonging to possibly more than one subfamily of Curculionidae), cerambycid beetles, and the wood-boring moth caterpillar of Leto venus (Hepialidae). Then there are nitidulid beetles, which apparently eat the sap of Virgilia but at the same time encourage fungi that go on to rot the wood (I get the impression that these fungi differ from the ones cultured by the weevils).
Possibly at least one species of termite is involved as well, and I wouldn’t rule out carpenter ants (Camponotus) although the latter don’t eat the wood. The beetles at least seem mutualistic at various levels with a suite of wood-rotting fungi, in the case of the weevils to the point of an actual fungus-growing interdependence.
The wood-boring moth larvae in Virgilia are a bit odd relative to their family Hepialidae, in their specificity. Most hepialids do not bore into wood, being more generalised in partly detritivorous diets, and many hepialid moths simply scatter their eggs while flying, rather than choosing specific sites to lay the eggs.
By contrast, the hepialid Leto venus almost seems specialised on Virgilia and specialised on wood-boring, which in a way parallels the remarkable incidence of fungus-growing weevils in Virgilia?
The giant carpenter bee (Xylocopa flavorufa) attends the flowers of Virgilia oroboides, pollinating the plant. Carpenter bees also contribute to the destruction of dead wood by tunnelling to make their nests. However, I have not heard of the giant carpenter bee affecting the wood of its host-plant, Virgilia. The coincidence of the word ‘carpenter’ with Virgilia may not actually mean much in terms of the destruction of the wood of this plant in the case of this bee.
It is interesting that there seem to be no foliage-eating caterpillars on Virgilia. The only butterfly I’ve heard of on this plant is the widespread (cosmopolitan) lycaenid Lampides boeticus (‘lucerne blue’), which attacks Virgilia along with many ruderal and domestic legumes, usually herbaceous, such as lucerne. Furthermore, this small lepidopteran attacks only the flowers, not the leaves.
I would like to know whether the foliage of Virgilia is affected by aphids and the ants that tend aphids, but I suspect not. I wonder if Virgilia has extrafloral nectaries that cause ants to patrol its foliage and to exclude herbivorous insects.
The seeds of Virgilia look like candidates for myrmecochory, but I do not know if they possess an elaisome. I suspect not, and that ants are not involved in dispersing and sowing Virgilia.
What emerges is a plant that is seemingly excludes insects from its foliage, allowing to grow exceptionally rapidly after disturbance (including fire) at the edge of the Afromontane forests of South Africa. After a brief life of rapid growth, the plant naturally dies. Its wood is soft and palatable. It is extremely rapidly broken down by
- a consortium of diverse insects (probably excluding carpenter bees despite the conspicuous association between Virgilia and a giant carpenter bee), and
- diverse fungi including a ‘fungus-growing’ community, somewhat analogous with fungus-growing termites in the tropics.
The bottom line:
What is extreme about Virgilia is the degree to which its foliage is free from herbivorous (?and sap-sucking?) insects, relative to the degree to which its wood is riddled by insects as natural participants in a live fast-die young strategy of growth. Virgilia is more than just one of the shortest-lived and softest-wooded trees of its size; it’s a particularly good example of how a plant can allocate only just enough energy to its woody support system to suffice, not wasting any energy on a woody stem system so durable that it remains years or centuries after the death of the tree.
Virgilia is what it would look like were all trees to make their wood available to wood-eaters immediately after the death of the plant: there would be no dead wood lying around, would there?
This highlights the question of why any tree would allocate lots of energy to dense, durable, insect- and fungi-resistant wood, thus sequestering resources in what would appear to be a gratuitous way.
(writing in progress)
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