Borers and Thai forestry
Earlier today I observed a chrysalis protruding from the wood of a pink shower tree (Cassia bakeriana, Fabaceae) at Dokmai Garden.
I pulled it, and to my great surprise the chrysalis was enormous:
This chrysalis is ten centimeters long and 18 mm at its broadest abdominal segment. The tree stem was 14 cm in diameter at the point I pulled out the chrysalis.
The wings and proboscis are clearly visible so this is likely to be a moth. The gigantic size suggests the family Cossidae. A quick look at Moths of Borneo gives candidates such as Xyleutes persona which is reported from the genus Cassia (and other genera) from India to New Guinea. X. strix is another large cossid.
Since we know that teak (Tectona grandis, Lamiaceae) may be attacked by Xyleutes ceramicus (Pholvica et al. 1992. Thai Journal of Forestry 11:8-15), I am worried that a teak monoculture may become economically adventurous. In nature, teak never grows in pure stands, but is mixed with a range of other forest trees. A monoculture may nurse incredible numbers of pests which may attack adjacent plantations. Since the borers reside inside the tree, there is not much a land owner can do when the attack is evident. Prophylactic sprays of the mild deterrent permethrin (originally from Chrysanthemum or Tanacetum) in the rainy season may prevent an attack, but according to the article mentioned above, ants kill 18 % of the moth’s eggs and another 80 % of the young moth larvae before they bore into the wood. The wrong pesticide used at the wrong time may remove the ants but not the pests.
My greatest fear is that since teak monocultures are essentially untested, they might be economically high risk ventures. Most teak plantations you see around Chiang Mai are from 1989 or younger. Only 1% of the Thai forests that were logged during the past century have been actively planted again. A mix with other economically valuable but unrelated hard wood species (e.g. Diospyros spp, Hopea odorata, Shorea spp, Xylia xylocarpa, Afzelia xylocarpa, Dahlbergia spp) would lower the risks for the land owner and also benefit biodiversity.
As can be seen from the top picture, the chrysalis is found near a pruning cut. A Swedish website, ‘Forest Damage‘, claims that the female of the related Swedish moth Cossus cossus (‘trädödare’ or ‘tree devastator’ in Swedish) prefers to lay her eggs near wood damaged from fire or machines such as mowers. If this is the case also with our Thai Xyleutes spp., then I should recommend that pruning in the rainy season should be avoided not only because of higher concentrations of fungal spores, but also due to the activities of these borers. Although owners of teak plantations may not care about air pollution or other people’s health and therefore often use fire to manage weeds, they may consider that fire may result in increased damage due to borers.
This sheath of frass, sawdust mixed with faeces in a silk tube, is commonly seen on stems of pink shower trees and timber trees of the legume (Fabaceae) family. Note the recent chewing of the bark and the position near a former damage to the tree.
In the night of the 9th of September I found this larva inside the frass sheath. It is a Lepidopteran (moth) due to its four pairs of prolegs in the middle of the abdomen and an anal pair of claspers. If this is a young instar (only 2.5 cm) of the Xyleutes or not is still an open question, although its resemblance with European Cossus hints this indeed may be a cossid larva. The Romans claimed that Cossus larvae fed with flour are delicious, so in order to identify the species I currently try to feed this baby jasmine rice flour and I keep its plastic box dark. Since some cossid species may take many years to develop into adults, we may have to wait for a definite identification.
For the home gardener, the presence of frass on your favourite Cassia should inspire you to make a night excursion to mechanically kill the larva. If there is a hole, use a metal wire to kill the larva inside. Ants and woodpeckers are your allies. I am sure there are parasitoid wasps and flies helping you too. When I reared a Cossus cossus larva in my childhood, it turned out to be parasitized by a fly. I mentioned it in disappointment to a university entomologist who remarked that parasitic fly has only been recorded a couple of times in Europe. My flies are kept at Uppsala University, but I think that parasitoid fly is just overlooked. People tend to spend their valuable time on other things than rearing larvae.
Other borers found within Dokmai Garden are the longhorn beetles Aristobia approximator (picture) and A. horridula (Cerambycidae). In general, saplings of timber species of the bean family (Fabaceae) seem very susceptible to borers. The mahogany relative Toona ciliata (Meliaceae) is also frequently attacked by borers (the cedar tip moth: Hypsipyla robusta, Pyralidae, Lepidoptera). It was recently discovered that saplings which develop in the shade have a much reduced incidence of infestation, compared to saplings planted in full sun (Sakchoowong et al. 2008. Kasetsart J. Nat. Sci. 42:435-443). When a Dokmai Garden specimen was under attack I cleared the galleries using a metal wire, feeling like a chimney sweep.
With over one thousand native tree species we have a lot to learn about northern Thai forestry.
Text & Photo: Eric Danell
(Precipitation report: yesterday on the 7th of September we received 32 mm of rain).