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Borers and Thai forestry

September 8, 2012

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).

8 Comments leave one →
  1. Dave Rolfe permalink
    September 9, 2012 9:20 PM

    Here in England we rear Cossus cossus on apples and brown bread. They will develop in a single season when reared this way. The down side is they smell to high heaven!

    • September 9, 2012 9:40 PM

      Thanks Dave! This larva does not seem interested in eating the rice flour so I may try some fruit and sawdust.


  2. September 9, 2012 11:10 PM

    Very interesting article Eric. There are some fabulous moths whose caterpillars bore into trees. I hope you’ll share a picture of the moth when it emerges from that massive chrysalis.

    • September 10, 2012 11:00 AM

      The chrysalis I found was empty, but I have been out a couple of nights trying to find flying specimens. One source said they might be diurnal. They do not come to our lights. If I get hold of a moth I shall absolutely take pictures.

      Cheers, Eric

  3. September 10, 2012 5:53 PM

    This article covers so much ground so I’ll just stick to the Teak bark borer. Over ten years ago U.S. post graduate student which considered in her thesis, inter alia, the economic viability of teak planted as a monoculture. She claimed losses of up to 80% of the value of the timber could be lost due to Xyleutes ceramicus.

    Eric’s claim that Teak plantations are new to Chiang Mai may be correct but the same cannot be said for Phrae and Uttaradit where logging has been going on for over decade.She also claimed as I recall that the borer was not a major concern in wild Teak and suggested mixed plantings. However earlier this year while browsing in the RFD library in Bangkok I came across an old report about the Teak logging industry. The comments there refuted the assertion that X. ceramicus was not a problem in natural forests.

    The student thesis also noted a possibly more serious problem, namely severe soil erosion in Teak forests possibly but plantations for sure. I have seen hideous soil loss on moderately sloping land in Phrae where a plantation suffered frequent burning. Even in Nan where I planted Teak together with a mix of other tree species on gently sloping land I have witnessed considerable movement of soil.

    In the wild Teak grows in what Thai foresters call Pa Benjapan (5 species forest) and at its most dominant Teak did not exceed 50% of the big trees in the study of the Mae Yom forest where the present government has announced its intention to build a dam at Kaeng Seur Den which has long been opposed by local folk and conservationists, and which is the most important large remnant of this kind of forest.

    Also, which foresters may easily over look, Pa Benjapan has bamboo species as a major component and although termites do not attack Teak they love bamboo and in the process of consuming dead bamboo move huge volumes of soil. So then arises how does this component of the forest, the bamboo with its dense net of surface roots and associated termites ameliorate or exacerbate soil loss?
    Then if we were considering the best way to grow a mixed Teak plantation we would have to consider the CO2 sequestration issue.

    Mark Emery, ChiangMai’s local bamboophile should be able to fill us in on this score as he is sure to have bought this reference:
    Ecological functions of bamboo forest: Research and Application by
    Zhou Ben-zhi, Fu Mao-yi, Xie Jin-zhong, Yang Xiao-sheng and Li Zheng-cai.

    • September 11, 2012 7:44 AM

      Thanks Ricky for filling in with valuable comments!

      Scandinavia has natural monocultures of Norway spruce and Scots pine. A couple of decades of experience from growing un-natural teak plantations is a short time, if we consider that major outbreaks of devastating insect attacks in Scandinavia happen at that frequency. I refer to death of trees, not damaged trees.

      An interesting remark is that a builder recently told me that Thai teak (from plantations, if legal) has a poor quality due to ‘the many holes in the wood’. I wonder if the ‘good’ wood came from the natural jungles in Laos and Burma? The builder did not know but that would be interesting to explore.

      As far as I know bamboo is a pioneer species and disappears when the canopy closes. Some Dendrocalamus bamboos may quickly reach a height of 20-30 meters and stay competitive for decades, but not centuries. Fires and land slides would create pockets of new growth. I doubt a climax forest of five contain significant numbers of bamboo, but unfortunately there is hardly any virgin forest left in northern Thailand. Comparing with high elevation old growth forests is wrong since they are very different from the lowland forests, which are extinct in their old growth form. The loggers came long before the scientists could make any detailed studies of virgin northern Thai lowland forests.

      Since a teak can become 1500 years old, and based on the many teak and makha (Afzelia xylocarpa) slices of gigantic proportions sold at the Ban Tawai antique street, a virgin forest of five must have been totally different from today’s scrub lands and juvenile plantations. I fear that sight will never come back, but we can always try to improve commercial plantations to lower the pressure on the national parks. With a steady increase of the world population future generations will reconsider the use of today’s national parks. Panta rei.


  4. September 11, 2012 9:58 AM

    In terms of seeing the possible return of Teak forests of old, i.e. recovering the appearance of geriatric virginity long after logging has ceased, the best, and perhaps the only candidate in Thailand is at the Mae Yom National Park and hence the importance of frustrating the government’s dam plans there.
    Returning to the X. ceramicus discussion, I suggest reading this article:
    Tropical Forest Insect Pests: Ecology, Impact, and Management – หน้า340 – ผลการค้นหาของ Google Books
    K. S. S. Nair – 2007 – Nature
    Pest profile Xyleutes ceramicus (Walker) (Lepidoptera: Cossidae) and related
    species Xyleutes ceramicus (Walker) (syn. Xyleutes ceramica, Duomitus …

    According to this X. ceramicus thrives as moisture levels rise in the forest understorey. Thus the frequent burning of Teak forests, while destructive of the soil and injurious to human health may reduce the level of infestation. Also near Bamboo understorey plants disappear which may act likewise.
    I have seen no discussion about insect attacks depending on tree age. If trees can be protected for 30 years, they can put down hole free heartwood beyond the range of the borer should it attack.
    Speaking years ago with Dr Somsuk Sukwong, former Head of RECOFTC he talked about the association of Teak with Bamboo and suggested I study and write about the subject and travel to Burma to see the forests there. That did not happen as I fixed my attention on growing the missing lowland riverine trees. Would someone like to take up Dr Somsak’s suggestion?

  5. Fred Elliott permalink
    February 27, 2013 1:37 PM

    Trying to find any pictures of the chrysalis or cocoon for Aristobia approximator. Trying to find a weakness in their life cycle.

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