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Black bears in the trees

July 16, 2018

Upon our arrival to Chiang Mai and our beloved Dokmai Garden I noticed a multitude of large, black and hairy larvae on the trunks of our trees. They reminded me of black bears or skunks from a Walt Disney movie. The same larvae were observed in great numbers also in the nearby jungle at Opkhan and at a local tree nursery.

During my five years in Thailand I never noticed it before. I asked the manager of the nursery if she knew what they are, but she only used the northern Thai word for a caterpillar in general.

Since I was slightly worried this was a harmful pest, I began watching the larvae. The first observation was that they appeared on all sorts of unrelated tree species which is quite unusual. A tree’s chemical defence is often so advanced insects usually have to specialize in certain genera or families of trees. In this case they thrived on most trees, including e.g. mango, strychnine and golden shower from different plant families.

Secondly, the larvae did not eat leaves, nor wood, still they spent several days on the trunks of trees. A close look revealed they were actually feeding on the trunk, but not damaging the bark.

The anatomy of the larva, i.e. leg arrangement with anal claspers indicated it was a Lepidopteran, a moth. As it turns out, this larva is most likely a lichen moth, probably the diurnal Macrobrochis gigas (Erebidae).

Relieved these bear-looking larvae just feed on lichens, algae and cyanobacteria covering the tree trunks, I now enjoy their presence. The hairs are most likely irritating so I should follow the weaver ants’ precaution and not touch the larvae, but they are cute!

The reason insects some years appear in abundance is due to a fortunate combination of ideal weather and low numbers of natural predators, disease and parasitoids. A peak in population density is usually followed by years of declining numbers due to increasing numbers of natural enemies and disease.

In conclusion, there is no reason to worry. Relax and enjoy this rare moment. Soon they will grow up and fly away.

Eric Danell

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9 Comments leave one →
  1. July 17, 2018 6:13 AM

    They are harmless to touch! I am always fascinated about their speed, when they move to the next tree.

  2. July 17, 2018 7:40 AM

    Thank you for that information Eric. Blessing to you, Bruce BebePhayao Permaculture Center

    • July 17, 2018 12:07 PM

      Hi BeBe – we miss you!

      Permaculture is becoming a widespread philosophy among young Swedes too, even regular gardening schools start teaching this topic. Five years ago it was all about spraying, so things improve quickly! Thank you for kind words and inspiration.

      Cheers, Eric

  3. Annelie permalink
    July 17, 2018 8:24 AM

    Hi Erik
    Thanks for the article. My friends in Doi Saket have their trees full of it. For the first time so far as we know.
    Greetings and have a great summerholiday
    Annelie

  4. Leif Ljungström permalink
    July 17, 2018 12:07 PM

    Björnspinnare? I remember them from my summers on Alvaret, Öland in the 50s. We were told not to tusch them.

    • July 17, 2018 12:15 PM

      Yes, this moth is related to Arctia caja in Sweden (same family), but that species feeds on leaves. We also have lichen moths in Sweden (lavspinnare), but they are smaller than this species.

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