Microbial evolution has turned a mild-mannered North American beetle into a tree killer in Asia. The red turpentine beetle, Dendroctonus valens, is a wood-boring species native to North America. D. valens is unremarkable and harmless in its native region. However once it migrated to China, it wiped out more than seven million pines in the past 12 years, and it’s poised to spread throughout much of Asia and Europe.
D. valens stows away in internationally traded timber products, which explains how the species, and 50 other Scolytinae species ended up in China. D. valens takes with it some bacteria, fungi and mites on its body. Once the insect drills into the bark of a tree to lay its eggs, the microorganisms are transferred to the plant’s vascular tissues. The fungus, Leptographium procerum, breaks down the phloem, the innermost layer of bark, and the insects consume both the fungus and the phloem.
D. valens‘ most common fungal partner has evolved new strains to make trees even sicker, and in turn attract more beetles. Jiang-Hua Sun, a biologist at the Institute of Zoology at the Chinese Academy of Sciences et al. published a study in the journal Ecology and worked with samples of L. procerum from China and North America. The Chinese strains are a subset of the ones found in the USA, confirming that the killer fungus stems from America.
However, there are new genotypes in the strains found in China, which are much more damaging than the ones originally found in America. The studied tree seedlings’ aroma even changed, once they were infected with the Chinese variants of the fungus.
The aroma is what attracted even more D. valens to the infected trees. This in turn develops a feedback loop until the trees succumb to the damage from the beetles and fungi. If D. valens manages to return to the US, there’s no telling how many pines will succumb to it.