Wednesday 12 May 2010

Oak Processionary Moth

Visiting Kew for a meeting today I was surprised to find a large tract of the grounds fenced off - enquiry soon confirmed my suspicion that this was due to attempts to control the Oak Processionary Moth, Thaumetopaea processionea. This insect, which is native to southern Europe, appeared in west London in 2006, apparently having been brought in on oak trees imported from southern Europe. Nests of hairy caterpillars are produced; as their name suggests they can travel from tree to tree nose to tail in long processions (pic from Wikipedia). They are capable of defoliating oak trees, if present in sufficient numbers. Although an unattractive sight this is not disastrous for large trees, but apart from the assault on our national tree the big problem comes from their effect on human health. The hairs are intensely irritant and can cause severe rashes on the skin, that are painful and take weeks to diminish: last year Tony Kirkham, Head of the Arboretum at Kew, showed me the result of his encounter with a caterpillar. It looked a lot worse in the flesh than it does in the picture. A further danger is that the hairs can detach from the caterpillars and be inhaled, inflaming the soft tissues of the nose, throat and lungs and causing all sorts of problems. The caterpillars are only dangerous for a few weeks in spring, and only attack oaks of sections Robur and Cerris, the white and Turkey oaks.

To try to prevent this pest spreading, there is a concerted effort in west London this year to try to eradicate it. This means that the authorities have had to difficult decision to spray all appropriate oaks at Kew and elsewhere nearby in West London, including Richmond Park, with insecticide. Not good on trees renowned for the biodiversity they support, but if it is successful then it is a price well worth paying.

The bottom picture shows the British Champion Quercus castaneifolia, a magnificent tree measured at 31 m in height in 2001, planted in 1846. A native of the Caucasus and a member of Section Cerris, it is potentially subject to attack by the Oak Processionary Moth.

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