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Horse chestnut miner and blue tits

Posted: Friday 11 October 2013
by Kate Bradbury

I recently attended a seminar on native plants. I was one of the speakers, arguing that growing some natives in the garden is essential for creating breeding habitats for moths.


I recently attended a seminar on native plants. I was one of the speakers, arguing that growing some natives in the garden is essential for creating breeding habitats for moths (and therefore caterpillars for hedgehogs, birds and other predators to eat). Other delegates advocated the value of non-natives to wildlife. Few of them mentioned moths, however, but one speaker who did was Kew’s tree expert, Tony Kirkham.

The moth in question was the horse chestnut leaf miner, Cameraria ohridella. In stark contrast to the picture I was painting of lovely moths causing barely any damage to garden plants, the horse chestnut leaf miner was said to be doing just that.

It arrived in the UK in 2002 and has since spread throughout most of England and Wales. It’s not the moth itself that causes damage, but its caterpillars, which ‘mine’ hollow tunnels through horse chestnut leaves, causing unsightly brown blotches. Mined trees apparently produce smaller conkers, and normally by late summer the brown patches are so numerous that the trees shed their leaves early (they seemed late this year - did anyone notice?).

Tony Kirkham told us that some more exotic varieties of horse chestnut are less appetising to leaf miners, and so we should consider growing them instead (we tend to think of ‘our’ horse chestnut, Aesculus hippocastanum, as native, but it was actually introduced in the 16th century from south-east Europe).

But the horse chestnut leaf miner might not be as bad as it seems. A recent 10-year study conducted by Forest Research suggests the moth may not cause long-term damage at all. Rather, bleeding canker is the biggest threat to the future of our conkers.

What’s more, the moth has a new enemy: the blue tit. Just as it learned to steal the cream from our milk bottles in the 1980s, and has discovered that solitary bee hotels are packed with tasty grubs to eat, so the blue tit has found that horse chestnut trees are absolutely loaded with caterpillars. It’s taking full advantage.

Caterpillars are an important food source for blue tits, which feed them to their young. It’s estimated that blue tit chicks eat 35 billion caterpillars a year. It’s not yet clear how many horse chestnut leaf miner caterpillars are being eaten by the birds, but if there’s potential for them to make a dent in their population while feeding their young, there’s hope for our conkers yet.

You can help scientists find out how effective the blue tit is in controlling the leaf miner by learning to spot the signs of blue tit predation on horse chestnut leaves and taking part in the Bird Attack! Survey.



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oldchippy 11/10/2013 at 10:38

Hi Kate the chestnut trees in the park had lost most of there leaves by the end of September, you could see clear blue skies through the branches ,I have one growing in a pot in the garden and the leaves have just started to turn brown,maybe the moths haven't found it yet it's about 15 years old,Oldchippy.

Richard Jones 22/10/2013 at 09:13

Just from personal observation, South London horse chestnuts seem to be less affected by the leaf-miners this year than previously. I wonder whether the cool weather in spring delayed them, or leaf-growth, and that by the time everything had caught up leaves were browning and falling anyway.