Tuesday 25 August 2015

Horse chestnut leaf miner

Peter’s pests
Horse chestnut leaf miner
At the beginning of July we visited Peter’s family in France. He had two alien invaders. Both now also found in the UK, they are recent arrivals. The cause of greatest concern was that of box caterpillar which I will post on tomorrow.

A contrast between Autumn tints and early Summer growth.

Compared with box caterpillar his horse chestnut leaf miner was a minor distraction! Perhaps it ought not to have been because in the UK it has been the matter of some concern as from first sightings in London in 2002 it has spread almost everywhere. It has been subject of ‘citizen research’ where gardeners and schools have participated in recording its spread.
Please forgive me if I observe that in the UK an alien invader has been attacked by an alien invader. To me the irony is that the horse chestnut is not native to the UK. It was introduced from the Balkans four hundred years ago and of course has spread everywhere. Most people now regard it as native. How long must an immigrant stay to be thought one of our own? Most of you will know that the concept of alien plants is foreign to me!

Horse chestnut premature leaf fall 

On the first of July Peter’s yard looked as if it was Autumn. The ground was covered with golden brown leaves from his two large trees. The trees were almost completely denuded. There will be no conkers this year.

Leaf miners on chestnut are small moth-caterpillars that tunnel under the leaf surface. In some cases thousands have been recorded on a single leaf. The good news from intensive investigation is that they do little long term harm to the tree and each year fresh new leaves appear in Spring. I find it difficult to believe that loss of Summer photosynthesis will do no harm at all. There must be some loss of vigour? Not necessarily a bad thing.
I find it of great interest that some plants such as Solomons’s Seal can be completely defoliated – in that case by sawfly – and suffer zero long term harm. I wonder in terms of natural selection what’s going on!

Autumn in July

Although the pest is primarily one of horse chestnut other members of the same genus aesculus can be infected – but not all. It has been recorded on sycamores growing near to large primary infection on chestnuts but would seem to be of no present concern.

The citizen recordings seem to show that it takes three or four years for an infection to build up to a maximum. If the maximum is as much as that on Peter’s trees that’s a lot.
It is impractical to spray large horse chestnut trees. The citizen research observed that the rate of establishment of natural predators and parasites at the present time is not very significant. Blue tits do make a very small contribution.

The caterpillar tit-bits might be very tasty but bluetits do not provide control

If there are no near horse chestnuts in your neighbourhood it might reduce future infections on your tree if fallen leaves are swept up and burned or covered and composted.

I discuss alien invaders here
About the citizen science horse chestnut project


  1. I have a large horse chestnut tree adjacent to my fence-line Roger, which although it always has quite a bit of leaf discolouration by this point in the season, thankfully, is not showing any signs of leaf miner. I think the answer to the recovery properties of Solomon's Seal probably lies in its rhizomatous root-stock.

    1. Thanks Rick,
      I guess like bulbs Solomon's Seal rapidly build up carbohydrate reserves and by July the leaves have done their job and in their native conditions perhaps even might be shaded. The sawfly may be doing the Solomon's Seal a favour turning tough old leaves to compost!

  2. I like the idea of that Roger.


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