Peter’s pests
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!
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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.
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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
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.
ReplyDeleteThanks Rick,
DeleteI 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!
I like the idea of that Roger.
ReplyDelete