MOST MOTHS GOOD, A
FEW MOTHS BAD.
The vast majority of moths are beneficial. Their caterpillars are a vital food source to birds, especially when feeding their young in the spring. Bats will eat the adults and night-flying moths are valuable pollinators. It is only a minority of species which can multiply to invasive proportions sometimes, that become pests or in other ways draw unfavourable attention.
THE BOX MOTH
The shock-horror story of 2018 was the publicity given to
the depredations of the Box Moth caterpillar.This attractive moth, a relatively
recent immigrant from SE.Asia has spread unrelentingly throughout Europe. First
seen in Germany in 2006, since then it has spread rapidly throughout Western Europe. It was first seen in Kent in 2008 and thence from
its rapidly established stronghold in the London area it has spread across
Britain. There are several records in Cornwall, South Wales and it was first seen
in Scotland in 2018.
'Normal' form of the moth. It also has a less usual brown (melanistic) form.
It is about 4cms across from wing-tip to wing-tip.
This moth is a member of the Crambiidae family
which includes the Grass Moths in Britain. Despite its size, it is a ‘micro’
and has several generations of caterpillars in a year. In Asia its host plants
are various species of Buxus and in its natural range it seems to live in
balance with its natural predators. In Europe, efforts to control it, with pheromone traps, chemical sprays,
nematodes, seem to be of little avail. Ironically the (also dreaded, invasive)
Asian Hornet seems to predate the caterpillars and they have been seen actively
searching for them in Box bushes.
The Royal Horticultural Society has sounded the alarm
because this is the second enemy to hit Box. The first is the fungal disease
causing Box Blight. The voracious hairy caterpillars of the Box Moth rapidly eat all the leaves, weakening the bush and if not killing them
leave them vulnerable to other stresses. The RHS is understandably concerned as
Box is such an important element in the design of formal parterres and other
design features in the big gardens of Britain and Europe.
We first saw Box many years ago, growing as an abundant understorey shrub in
the woodlands of SE France in the limestone Vercors region. The hill villages
made much of the shrub, trimming it into various shapes at the roadsides. If
the moth strikes that area, as seems very likely, its character will be altered
to a degree.
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| Trimmed box at roadsides. |
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| Quirky topiary |
And I quote from last year’s trip to the south of France:
“We made
our way out to the valley and stopped for a drink in the shade by the river at
Versols-et-Lapeyre. We at once realized we were enveloped in a positive
blizzard of moths flying across the field and settling on the foliage of a row
of Lime trees growing along the river bank. They were Box Moths. As soon as
they were disturbed they flew up in a cloud, to settle again among the leaves.
It was an extraordinary sight.
Later, as
we drove out of the Sorgues valley and up onto the plateau again, all the Box
bushes at the edge of the woodland beside the road could be picked out because
of their total defoliation. There was no sign of the caterpillars. They had
either moved on or pupated. Up on the top of the plateau, the bushes were
untouched. Either the caterpillars hadn’t got that far, or they cannot survive the
winds and harsh winters up there .”
Totally defoliated Box bushes at the woodland margins.
PINE PROCESSIONARY
On our early excursions in southern Europe in the 70’s we
were mystified by frequent buff-coloured fibrous-looking bags hanging in the
branches of the pine trees. Looking more
closely they seemed to be made of tough felted web, but they contained nothing
but fragments of old pine ‘flowers’ and caterpillar frass or droppings.
We later learned that they were the protective ‘nests’ of
the communal Pine Processionary moth, but it wasn’t until we stayed in the
Algarve in southern Portugal in the winter that we found these nests occupied
by their caterpillars.
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| Webbing nests of Pine Processionary in pine trees. They are about 6" long. |
The caterpillars venture out of their ‘nests’ to feed, mainly at night,
on the pine needles and as they mature they can be seen in a nose-to-tail
procession, going down the tree trunk to the ground in search of a suitable
place to pupate among the needles.
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| This caterpillar is nearly fully grown, about 4 to5 cms long. |
It is at this stage the caterpillars may be picked up by
curious people or investigating dogs and the toxic hairs can cause serious
allergic rashes and reactions in both humans and dogs. And in ‘infestation
years’ the caterpillars can cause serious defoliation in commercial pine
plantations. They are considered pests and measures are taken to warn and
protect both trees and the public.
Widespread throughout Europe, the moth
is a rare immigrant to Britain but there are no records for Cornwall.
The closely related
and rather similar OAK
PROCESSIONARY
is also a
rare immigrant but has now become established in the London area and measures to
control it don’t seem to prevent its spread. It was first recorded in Mawnan
Smith in 1983 and accidentally introduced in London in the mid 2000s. It is
resident in the Channel Islands and throughout Europe.
It defoliates Oak trees, has a similar life story to Pine
Processionary, making its protective nests not in the foliage but on branches
and trunks, and like the Pine Processionary, the caterpillar has toxic hairs which can cause
serious problems for curious children or pets and tree workers.
BROWN-TAIL
This is another moth which in some summers may get publicity
because of its extensive and conspicuous webs in roadside hedges of mostly
hawthorn and blackthorn. The communal caterpillars make the protective web, and
forage beyond it as they get older causing complete defoliation as they
advance. This is another hairy caterpillar, the hairs very liable to be shed
and spreads in the wind, giving toxic reactions. This phenomenon has been
known for centuries and local control
measures have long been put in place but
apparently with little effect.
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| Web of Brown-tail moth caterpillars in hedge. |
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| Brown-tail moth 'playing dead'. |
As well as being widespread in Europe, this attractive moth
with a conspicuous brush of rufous-brown hair on the tip of the abdomen is resident
in Britain. It is most frequent in SE England, mostly near the coast. There are
some records in Cornwall, mostly from the south coastal scrub habitats.
 |
| Brown-tail Moth |
When in their native habitat and subject to their natural predators, moth populations normally exist in equilibrium unless in some years there is a combination of circumstances which leads to a few species multiplying to excessive numbers. In the case of species with toxic hairs this will lead to conflict with people and their pets.
If introduced (usually accidentally by the plant trade) to areas where they are not native and without their natural predators, problems may arise.
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