Leaf-mining moths going conkers…

C. ohridella distribution records in 2010. Red
dot indicates original site of infestation. (Forestry Commission)

While strolling around the campus grounds, have you been wondering what’s happening to our horse chestnut (Aesculus hippocastanum L., Sapindaceae) trees? Are those brown leaves really showing signs of an early autumn? These leaves are actually changing colour for a very different reason; our horse chestnuts are under attack from the horse-chestnut leaf miner (Cameraria ohridella), a highly invasive moth.

C. ohridella first established in the UK around Wimbledon (in 2002), and is thought to be spreading at around 25 miles per year. The moth is now present across much of England, extending some 190-230 km from the site of the original infestation…

The burrowing leaf-mines of C. ohridella on an A. hippocastanum leaf(let).
(C) Waheed Arshad 2013

The larva of this moth feeds in a tunneled mine between the leaf’s upper and lower epidermal layers, damaging tissue and reducing the leaf’s photosynthetic capabilities. The attack is relentless; hundreds of larvae rampage through the leaves and cover them with brown scars. The leaves wilt, drop off, and the cycle is repeated several times in one growing season.

But what does this mean for the horse chestnut’s future? Nurseries’ sales of saplings have reduced by around 97% in the last decade, so the prognosis seems bleak. Our tree species could well be at risk from vanishing altogether, but there is a potential saviour – a native hero defending our landscape from attack… the blue tit (Cyanistes caeruleus). They peel open the leaf mine to extract the pupa inside, so the voracious caterpillar-eating diet of the blue tit is in luck, and passing on this feeding behaviour to future generations may provide some pest control of our beloved trees.

Severely damaged leaves shrivel by late summer, abscising well before normal leaf-fall. (C) Waheed Arshad 2013

Although damage from C. ohridella is highly disfiguring, the larvae do not kill the tree, even after many cycles of infection. They could, however, increase the tree’s vulnerability to other stresses (e.g. prolonged summer drought). There are currently no effective control measures available, but recent work has involved the development of an integrated pest management approach: pheromone traps. The idea is to trap the adult moths and drastically reduce their population, leaving females unfertilised and fewer eggs which hatch into damage-causing larvae.

These magnificent giants are as iconic as the game they created, but is that game now falling out of favour being horse chestnuts are losing their looks…?