Nature Features

The Horse Chestnut Leafminer (Cameraria ohridella) has been confirmed in Guernsey for the first time. This small moth mainly attacks white flowered Horse Chestnut trees, with its larvae tunneling through the trees leaves.

Heavy infestations can lead to premature defoliation which over successive seasons can severely weaken trees. 

According to Nigel Clark, the islands Plant Health Inspector, the arrival of this pest was not entirely unexpected:

"This pest was first recorded in Macedonia back in 1985, since this time it has been spreading rapidly through Europe, arriving in Northern France in 2002. It was only a matter of time before we had our first finding."

The marks of Leafminer infestation.

Horse Chestnut leaves are also susceptible to fungal infections. Leafmines can be distinguished from these infections by holding leaves up to the light. Leafmines appear translucent with specks of frass or larva present. Fungal lesions in contrast are brown.

However, it should be noted that late in the season heavily mined leaves may dry out, shrivel and may also turn brown.

With further introductions likely and the impracticability of using pesticides on mature trees there will be no attempt to eradicate this pest.

Islanders are encouraged to contact Plant Health on 234567 if they spot leaf mines on Horse Chestnut trees so that the spread of this pest can be monitored and gardeners can be advised of practical measures that can be taken. 

The moths overwinter as pupae in fallen leaves so the best method for gardeners to control any infestation would be to collect fallen leaves and dispose of them by burning or composting. 

Facts, Figures and More Information

• The Horse Chestnut is an important amenity tree, popular with school children keen to indulge in the playground game of conkers.

• In Western Europe three generations of the moth per year can be expected. Adults emerge from April.  The adults are up to 5mm long. The forewings have brown and white stripes and the hindwings have long fringes. 

• Eggs are laid from May through until August. Each female moth can lay 20-40 eggs.  The larvae that emerge from the eggs complete their development in around 4 weeks before forming into pupae. Adults can emerge from pupae after 2 weeks to start the cycle again although pupae can remain dormant for 6-7 months allowing them to overwinter.

• The adult moths are able to disperse by flying although the greatest risk of dispersal is as pupae in infected leaves moved on vehicles. This has caused significant problems in urban areas in mainland Europe where the Horse Chestnut is a popular amenity tree.

• Around 15 species of natural enemies (mainly parasitic wasps) have been identified. However, only low levels of parasitism and predation within established populations have been reported.

• The Sycamore tree is an alternate host for this pest.