Are moths killing Horse Chestnut trees?

Horse Chestnut Leaf Miner

Micro-moths tend not to attract much attention from the general public, apart from the often overblown hysterics about clothes moths that regularly appear in the media. But in recent years few will have failed to notice the impact of the Horse Chestnut Leaf Miner Cameraria ohridella, a tiny moth (wingspan of <1cm) arrived in Britain in 2002 and has spread rapidly to almost all parts of England and Wales. 

The caterpillars of this moth live in brown blotchy mines they hollow out inside the leaves of white-flowered Horse Chestnut trees and can occur in spectacular, or perhaps alarming, abundance. The moth has two or more broods each year leading to increasing damage to leaves during the tree’s growing season. Typically, in recent years, Horse Chestnut trees occupied by the moth tend to appear brown by late summer and leaves start to fall as if autumn has come early. This is not the natural ageing of the leaves but the brown damage of successive generations of caterpillar miners. 

Understandably such visually dramatic damage has generated a lot of concern. Horse Chestnut, though not a native UK tree, is an important tree in many landscapes across the country, especially in urban and suburban areas. It is also important culturally, of course, to umpteen generations of conker-wielding children. It is a natural reaction to assume that this annual damage must be harming, perhaps even killing, the affected trees.

However, until recently there was little evidence to back up this assumption.

New studies conducted by Forest Research has addressed exactly this question. In a 10-year study in southern England, the scientists assessed the impact of the Horse Chestnut Leaf Miner on the general health and growth rate of trees. Although in extreme cases up to 75% of the leaf area of individual trees was damaged by larval mines, on average this figure was around 20% by September. In spite of this damage, the presence of the moth had no impact on the growth rate or general condition of studied trees.

In contrast, a bacterial disease called Bleeding Canker Disease, was found to have infected almost half of the white-flowered Horse Chestnut trees in the study and to have led to the death or removal (for health and safety reasons) of 11% of the trees. Surviving infected trees showed decreased growth rates.

Importantly, the study also investigated potential interactions between the two organisms. Even if the Horse Chestnut Leaf Miner wasn’t causing the death of trees itself, it might be rendering them more susceptible to infection by or more likely to succumb to Bleeding Canker Disease - but no evidence for any of these effects was found. In contrast, trees with higher levels of leaf mines had a lower incidence of the disease, though the researchers believe this is a coincidence due to greater densities of the moth on larger trees and higher prevalence of Bleeding Canker Disease in smaller trees.

So, despite the undoubted visual damage caused by C.ohridella there is no evidence that it is harming Horse Chestnut trees. The real danger to the future of this tree in the UK is the Bleeding Canker Disease.

Interestingly, my personal observations this year are of far lower levels of damage to Horse Chestnut leaves from the moth. The mature tree opposite my house is still gloriously green with conkers forming well, whereas in past years it would be looking decidedly autumnal by now. Perhaps the moth’s populations were knocked back by the long, cold winter or were late emerging resulting in less damage than usual for the time of year. I’d be interested to know if others have noticed the same effect this summer. 

Richard Fox

Surveys Manager

Follow me on Twitter: @RichardFoxBC