Can bird attack save conker trees from alien moths?

At the end of last month, researchers from the NERC Centre for Ecology & Hydrology and the University of Hull appealed for volunteers to help in their latest citizen science-based study into the effects of an alien moth species on our horse chestnut trees, which are perhaps better known to many as conker trees.

The public are invited to take part until the 23^rd September, by which time enough data will hopefully have been gathered to provide the professional scientists behind the project with evidence for the response of native birds to the alien invasion.

The horse chestnut leaf miner, Cameraria ohridella, was first described as a new species in 1986 after being observed in Macedonia in 1984, but the range of the moth subsequently grew very quickly across Europe. I spoke to Dr Michael Pocock from the NERC Centre for Ecology & Hydrology, who runs Conker Tree Science along with colleague Dr Darren Evans of the University of Hull, and began by asking him about the reasons behind our inability to control the advance of the moth since it reached our shores.

"The moth first appeared in the United Kingdom in 2002, in Wimbledon," Dr Pocock explained. "Since then it has spread very rapidly. One of the reasons it has been so difficult to control is simply because there are so many of them. This is partly because – unlike many native species which may cause damage to trees and other plants – they have multiple generations per year."

Dr Pocock has estimated that over the course of a season more than a quarter of a million leaf miner moths could potentially be reared from one large, badly infested horse chestnut tree. Although the leaf-mining larvae of the insects do not kill the trees they infest, they may weaken their systems of defence against other threats and cause smaller conkers to be produced.

The main impact of the leaf miner caterpillars, however, is on the appearance of affected horse chestnuts which is cause for concern given that these trees have been planted in parks, gardens and streets over the last 150 years  for their stately aspect and attractive flowers and foliage in particular.

Horse chestnuts are also affected by a disease known as bleeding canker. Although the effects of bleeding canker are not as obvious to the eye as the effects of the leaf miner moth, it can cause serious damage and may often require that the tree is felled for safety reasons. This is bad news for the United Kingdom’s horse chestnuts, as Dr Pocock pointed out.

"It is often the case that local authorities have been advised to cut down horse chestnuts which have been affected by bleeding canker," he said. "It may be that the trees become more susceptible to bleeding canker when they have been attacked by the leaf miner.

"Of course, local authorities and householders then choose not to replant horse chestnuts because they look so awful. The leaves of trees affected by leaf miner moths have turned very brown and patchy by the middle of summer. Overall the long term prognosis for our horse chestnut trees isn’t good."

Another reason that the moth might have spread in such a rapid and unchecked manner, and the focus of previous Conker Tree Science missions, is the lack of response from native species which are conceivably natural predators of the leaf miners.

"In the past we’ve asked people to look for some of the natural pest controllers of this moth – tiny parasitic wasps which lay their eggs inside the moth’s caterpillars," Dr Pocock said. "Often there are many wasps around attacking the native insects which damage plants, but there seem to be remarkably few which will target the horse chestnut leaf miner."

It is important to monitor the success of natural predators of the moths and their caterpillars, Dr Pocock told me, because no other way of controlling the moths has been found, including the use of chemical pesticides.

"The sort of sprays that you might, unless you operate on organic principles, apply to garden shrubs and plants just cannot be scaled up for use on trees. There doesn’t seem to be any method which is particularly effective. People have come up with various suggestions, but due to the scale of the problem it has not been possible to apply them.

"Our only real hope, therefore, is to note whether natural predators can adapt to utilise the horse chestnut leaf mining moth as a new food source. They will never eradicate it – I’m sure of that – but what they may do is regulate its numbers if they can adapt."

The idea for the current study, Dr Pocock explained, came from the volunteers themselves.

"Over the past couple of years people have been submitting data, monitoring the level of damage the moth is causing and the activity of parasitic wasps. They started to come back to us saying that they had noticed birds foraging in the trees. That got my colleague, Darren Evans, and I thinking it seemed a very valid question.

"As with the other questions we have tried to address with Conker Tree Science, we can’t answer it alone. We really need the big picture across the country to understand the level of attack by birds such as blue tits on the caterpillars when they’re still in the leaves. Are the blue tits and great tits adapting to this new food source or not? If they are adapting, in what ways are they adapting? Do they always choose the trees which are most heavily infested? Is this a behaviour which is spreading through the country?"

Approximately 8000 people have contributed to previous Conker Tree Science missions in order to provide researchers with the data they need to draw meaningful conclusions about the impact of the moth and, now, the impact of native predators of the moth and its young.

Work on previously collected data is ongoing, but preliminary observations suggest that clearing leaf litter can have a very significant effect, as Dr Pocock outlined.

"Last year, we asked people to look at what was under horse chestnut trees and to record the damage to the trees. People had been telling us that they cleared leaf litter from under horse chestnuts and that this had an effect, which is not surprising because the moths over-winter in the leaf litter. That is potentially a very straightforward, practical solution for individual trees, but we’re still looking at the data.

This is one of the largest projects of its kind to have been undertaken so far in the United Kingdom, and Dr Pocock is first to recognise the value of citizen science – on a much wider scale than the horse chestnut – in helping professional scientists get to the root of various scientific problems.

"One of the great things about citizen science is that it can generate huge amounts of data," he enthused. "From my point of view, I want to analyse the data in the best possible way in order to get the best possible understanding from it."

The current ‘Bird Attack’ mission involves members of the public monitoring the extent to which birds such as blue tits have been preying on the leaf miner caterpillars, as distinctly evidenced by the mines being torn open. I asked Dr Pocock whether he believes that citizen science projects can help find a solution to the leaf miner problem, and whether native birds could be that solution.

"There is nothing we can do to speed up the response in the behaviour of the blue tits," he pointed out. "However, we can aid our understanding of how our native animals respond to these alien invaders, in terms of how quickly and in what ways they respond. There are lots of species coming into the country and becoming established which might potentially become invasive.

"As to whether we’ll get a solution, clearing leaf litter seems to be very beneficial, as I said. As far as the likely impact of birds such as blue tits goes, I think we’ll just have to wait and see what the data shows."


Visit the Conker Tree Science website for more information and to get involved