Citizen science charts horse chestnut tree pest spread

Horse chestnut leaf-miner (Image: Dave Kilbey) The horse chestnut leaf-miner moth was first recorded in the UK in 2002

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A citizen science study, involving more than 3,500 people, has revealed the spread and establishment of the horse chestnut leaf-miner in the UK.

It also suggests that a native species of wasp that preys on the tiny insect will not be able to curb its impact.

Caterpillars of the non-native moth tunnel through leaves of infested trees, causing them to turn autumnal brown, even in the middle of summer.

The study and its findings have been published in the journal PLoS One.

"It has now basically got everywhere south of Newcastle," explained co-author Michael Pocock from the Centre for Ecology and Hydrology.

Dr Pocock and colleague Dr Darren Evans, based at the University of Hull, wanted the public to help them answer two questions.

The first asked whether the level of damage to the trees (Aesculus hippocastanum) increased with the length of time the moth (Cameraria ohridella) had been in an area.

"What we found was that it takes three years after the moth first arrives in a location for the levels of damage to basically reach a maximum, which causes trees to look as if autumn had arrived early," Dr Pocock explained.

"The leaves will turn brown by the end of August, instead of naturally browning sometime in early November."

People power

The second question the pair asked was whether the rate of predation of the leaf-miner by parasitic wasps increased with the length of time that the moth had been present.

Other threats facing the UK's trees

Acute oak decline - tree with cankers (Image: Forestry Commission)
  • Phytophthora ramorum - fungal pathogen that infects the commercially important conifer species, Japanese larch
  • Acute oak decline (AOD) - An aggressive bacterial disease that can kill an infected tree in just four or five years
  • Ash dieback - "a serious disease of ash trees", caused by a fungus called Chalara fraxinea, and can kill infected trees
  • Great spruce bark beetle - breeds under the bark, weakening the infected tree and in extreme cases, can kill the tree

(Source: Forestry Commission)

Dr Pocock said: "What we found was that yes, the levels of predation do increase with the length of time the moth has been present but it appears to remain at very, very low levels.

"Because they do not seem to be attacking it at particularly high levels, I think that helps explain why this moth has been so successful and why it spreads and establishes so quickly."

One of the experiments the professional scientists asked people to do was to seal an infested leaf in a plastic bag and wait for the insects to emerge.

The results from the experiment carried out by thousands of people, including hundreds of schoolchildren, offered an indication that the parasitic wasps were not numerous enough to effectively control the leaf-miner populations.

Dr Pocock observed: "It seems almost like magic for children and other people to put a damaged leaf in a plastic bag, wait two weeks and then see insects - the adult moths or their pest controllers - emerge, but making these discoveries was a valuable contribution to understanding why some animals become so invasive."

He added that using a "hypothesis-led" approach to citizen science offered a "win-win" scenario.

"Our study was tightly focused, people knew exactly what they could do. It was designed in such a way that it was very simple and anyone could do it and provide accurate results.

"Within science generally, there is a lot of emphasis on scientists not being stuck in ivory towers," he said.

"Scientists need to engage very widely but I think one of the problems is that scientists still tend to be heavily evaluated in terms of winning grants and doing research.

"The advantage for me doing something like this is that I can do an activity that is engaging people with science but I can also be doing research at the same time."

The study's findings suggested that the outlook for the ornamental tree species, which was first introduced to the UK during the 17th Century, was not favourable.

Horse chestnuts in the UK, until 2002, were considered disease-free and widely planted in parks and gardens.

However, the arrival of the leaf-miner moth and a disease called "bleeding canker", which can kill an infected horse chestnut, meant that local authorities were reluctant to plant them.

Sales of white-flowered horse chestnut saplings have plummeted in recent years.

"This does suggest that the long-term prognosis for these beautiful trees is not actually that good and they will become rarer and rarer," Dr Pocock suggested.

Dr Pocock said that he and his colleagues hoped in the near future to publish advice on what people could do to help their horse chestnuts.

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