Friday, 27 March 2015

What ails our horse chestnut trees and can we save them?

A guest blog from pupils of La Sainte Union School, Camden, London.

Dr Michael Pocock, an ecologist at CEH, is one of the scientists behind Conker Tree Science. Thanks to a Royal Society partnership grant, he recently shared some of his knowledge and skills with students at a school in London.
One pupil’s scientific drawing of the tiny leaf-miner larva

Michael writes, “Over the past few months I have been visiting Dr Pari Collis and her pupils at La Sainte Union school in Camden, London as part of a Royal Society partnership grant. Based on my experience with Conker Tree Science, I helped the girls begin a project on the horse-chestnut leaf miner, but with their teacher they took it so much further than I thought they would.

"Not only did they undertake careful ecological studies, but they also covered biochemistry and environmental ethics, mixed with a little bit of German, maths and art! My final visit to the school was last week when they gave an excellent presentation of their project in front of an audience of teachers and parents.
"It has been a privilege to work with Dr Collis and the girls over the past year and share moments of scientific discovery and excitement with them. I would highly recommend using the Royal Society partnership grants for any scientist to link with a local secondary school."

The pupils at La Sainte Union school have written up their project and submitted it for peer review at the Young Scientists Journal. Below is an abridged version of their full article:

"Horse chestnut trees are ornamental trees and were largely planted for their attractive shape and beautiful flowers which make them really desirable in parks and village greens. We noticed that there was extensive early browning of the horse chestnut trees in our school grounds, which we found was caused by the horse-chestnut leaf-miner, Cameraria ohridella. Both the horse chestnut tree and the Cameraria ohridella are invasive species but we consider the moths to be pests because they harm the trees that we chose to plant.

We worked in groups to visually estimate the number of horse chestnut leaf miner in one tree. We counted the number of the leaf miners in a small section of the tree and then scaled up to estimate that there were 250,000 – 500,000 leaf miners per tree.

Making careful observations of  horse-chestnut leaves
The common horse chestnut Aesculus hippocastanum is very susceptible to infestation by the leaf miner, but we found that the red-flowering hybrid (Aesculus x carnea) is very resistant. The mass of the red flowered horse chestnut tree leaves was approximately twice that of the white flowered trees (0.02 compared to 0.01 g per cm2). The caterpillars seem unable to feed successfully on these leaves.

We wanted to find out if anything could be done to stop the moths. We investigated the following possibilities of control:

  • Natural predators (parasitoid wasp and blue tit);
  • Pheromone traps that attract and kill male moths;
  • Collecting and burning or burying fallen leaves in autumn to destroy the overwintering pupae.

Natural pest controllers: Leaves from an infected horse chestnut tree were collected and stored in zip-lock bags for two weeks on a cool and dark shelf. We then recorded the number of adult Cameraria ohridella moths and parasitoid pest controllers. There were 171 wasps altogether (19% of the total) which means 171 horse chestnut tree leaf miner larvae were killed by the larvae of the parasitic wasps.

Pupils ready for the big sweep to 
collect fallen horse chestnut leaves

Blue tits have discovered that horse chestnut trees are absolutely loaded with caterpillars. Caterpillars are an important food source for blue tits, which feed them to their young. If blue tits were to start eating a substantial amount of the caterpillars it would help the horse chestnut tree. We examined the leaves for v-shaped tears as evidence of ‘bird attack’ and found 0 to 57 bird attacks per leaf.

Pheromone traps: The main component of the sex attractant (pheromone) released by the females of the horse chestnut leaf miner has been identified as E,Z-8,10-Tetradecadienal12. We used pheromone traps to catch male moths which reduces mating and therefore egg laying. We weighed the content of the trap and estimated that 30,000 moths had been captured over a period of two months. This is about a tenth of the number we had estimated on each tree in July after the 1st generation.

Clearing leaf litter: Early in the season (July), we noticed that the browning of the leaves is more prominent at the base of the tree. This is consistent with the moths emerging from the fallen leaves and spreading upwards first to the lower leaves. We collected fallen leaves from under the horse chestnut tree and found an average of 100 pupae per leaf. If we estimate that there could be at least 100 fallen leaves in the vicinity of the tree, then 10,000 moths could emerge. If half of these are females, which lay 30 eggs each, we could expect 150,000 moths at the end of the first generation and 2,250,000 eggs at the end of the second generation. In reality the number will be smaller because not all pupae, moths or eggs will survive. Nonetheless the number of potential moths is formidable bearing in mind that the calculation is based on just 100 leaves.

It may be time for the UK to follow Berlin’s example where there is a programme encouraging everyone to take part in raking up and clearing every single horse chestnut tree leaf. It is considered to be every citizen’s civic duty to participate in the clearing of the leaves. Involving the community in this way may have many social benefits encouraging social interaction, interest and responsibility for the environment."

Michael Pocock and several of the students
By: Tito A, Ursula A, Elisabeth A, Sharon B, Ariane F, Grace G, Catriona G, Zoe H, Oghogho I, Dea L, Mia O, Lara R, Hannah S and Ellie T (La Sainte Union School, London).

Dr Pari Collis is the science teacher at La Sainte Union school who invited Michael to visit and she concludes:  We really enjoyed working with Dr Michael Pocock.  Michael’s enthusiasm was absolutely infectious and it kept everyone on board. The girls did outdoor science, learnt how to be observant and use scientific method.  They improved their communication skills, worked as a team and showed commitment to the project. Under Dr Pocock’s guidance they also learnt to read original scientific papers for themselves. Most importantly they took an interest in their surroundings, seeing them in a different light, and in particular have become extremely fond of horse chestnut trees and conkers.  They discussed the project and shared their enthusiasm with their friends and family. It has been a great pleasure to have worked with Michael and to share and discuss our findings with him.  

Additional information

Michael has also recently written about his life in science for Catalyst magazine, which is produced to inspire secondary school pupils about science.

Royal Society Partnership Grants

Conker Tree Science

Staff page of Dr Michael Pocock

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