Records of leaf damage caused by and parasitism of Cameraria ohridella in Britain in 2010 collected with a citizen science approach, plus validation of the data

Records of leaf damage caused by and parasitism of Cameraria ohridella in Britain in 2010 collected with a citizen science approach as part of the Conker Tree Science citizen science project, plus validation of the data. Over 3500 people in Great Britain provided data at a national scale on an invasive insect (horse-chestnut leaf-mining moth, Cameraria ohridella Deschka & Dimic; Lepidoptera: Gracillariidae) in order to address two hypotheses. Specifically:

(1) whether the levels of damage caused to leaves of the horse-chestnut tree, Aesculus hippocastanum L., and
(2) whether the level of parasitism of C. ohridella larvae were both greatest where C. ohridella had been present the longest

Participants recorded leaf damage on an ordinal scale (0-4) during the summer (1st July to 15th October 2010). In order to assess the levels of parasitism of caterpillars of C. orhidella, we invited people to rear insects from horse chestnut leaves infested with C. ohridella. Participants sampled leaves during the first week of July 2010 (i.e. the first of the moth's gererations that year) and stored them in sealed plastic bags for two weeks. We then asked participants to report the number of leaf-mines, and to identify and count the insects in each category: adult C. ohridella moths, parasitoids, and other insects. Anyone could take part in rearing parasitoids, but we particularly focused on school children aged 8-11 by working with a team of eight trained volunteers across the country who directly contacted schools and led lessons in classes. The volunteers did not provide directive guidance during the time that the children were counting adult moths and parasitoids, so the data were not biased by our supervision. At the completion of the activity, we retained a randomly-selected subset of 669 samples that the children had counted. We also retained an additional 75 samples in which children had reported parasitoids.

For all of these samples an expert blindly assessed the counts of leaf mines, adult C. ohridella moths and other insects. In order to assess how many years that C. orhidella had been present in a location, we used a long-term dataset collated by Forest Research (used with permission). These data showed under-sampling of the range of C. orhidella after 2006, so we also modeled the predicted arrival of C. orhidella based on a demographic model of spread parameterised in continental Europe by augementing the known distribution with a model of short-distance spread by the model. We ran the model twice, assuming two and three generations of C. ohridella, respectively.

The project was supported by the Natural Environment Research Council and undertaken at the University of Bristol, UK.