Citizen science – a novel approach to investigating horse chestnut leaf miner

Many studies undertaken by scientific researchers are – by a sort of default – quite limited, in terms of the study’s spatial scale. Whilst this is not necessarily a bad thing, when the point of the research is to use a specific area (such as a city) as a case study, it can limit the effectiveness of problems that occur across a very large geographical area. One such example would be Cameraria ohridella (horse-chestnut leaf miner) presence within the UK, where it is an invasive pest (having arrived in 2002) progressing across the UK that may aggressively defoliate Aesculus hippocastanum (horse chestnut) and thus cause serious tree health problems.

An example of horse chestnut foliage being attacked by the leaf miner. Source: The Wild Diary.

Because the pest is found in many areas of the UK, understanding its characteristics as a meta-population is important, but tricky (in terms of cost and time) if relying upon the standard method of research. To remedy this issue, a form of science known as ‘citizen science’ can be used, which directly involves members of the public, across very large areas, gathering data and relaying it back to scientific researchers, where the data can then be processed. And would you know it, the study looked at here employs such a form of science, with the aim of understanding: (1) more about how serious leaf damage is from Cameraria ohridella, when comparing leaf damage extent to how long the miner has existed within the local geographical area, and (2) whether parasitism of the leaf miner (by parasitic wasps) is higher in areas where the leaf miner has existed for longer periods of time. The research project was dubbed ‘Conker Tree Science‘ (and is still ongoing, in a slightly different format).

In order to bring in accurate information from members of the public, which could be processed and transformed into conclusions, those aiding with the survey (around 875 people, for the first part) were asked to complete a few basic tasks. The first was to determine an average level of leaf mining damage upon foliage of the horse chestnut, with the aid of a very basic diagram (shown below). Such recording was undertaken during June – September of 2010, and alongside the numerical scoring the surveyors also submitted photos of the foliage inspected. This enabled an experienced individual to determine whether recording was accurate, and if not, bias would be accounted for in the data analyses. No bias was found, so the data gathered by the survey participants was considered to be accurate.

Ranging from 0-4, with 0 being no damage and 4 being major damage, survey participants reported to what extent the horse chestnut they were inspecting was mined.

Where a member of the public had stated the leaf was at least partly damaged, the researchers compared the location of the tree to how long Cameraria ohridella had been known to exist in the area. Of course, the time of year was factored into account, as leaf mining will progressively worsen as the larvae progress through their instar stages; as will it generally worsen as the summer progresses (there may be more than one life cycle of the pest in one summer). Furthermore, because the presence of the leaf miner may have been under-recorded prior to this study taking place (as people became used to its presence,a nd no longer recorded it), the researchers constructed a model that would predict where the miner should have progressed to in the years following 2002. Therefore, if reports came in from areas where there were no records of the leaf miner, the model would allow for comparisons to be made between mining severity and how long the miner had likely been present for in that locality.

With regards to ascertaining whether parasitism increased with how long the leaf miner had been present within the local environment for, survey participants were also asked to take small cuttings of foliage during early July 2010 and seal those cuttings in a plastic bag for two weeks. After this time, a count for Cameraria ohridella, parasitic wasps, and any other insects (in case of contamination) was undertaken. In order to identify the different organisms, the researchers provided an identification key to all survey participants. Much of this part of the data collection was done by schoolchildren, with the aid of trained researchers. Following counting of all insects, the data was compared to that gathered by experts, in order to check for bias. No significant issues were found with regards to identifying the leaf miner, so the data gathered by the school children was used in the study without being corrected in any manner. For the parasitic wasps however, it was found that the school children failed to fully identify them in some instances (under-estimation).

After collating the results and analysing them, the researchers found that the damage caused by Cameraria ohridella rose for the first three years; at which point, it begun to quickly ‘flatten’ (and thus, damage then remained rather constant, albeit significant). This is shown in the below graph. In this sense, in the fourth year, the leaf miner will most likely be causing maximum damage, and from here-on-in, such maximum damage will routinely occur (of course, it will still vary from year-to-year).

Foliar damage associated with Cameraria ohridella, in relation to how long it had been present in the local area of the UK.

As for whether parasitism of the leaf miner increased over time, the 2,208 cases of reared insects (1,810 of the cases were from school children) showed that the rate of parasitism increased aongside how long the leaf miner had been present in an area for. Consulting previous literature of parasitism on the leaf miner, the researchers suggest that initially pre-pupal stage generalist parasites will use the leaf miner as a host, though after a few years more specialised pupal stage parasites will succeed into the trophic system. However, once specialised parasites do arrive, it is not expected for there to be a continual rise in their population abundance, as research in mainland Europe has shown that parasitic wasp populations plateau after a some years. Therefore, parasitic wasps may not be able to greatly limit the damage caused by Cameraria ohridella.

Such results are certainly interesting, though the manner in which they were obtained is equally so. What we can draw from this is that citizen science certainly has the ability to work, and across a large geographical area; all whilst costing little money and ‘outsourcing’ the time spent on data collection to willing volunteers. This can be good as it engages people with the scientific process, thereby removing it from its pedestal and giving science a form of accessibility. The act of engaging with school children was particularly pleasing to read about, as one never knows whether such research could evoke a greater interest in scientific research for some of those children. Granted, citizen science isn’t a ‘cure-all’ approach, as there are many limiations. The researchers remark that one main limitation in this study was the inability to directly sample wasp parasites, and thus specific species couldn’t be identified. Furthermore, the data is only as good as those collecting it, and because volunteers are unlikely to be amateur gall enthusiasts, data collection must be simple, swift, and succinct. There’s also the need to verify the data after it has been collected, unlike with scientific researchers who will know how to gather data prior to gathering it (and thus eliminating bias, ideally).

Nonetheless, a good study, and hopefully citizen science can be used in the future for other projects!

Source: Pocock, M. & Evans, D. (2014) The success of the horse-chestnut leaf-miner, Cameraria ohridella, in the UK revealed with hypothesis-led citizen science. PLoS One. 9 (1). e86226.

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Citizen science – a novel approach to investigating horse chestnut leaf miner

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