The data collected can be examined for long term trends linked to conservation. Changes that are as expected and those not anticipated.
It is therefore important to select what to monitor, how, where and over what time.
The Lake District is special for many reasons and one of them the presence of the Netted Carpet Moth (Eustroma reticulatum). A very rare species it is primarily known to only a few areas here, though, recently new sightings have been made in North Lancashire and possibly some in Wales too.
A possible explanation for its rarity is that its eggs are laid, and its larvæ feed, on only one plant, Touch Me Not Balsam (Impatiens noli-tangere), itself a nationally scarce species.
Therefore, conservation of Netted Carpet Moth is also conservation of Touch Me Not Balsam.
The UK's only native impatiens, it favours moist, nutrient-rich soils and is commonly found in damp open woodland with dappled shade, alongside streams and where there is regular ground disturbance.
With few associated species it often forms pure stands of several hundred individuals. Though here in the Lake District sites of much larger populations numbering many thousands of individuals are known, particularly around Coniston and Windermere.
Recent conservation work by the National Trust in the Lakes has resulted in a significant increase in the population of Touch Me Not Balsam.
Winter grazing of cattle has been introduced to woodland sites known to have the balsam amongst its ground flora. The cattle trample the ground providing the much needed ground disturbance to spread the seed. The cows are then removed from the woodland before germination takes place.
No other method of ground disturbance has proved quite so successful as cattle.
Even with such successful conservation efforts as this the numbers of Touch Me Not Balsam are still known to fluctuate from year to year.
A kind of 'predator - prey' relationship between the balsam and the Netted Carpet Moth occurs whereby as populations of balsam increase so too do the numbers of larvæ feeding on the food plant until a critical point in the larvæ population when the numbers of balsam start to deplete. As the population of the food plant decreases so too do the numbers of larvæ feeding on the balsam until another critical point whereby the balsam can begin to recover and increase its population and the cycle begins again.
Such is the delicate relationship between the moth and its food plant. And thus the need to monitor populations of both annually.
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So, Netted Carpet Moth eggs are laid during July to August meaning the caterpillars are fully grown by early to mid-September. And this is the best time to survey.
We joined a group of Rusland Horizons volunteers for the annual survey of Netted Carpet Moth at Brantwood on the east shore of Coniston.
The survey was led by the National Trust's Wildlife and Countryside Advisor John Hooson and Dr. Paul Hatcher of Reading University. Together they have been monitoring the numbers of balsam and moth here for the last 20 years.
Brantwood gardens are managed specifically with Touch Me Not Balsam in mind and head gardener Paul was keen to take us to spot where the recent felling of a tree, providing both light and heavy trampling of the ground beneath, had resulted in an explosion in the population of balsam.
We began with a quick tutorial of how to spot the caterpillars which are to be found on the underside of the leaf and can camouflage themselves to look like a seed pod or a leaf stem.
The method of monitoring with which we began was to count the number of larvæ per site. We paired off into groups, picking a site of the garden each, and first estimated the number of balsam plants on our site. Then we counted the number larvæ we found on all the balsam, that we could reasonably reach, within our site, without trampling any of the neighbouring plants. We would then re-group and give Paul Hatcher the number of plants estimated on our site, the percentage of those plants counted and the number of larvæ found.
These figures will be used by Paul later to estimate the number of larvæ in each site and in the gardens as a whole as well as having the actual number seen on the day. These figures will in turn will be used to estimate the number of Netted Carpet Moth at Brantwood.
In the afternoon we moved to another location along the Coniston east shore called High Barn Wood. This site has seen winter grazing of cattle since 2001 which has resulted in widespread ground cover of almost pure stands of Touch Me Not Balsam.
With such large numbers of plants over a much larger area we used a different method of surveying here. This time we worked individually each picking an area of woodland and checking a total of one hundred plants in that area. This gave us the number of larvæ per one hundred plants. We did this in three or four different areas and again gave all the results to Dr. Hatcher who, again, would use the figures, later, to scale up an estimate of the density and number of larvæ in the whole site as well as having the actual number seen in the woodland.
Surprisingly the numbers of larvæ found in High Barn Wood appeared to be less than those found in Brantwood gardens. Though as Dr. Hatcher explained it is often the case that large areas of balsam yield a lower density of larvæ, though overall higher numbers, than smaller sites.
After 20 years or more surveying the moth and its food plant, however, he has yet to observe any clear correlation between site and numbers of caterpillar. A large, seemingly ideal, habitat may produce only a handful of caterpillars one year where as a small patch of balsam next to a path produce pleasingly high numbers. And then, the following year, the opposite.
Musing on why some sites appear to be more favourable than others to the presence of Touch Me Not Balsam - vital, remember, for the existence of the Netted Carpet Moth - Paul Hatcher noted that the woods around us had been coppiced for charcoal production in years gone by. That the combination of a 10 year coppice cycle, providing dappled light, plus the trampling of the ground by those working the woods would provide ideal conditions for the balsam to grow. And that the presence of Impatiens noli-tangere as the primary ground flora in a woodland may be a strong indicator of historical industrial woodland.
And so, from this years survey, it appears that the overall number of larvæ in the Lakes is increasing year on year. Which is encouraging news to both surveyor and conservator.
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Incidentally, the Netted Carpet Moth itself can be surveyed at dusk in early July to mid-August. Or, more precisely, according to one intrepid volunteer, during the last week of July and first week of August at 9:15pm. So there you go.
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| An explosion of balsam where a tree once stood at Brantwood gardens |
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| Two for the price of one on rare larvæ |
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| A seed pod. Or is it a caterpillar? |
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| A caterpillar. Or is it a seed pod? |
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| Larva pretending to be a leaf stem |
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| The group disperse into a sea of balsam in High Barn Wood |
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