Peak Blue Shieldbug counts on the Kingmoor Sidings Boardwalk, Carlisle, 2016

INTRODUCTION

 

Peak Blue Shieldbug counts on the Kingmoor Sidings Boardwalk, Carlisle, April to September 2016.

 

In 2015 I accumulated a mass of data on Blue Shieldbug sightings in the Carlisle area and presented the information in histogram form in an attempt to give an indication of the life-history of the species in this locality. Basically each month was divided into three segments, the first two of tens days duration and the third comprising the rest of the month; the total number of counts summed from all sites were then included in each interval, with adults and nymphs being treated separately (for convenience this has been copied into the host album).

 

Although the results gave a true indication of my observations the chart bore little resemblance, structurally, to the one presented by R D Hawkins in Shieldbugs of Surrey - my only point of comparison available at the time. The most obvious difference was the lack of a clear double peak in the adult population, as the vast majority of my adult sightings occurred later in the year, whereas in the reference work a substantial proportion occurred before the mid-year dip.

 

A number of reasons for this discrepancy have been identified, but the most significant factor is that at the start of the year I had minimal experience of finding Blue Shieldbugs and in fact had only seen two of them before - both found at the same location in December 2014; however, as the year progressed I accumulated more and more information about the sites favoured by this species, and so I became much more proficient at finding them. Also, whenever a "hotspot" was discovered it was subsequently re-visited throughout the rest of the survey period which clearly had a distorting effect on the results.

 

In order to avoid this issue the original intention for 2016 was to repeatedly survey a small number of known Blue Shieldbug sites from start to finish of the survey period and only include records from these sites in the analysis - bugs found elsewhere would merely be reported. However, this was ultimately judged to be too time-consuming, and instead the decision was made to derive the data from a single location. For reasons described in the following section the site chosen was the Kingmoor Sidings boardwalk.

 

To ensure that bugs were not counted more than once during any given ten-day period only the number counted in the highest scoring survey was used in the chart, regardless of the number of surveys carried out. Although this substantially reduced the record-keeping burden it's a fundamentally different approach to that adopted previously, and is considered further later.

 

It can be seen that the nymphs are divided into two categories: early and final-instars. As no first-instar nymphs were seen, and as only a single second-instar was found, in practice the first category essentially comprises third and fourth-instar nymphs.

 

The inset photographs were all taken on the boardwalk. In the case of the nymphs, they were temporarily transferred onto a piece of cm/mm scale graph paper to facilitate measurement.

 

A number of noteworthy findings were photographed over the monitoring period - eg a Blue Shieldbug nymph feeding on an adult. Although these photos have already been posted on Flickr, and are not directly relevant to the study, a selection have been copied into the host album in case anyone is interested.

 

THE KINGMOOR SIDINGS BOARDWALK

 

The Kingmoor Sidings Nature Reserve is positioned on the outskirts of Carlisle and a sketch map is included in the album. The boardwalk is located in Area 2, at ca NY385580. A number of photos giving an indication of the surrounding vegetation are also included in the album. The total extent of the boardwalk is about 212m, with the width ranging from 0.45 - 0.52m (with a weighted average of 0.5m) giving a total upper surface area of about 106 sq m.

 

There are a number of reasons why it was chosen as the site for this study, but the most important factors were that Blue Shieldbugs were frequently found here over the previous year, and that Kingmoor is my local nature reserve, with Kingmoor Sidings being just a few minutes' drive away. Also there's an excellent (free!) car park just across the road in Kingmoor South.

 

The choice of the boardwalk, as opposed to an area of vegetated terrain, was also critical as it allowed a comprehensive - and repeatable - survey to be carried out in under 10 minutes, with a fair degree of confidence that any bugs present had been spotted.

 

How the boardwalk Blue Shieldbug population at any given time (or even time-averaged) relates to the population present in the surrounding terrain is not a question I feel competent to answer; however the basic premise - possibly flawed! - is that the shape of the life-history chart derived from the boardwalk data is indicative of what's going on in the "natural" environment. (This issue it speculated on later.)

 

COUNT DURATION, FREQUENCY AND VOLATILITY

 

Although I found my first Blue Shieldbug of 2015 on 19 March (on Scaleby Moss), boardwalk monitoring only started in earnest 14 April this year when six Blue Shieldbugs were found in close proximity in a small area of Kingmoor North that I'd been studying for other purposes. Monitoring formally ceased on 20 October after having seen nothing since 24 September. As I've found Blue Shieldbugs in this area of Kingmoor Sidings as late as 5 December (in 2014), I accept that there's a possibility that the odd one may still turn up later in the year. In which case I'll just append the information to the text, without amending the chart.

 

Ideally I'd have liked to carry out surveys at fixed intervals over the monitoring period but this just didn't happen! Although on some days I specifically set out to visit the reserve to carry out a count, most of the surveys were done as I just happened to be passing the Kingmoor South car park en route to other sites I routinely visit. If time allowed two counts would be undertaken - one on the way in and one on the way out. Although comprehensive records were not kept (see following section), I'm confident that at least six counts were carried out over every ten-day period, and in some cases perhaps up to 15.

 

The most troubling aspect of the study was the extreme volatility of the counts. For example on 20 June virtually nothing was seen during a survey carried out on the way into the reserve; but when the count was repeated about an hour later on the way out, under nominally identical conditions, nine adults and five nymphs were found! At times this made me question whether anything of any value was being achieved. In fact it was only because the opportunity cost was so minimal, with a survey taking less than 10 minutes, that I carried on with the project.

 

RECORDS AND RECORD KEEPING

 

Rightly or wrongly, I made the decision fairly early on that the parameter I'd be plotting on the life-history chart was peak counts. This significantly reduced the requirement for record keeping - for example if five adults were counted on day one of a ten-day period, then there was no need to record any further sightings of adult bugs over that period till a higher number was achieved. On reflection I regret this decision, because if comprehensive records had been kept then it would have been possible to analyse the data in several different ways (for example using average counts over each 10-day period) which might have been revealing.

 

Also record keeping wasn't as good as it might have been for the nymphs. Although I initially recorded the numbers of third and fourth-instars separately, on some occasions I only recorded the total number of thirds and fourths together - although final-instar nymphs were always recorded as a separate item. For this reason only two classes of nymph - "early" and final-instars -have been charted.

 

Adults were found from 14 April to 24 September, and nymphs as follows:

 

Second-instar nymph: 20 June (single record)

Third-instar nymphs: 12 June - 30 June

Fourth-instar nymphs: 12 June - 16 July

Fifth-instar nymphs: 30 June - 5 August

 

RESULTS

 

Despite the reservations expressed above about various aspects of this study, the forms of the life-history charts, for both adults and nymphs, are broadly consistent with expectations for shieldbugs. Specifically, the adult chart is twin-peaked with a mid-year dip, indicative of last year's population dying off before this year's adults made a showing. In the case of the nymphs, the rise and fall of the early-instars is followed - as expected - by the rise and fall of the final-instars, with a noticeable degree of overlap. One might rationally have anticipated a gap between the peaks of the final-instar nymphs and this year's adults but this isn't apparent. However, it's noted that these peaks were coincident for a number of shieldbug species covered in R D Hawkins' Shieldbugs of Surrey as well.

 

Had average as opposed to peak shieldbug counts been used in the study, then presumably the charts would have been much smoother, but as noted above this option was precluded due to lax record keeping!

 

The question remains of how the boardwalk Blue Shieldbug population at any given time (or even time-averaged) relates to the population present in the surrounding terrain. In particular, are the densities comparable, or are the bugs attracted to the boardwalk resulting in higher counts?

 

Although Blue Shieldbugs were seen adjacent to the boardwalk on a number of occasions, no attempt was made to survey this terrain and so there is no directly relevant data available to answer this question. However, a number of surveys were recently carried out on a broadly comparable 0.5 hectare site in Kingmoor North (designated Area 5), and highest number of Blue Shieldbugs recorded was 17, or 0.0034/sq m, on 4 September. Applying this figure simplistically to the boardwalk area of 106 sq m gives an equivalent bug count of 0.36. As four Blue Shieldbugs were recorded on boardwalk on 1 September (the nearest date for which a count was recorded) this might seem to imply - all things being equal - that the boardwalk count was artificially high. However, if an allowance is made for the fact that only a proportion of the Kingmoor North bugs would have been seen, and that a more typical boardwalk count for early September was one rather than four, then the case is less clear cut.

 

For what it's worth my own view is that the boardwalk counts are probably artificially high with reference to the surrounding terrain. This could be for a number of reasons - for example on several occasions Blue Shieldbugs were seen feeding on the carcases of insects crushed by the boardwalk foot traffic, and so perhaps bugs are drawn here for an easy - if risky - meal? (At least 20 crushed Blue Shieldbugs were found on the boardwalk over the period of the survey!)

 

However, even if this were true, there's no reason to believe that it invalidates the findings qualitatively.

 

(Text to be checked - and probably revised!)

Done