Peak Forest Bug nymph counts on Finglandrigg Wood fenceposts, Apr 2015 - Dec 2016

Peak Forest Bug nymph counts on Finglandrigg Wood fenceposts, Apr 2015 - Dec 2016.

 

INTRODUCTION

 

Forest Bugs are the only UK shieldbugs that pass the winter as small nymphs, reputedly high up on the trunks and branches of forest trees - generally oaks. Consequently, they get off to a flying start in the springtime, with the nymphs reaching final-instar stage in early summer, and the adults dying off well before the end of the year.

 

I found my first Forest Bug nymphs on 4 April 15 on wooden fenceposts at the edge of woodland in the Kingmoor Sidings Nature Reserve, Carlisle. These were later identified as second and third-instars. The following week more early instars were also found in comparable locations in Finglandrigg Wood. This soon became my favoured site for Forest Bugs, and repeated fencepost surveys were carried out till fifth-instar nymphs were found on 14 June. Adults were first spotted on the leaves of nearby trees on 10 July. Although no further searches were carried out for this species till much later in the year, adults continued to be found in some numbers over the next few weeks, with the last specimen being discovered on a post some distance from woodland on 30 September.

 

I then switched my focus to other species, assuming that the Forest Bug story was over for the year. However, on 22 November 2015 I was surprised to find a handful of Forest Bug nymphs in Finglandrigg Wood whilst checking out fenceposts for late Bronze and Hawthorn Shieldbugs. A more thorough survey was carried out a few days later when a total of 27 were found: One first-instar and 26 second-instars.

 

Although the original objective of the searches had merely been to find and photograph as many of the nymphal stages as possible, after this discovery it struck me that it might be an interesting exercise to attempt to construct a life-history of Forest Bug nymphs based solely on fencepost counts. Consequently, the monitoring was continued till the first third-instars of 2016 were found. However, due to a number of gaps in the record - in particular, the initial build-up of the second-instar population - the decision was then made to continue till at least the end of 2016. In fact I still don't have any immediate intention of calling a halt to the program, but I thought it was probably worthwhile summarising the findings so far, ie up to 31 December 2016.

 

As noted later, by far the most fascinating part of the study relates to the second-instars, which were found from September to May. Consequently, it's my intention to cover these nymphs in more detail in a follow-up report next year when more data have been accumulated. This is unlikely to be completed before next autumn as one of my objectives is to see how early in the year the new generation of nymphs can be found in Finglandrigg Wood - currently this stands at 7 September (2016).

 

During the surveys a number of beetles, bugs and moths new to me at the time were found and photographed on the fenceposts. 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 for convenience. Also included are some shots of the relatively few adult Forest Bugs also found on fenceposts during the surveys.

 

FINGLANDRIGG WOOD FENCEPOSTS

 

Photographs showing the main sections of the fenceposts included in the surveys, and a map showing the corresponding locations, have already been submitted (see album). The number of posts examined varied from visit to visit but a full survey covered approximately 430.

 

The question might be asked as to why nymphs were only counted on fenceposts when the population levels were presumably considerably higher on the trunks and branches of the forest trees? In addition to the obvious impracticability of surveying the trees themselves, nymphs on fenceposts are considerably easier to spot and - with the exception of the tiny first-instars - I'm confident that a reasonably high proportion of the nymphs present above the level of the surrounding vegetation were counted. The question of whether fencepost data provide any useful information on the wider Forest Bug nymph population is addressed later.

 

With the trees in full leaf it can be inferred from the map and location photos that many of the fenceposts will receive very little in the way of direct sunlight - in particular the line of posts through the block of woodland to the south of Little Bampton Common (Photo 8). However, once the leaves have fallen most of the posts are exposed to some sunlight to a greater or lesser degree for at least part of the day. Although the odd specimen did appear to be sunbathing it's not clear to me how important this is for Forest Bug nymphs.

 

SURVEYS

 

As it was not my original intention to produce life-history charts no groundrules were established at the outset regarding the extent or frequency of the surveys. However, in practice all of the surveys for which data were recorded covered at least the areas marked in red on the map, taking typically 90 minutes or so to complete. When time permitted surveys were often extended to include some or all of the areas marked in yellow as well, adding up to a further hour or so to the duration of a count.

 

The frequency of monitoring also varied throughout the program, with 19 surveys carried out in 2015 and 34 in 2016. However, this discrepancy is largely due to the fact that no monitoring was carried out in the first three months of 2015, and that the post-summer early-instar nymphs were found relatively late in the year, with the counts recommencing in November as opposed to September in 2016. Once these factors are taken into account, the survey rate over the active periods of the two years were both roughly the same, ie about once per week.

 

PRESENTATION OF RESULTS

 

It can be seen that the results are presented in two charts, the upper one covering first and second-instars with the later instars covered in the lower chart. Also the time-axis extends from September to July, which covers the period from the first sighting of the early-instar nymphs to the last sighting of the final-instar nymphs.

 

As in the case of the 2016 Kingmoor Sidings Blue Shieldbug study reported earlier, only peak counts have been presented; however it's my intention to look at other ways of analysing the data in the case of the second-instars when these are looked at in isolation at a later date.

 

Due to the reduced monitoring frequency it wasn't found to be practical to divide each month into three segments, as done previously. Consequently two segments were employed, the first comprising the first 15 days of the month, and the second the remainder of the month. For example, third-instar nymphs were found over the period covering the second half of April to the first half of June.

 

It can be seen that photographs of each of the five nymphal stages are inset in the charts, taken on, or adjacent to, some mm/cm scale graph paper to give an indication of size. Obviously there's a spread of sizes within each class, but not to the extent of presenting any identification difficulties.

 

I should point out that for various reasons no counts were carried out in the first half of February, and so the corresponding gap in the upper chart should be ignored.

 

RESULTS

 

The upper chart clearly shows the build-up in the second-instar population from early September, followed by a variable but continuous count through the winter months, followed by a decay in numbers over the spring period, with the last sightings being made in the first half of May. So the only months for which second-instar nymphs were not found were June, July and August! First-instars were recorded from September to November; however, only three specimens were found - perhaps due to the difficulty in spotting these tiny nymphs - and so I don't think much can be read into this.

 

The lower chart shows the sequence of third, fourth and final-instars, extending from the second half of April to the first half of July. I should point out that virtually all of these counts were derived from the 2015 surveys, with very few of the later instars being seen the following year. For example although 19 third-instars were found on 8 May 2015, only one was found on 11 May 2016. In fact as only bounding values are shown, the sole contribution to the chart from the 2016 surveys was the first entry, for the second half of April.

 

It's perhaps worth noting that although only one year's worth of data have been recorded for the months of January to March it seems unlikely that the upper chart will be significantly affected once the 2017 counts for this period are completed. This is because the second-instar counts have been relatively low since early December 2016, and in fact not a single nymph was found on the 1 January 2017, compared with 11 on 29 December 2015. So unless numbers pick up sharply the new data won't register on the chart which only shows bounding values (the sole exception is for the first half of February as no counts were previously carried out over this period).

 

For the record, over the period from 13 April 2015 to 31 December 2016, the five nymphal stages were found between the following dates:

 

First-instars: 17 September to 25 November (based on only three records)

Second-instars: 7 September to 6 May

Third-instars: 19 April to 7 June

Fourth-instars: 25 May to 14 June

Fifth-instars: 14 June to 10 July

 

As noted earlier, it's my intention to cover the second-instar stage in more detail in a later posting.

 

SOME UNANSWERED QUESTIONS

 

Below are some questions I've been asking myself about the fencepost nymphs. I can't answer any of them definitively but I've made a few suggestions anyway!

 

How did they get there?

 

Although I found the odd adult Forest Bug on fenceposts, I think there's little doubt the nymphs must have come down from the leaves, branches or trunks of the nearby trees. As they obviously can't fly, they presumably either fall off, are blown off or leave of their own volition, before eventually climbing up a nearby post - perhaps thinking it's a tree trunk? All I can add to this from my own observations is that on one occasion I found a second-instar climbing on to the base of a post from blade of grass. In fact this was one of the few times when I saw a nymph actually moving without first disturbing it!

 

As nymphs of other shieldbug species - Birch, Parent Bug and Bronze - were also seen on fenceposts, it's hardly a remarkable observation in itself. However, it's noted that the nymphs of these other bugs were found in considerably smaller numbers.

 

Why are they there?

 

The most likely explanation is probably random chance! However, one possible advantage of this habitat is that birds are know to feed on bugs in trees over the winter period. Consequently a nymph perched on the side of a fencepost - where many are found - is perhaps less likely to be predated?

 

How do they survive there?

 

Although Forest Bug nymphs can obviously cope with cold conditions a bare fencepost is a much more exposed environment than the bark of a tree. As nymph counts were noticeably lower following long periods of cold weather, and as very few dead specimens were found, perhaps they seek shelter elsewhere under such conditions?

 

The other relevant issue, of course, is how do they feed? Of all the hundreds of nymphs examined over the monitoring period on only one occasion (10 April 16) did I see a second-instar which might have been feeding. This was largely inferred from the fact that it appeared to have its rostrum extended due to its peculiar stance. Unfortunately it dropped to the ground whilst I was attempting to photograph it, and so this can't be confirmed.

 

There is an interesting section in Shieldbugs of Surrey by R D Hawkings addressing the question of how Forest Bug nymphs feed (in trees) over the winter months, and although the bugs were thought to be largely vegetarian, opportunistic feeding on dead insects is mentioned as a possibility. Although there would appear to be no shortage of such potential food items on the fenceposts, perhaps the nymphs go elsewhere for sustenance?

 

How long do they stay there?

 

It seems implausible to me that a second-instar found on a line of fenceposts in September might still be there the following April! One possible way of providing some data on this question is to select a section of fencing and periodically photograph every nymph found there over a period of time. Although I can foresee a number of potential problems with this scheme, I might give it a go anyway!

 

How representative are they of the wider nymph population?

 

Although I don't think this question can be reliably answered I think it's reasonable to infer that the fencepost nymphs moult from one instar to the next at the same time as those elsewhere in Finglandrigg Wood. Thus, with the exception of the first-instars (due to lack of data), I have no reason to doubt the validity of the life-history charts in terms of the duration of each of the nymphal stages, when due allowance is made for the limited number of counts.

 

3 January 2017

 

(Text likely to be revised at some stage!)

 

 

Done