An interim report on the regular sampling of the moth population at Fen Street, Hopton, Suffolk

AN INTERIM REPORT ON THE REGULAR SAMPLING OF THE MOTH POPULATION AT FEN STREET, HOPTON (OVERLOOKING MARKET WESTON FEN), SUFFOLK MICHAEL R .

HALL

The aim of this project is the long term monitoring of changes in the moth fauna, both overall and of individual species, at a single site by regular, consistent sampling. Results from the project can then be compared with those from a much more widespread, but less regular or consistent, trapping programme throughout the surrounding area. This project was conceived and instituted by the late Prof. Harry Caswell, to whose memory, inspiration and guidance this interim report is dedicated.

The intention is to sample on a lunar monthly basis, for two hours from dusk, for a period of 10 years. Practically, sampling has been carried out on a four/five week cycle on a suitable night as close as possible to the new moon, which has meant that to date records have been obtained on only 11 nights in each 12 month period.

The Trapping Method and The Site The trap consists of vertical and horizontal sheets giving a supported back 210cm long by 150cm high, with an equal sized ground sheet in front illuminated with a 125 watt mercury vapour u/v bulb at ground level. The bulb is positioned on the mid line of the ground sheet, 50cm in front of the backing sheet. The trapping site is on a meadow overlooking the north eastern end of Market Weston Fen, at grid ref. TL 983789. It is situated adjacent to Fen Street, Hopton, backing on to a hawthorn hedge and facing the fen, which is 50m away at its nearest point. Here the fen is extended as a neck along the course of a stream, with lighter land rising beyond.

The light is turned on at dusk, which is decided subjectively, and for the next two hours all moths that come to the sheets (front or back) or settle in the grass just off the ground sheet are caught and retained till the end of the trapping session. Most moths are identified at the time, the number for each species recorded, and then all released when the light is extinguished. Those that cannot be identified on site are retained for subsequent determination and this information added to the records. During peak times there has been additional assistance with the catching to ensure that all individuals attracted by the light were recorded. On each night the temperature at the start, after one hour, and at the finish of the trapping session is recorded. This is taken to the side of the sheet, at 60cm above ground level.

Details are given here only of catches from the Fen Street trap. However, some comparisons are made with species and populations from catches being made by the author from other sites throughout Norfolk and Suffolk, as part of the on-going survey being carried out by the Norfolk Moth Survey and the Suffolk Moth Group. These can only be subjective, however, as trapping techniques are different in the wider survey.

Trans. Suffolk Nat. Soc. 26 (1990)

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T H E MOTH POPULATION OF FEN STREET, HOPTON

The Records During the four years to date - from October 1985 to October 1989 - a total of 301 species of moths have been recorded. Of these, 52 have been 'micros' and 249 'macros'. In each year there have been some species exclusive to that year, often represented by just a single moth, and only 18% (54 species) have been recorded each year.

Table 1 Number of species recorded and regularity of appearance Total number of species recorded 1985 to 1989 Number of species recorded in all 4 years Number of species recorded in any 3 years Number of species represented by a single moth

301 54 56 58

Of the 58 species represented by singletons, 10 have been 'micros' and the remaining 48 'macros'. One of the 'micros' that was particularly interesting was the Tortrix Sparganothis pilleriana D. & S., which was recorded on the 16th July 1988. Its described distribution (Bradley, Tremewan & Smith, 1973) is from the southern counties of England and South Wales, where it is known from both coastal marshes and inland bogs, the two habitats from which it was recorded in Suffolk in 1988 (Watchman, 1989). Other fen and marsh specialities were also only recorded as singletons, the flame wainscot, Senta flammea Curt., in 1987, and the silky wainscot, Chilodes maritimus Tausch., in 1986. Also 1986 was the only year in which the reed dagger, Simyra albovenosa Goeze, was seen, but it did occur on the 15th June, 6th July and 4th August with a total of four individuals coming to light. This surprisingly low frequency of species that are known from Market Weston Fen, and come to light in greater numbers to traps set in the fen, questions the view that the attracting range of a 15W actinic tube may be over 200m for some species, if presented with an unobstructed view of the trap on a moonless night (Bowden, 1982). The more powerful 125W bulb used at Fen Street had an unobstructed range of well in excess of 300m with a substantial area of the fen no more than 75m away, and all recording nights were moonless..

One least yellow underwing, Noctua interjecta caliginosa Schaw., was recorded in 1986 and, although an inhabitant of fenland, it is a species also to be found locally on sandhills and waste ground. Both the common heath, Ematurga atomaria atomaria L., and the autumnal rustic, Paradiarsia glareosaglareosa Esper, were recorded as singletons in 1986. These, together with the grass emerald, Pseudoterpna pruniata atropunctaria Walk., seen 16th July 1988, the white colon, Sideridis albicolon Hb., which was recorded - 2 n d July 1989, and the heart and club, Agrotis clavis Hufn., on 4th June 1989, are more usually associated with heathland and would probably have come from the lighter land beyond the neck of the fen. The heathland species may be more mobile than the strictly fenland moths, and thus stray more

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Suffolk Natural History, Vol. 26

readily into the sphere of influence of the lamp. However, all these speciesand particularly the autumnal rustic and the grass emerald - have been more noticeable and numerous during the last two years of the more widespread recording programme and their populations could be increasing.

Two species having a wider range of habitats, but nevertheless not excessively common, the brown scallop, Philereme vetulata D. & S., one in 1988; and the triple-spotted clay, Xestia ditrapezium D. & S., one in 1989, are both moths that have also been recorded more frequently in the last year or so of the county surveys.

Several typically woodland moths have been recorded as singletons, the most notable being the frosted green, Polyplaca ridens Fabr., in 1986, the pale oak beauty, Serraca punctinalis Scop., in 1987, and the purple clay, Diarsia brunnea D. & S., in 1986. Similarly a moth of open hedgerows, the maple prominent, Ptilodontella cucullina D. & S., appeared as a singleton in 1989. It was not unexpected to record these species at a low incidence at this site as this level corresponds with their general incidence in Norfolk and Suffolk.

The brick, Agrochola circellaris Hufn., which was recorded as a singleton in 1987, is a moth that has declined quite noticeably in the last few years. In the early 1980s it was frequently represented by several specimens at any one site in virtually every year, but in the last two or three years it has hardly been seen at all. The larvae prefer the flowers and seeds of Wych Elm, and also Ash, (Skinner, 1984), and a lack of this foodstuff may explain the general decline of this moth.

It is surprising that the lackey, Malacosoma neustria L., common wave, Cabera exanthemata Scop., nut-tree tussock, Colocasia coryli L., and beautiful hook-tip, Laspeyria flexula D. & S., seen in 1986, together with the engrailed, Ectropis bistortata Goeze, and early grey, Xylocampa areola Esp., in 1988, and the scalloped oak, Crocallis elinguaria L., in 1989 have only been seen as singletons. All these species are widespread throughout Norfolk and Suffolk and I would have expected to have seen all of them represented each year at the Hopton site, if only in ones and twos.

Both the poplar hawk-moth, Laothoe populi L., and the privet hawkmoth, Sphinx ligustri L., have only been recorded as singletons, but this is not entirely unexpected as the hawk-moths tend to fly late in the night and the trapping sessions often end before they can be expected to be on the wing.

As would be expected, the vast majority of the species that have been seen in each of the four years are generally common, and it is in this group that those species recorded with 10 or more individuals on any one night (see table 2) are to be found. By far the most regular and numerous moth to date has been the flame shoulder, Ochropleuraplecta L., although even this moth was much less in evidence in 1989. Contrary to experience elsewhere, the smoky wainscot, Mythimna impura impura Hb., has been much more numerous than the common wainscot, Mythimna pallens L., at the Hopton site. Numbers of the drinker, Philudoria potatoria L., the shuttle-shaped dart, Agrotis puta puta Hb., and the dark arches, Apamea monoglypha Hufn., have all declined as the project has proceeded, which is a reflection of the wider picture.

Trans. Suffolk Nat. Soc. 26 (1990)

T H E MOTH POPULATION OF FEN STREET, HOPTON

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Table 2 Yearly totals of recording hours, species and those species occurring with some abundance or as singletons 1985/6 Recording nights/year* Hours recording/year** Species/year Moths/year Index of moths/species for each year Species recording 20+ moths on any one night Species with 10-19 moths on any one night Species represented by just one moth in the 4 year period Species found only in one year

11 22 181 890

October-September 1986/7 1987/8

1988/9

11 21VS 184 928

11 21 Vi 153 633

4.9

5.0

4.1

3.8

7

2

3

1

11

20

11

8

20

15

11

12

47

48

31

17

11 22 139 528

"in each year one night (in either January, February or March) was cancelled due to excessively hard frost. ** on one night in both the second and third years trapping was curtailed by heavy rain.

One moth that shows how the population of a species can fluctuate at a single site in a comparatively short time is the straw dot, Rivula sericealis Scop. In 1986 one was recorded on the 15th June, five on the 6th July, and seven on the 4th August. In 1987 there were five on the 28th June, 14 on the 2nd August and ten on the 23rd August. In 1988 three on the 12th June were followed by 31 on the 16th July and just one on the 16th August (which, incidentally, was the first moth to appear at the light). In 1989 none were recorded at all, and yet the moth was common to numerous at many of the other sites surveyed. This sort of variaton between years, and sites in the same year, makes it very difficult to draw any meaningful conclusions about changes in the status of any particular species from just single site or single year data. There is every expectation that the straw dot will be recorded in 1990 at something like the level of 1986/87.

When more general comparisons are made between the 4 years completed so far, there is a definite decline in both species and numbers recorded. This is both in overall terms throughout the year and in the peak numbers of a species.

However, there are still new species being recorded each year and species seen in 1989 that were not seen in all of the preceding years. More than anything else this would seem to confirm the comparatively limited range that is sampled by a moth trap and indicate the necessity of long term, regular trapping programmes when attempting to investigate changes in species and populations.

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Suffolk Natural History, Vol. 26

The marked drop in numbers of species and individuals between the second and third years and again between the third and fourth years is something that was also found in the more widespread trapping programme.

Whilst these figures seem to indicate a general decline in both species and overall moth numbers, when the temperatures during the recording periods are also considered these also fluctuate in the same way.

General experience from 200 nights recording at 49 different sites across Norfolk and Suffolk during the same 4 year period has indicated that 15°C is a critical 'barrier' temperature. Below this level the number of both species and moths flying is greatly reduced, irrespective of habitat. That same widespread programme has experienced a similar period of generally colder nights throughout much of 1989 (in spite of the hot, sunny days) and a proportion of 1988.

Table 4 shows that the number of both species and moths recorded at any one time depends on time of year as well as the temperature. There are more species that fly at the end of July and beginning of August than there are in late June. However, it is only when similar suitable conditions prevail Table 3 Temperature range during trapping hours and number of occasions when temperature above 15°C 1985/6 Number of nights when temperature above 15°C (for all or part) Trapping hours above 15°C Trapping hours 11-15°C Trapping hours 6-10°C Trapping hours below 6°C

3 6 6 5 5

October-September 1986/7 1987/8

4 8 6 2 5V4

5 5 9% 3 4

1988/9

3 2V2

111/2 6 2

Table 4 Details of single night in each year when most moths were recorded - 'best night'

Date of 'best night' Number of species % of total species Number of moths % of total moths Temperature range on 'best night' °C

Trans. Suffolk Nat. Soc. 26 (1990)

1985/6

October-September 1986/7 1987/8

1988/9

4/8/86 80 44 441 50

28/6/87 72 39 178 19

16/7/88 80 52 295 47

30/7/89 53 38 185 35

18-15

22-5-21

18-16

17-12-5

THE MOTH POPULATION OF FEN STREET, HOPTON

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throughout such a period that this becomes readily apparent from trap records.

Conclusions Throughout the four years for which records have been compiled a large number of species of moths have appeared fairly regularly and have been recorded at between five and 15 individuals a year (when they have occurred). Many of these are generally distributed moths that are to be found over most of eastern England. These moths include the buff arches, Habrosyne pyritoides Hufn., riband wave, Idaea aversata L., small fan-footed wave, Idaea biselata Hufn., red twin-spot carpet, Xanthorhoespadicearia D. & S., small phoenix, Ecliptoperasilaceata D. & S., v-pug, Chloroclystis v-ata Haw., green pug, Chloroclystis rectangulata L., bordered pug, Eupithecia succenturiata L., peppered moth, Biston betularia L., iron prominent, Notodonta dromedarius L., swallow prominent, Pheosia tremula CI., pale tussock, Dasychira pudibunda L., small square-spot, Diarsia rubi View., nutmeg, Dicestra trifolii Hufn., bright-line brown-eye, Lacanobia oleracea L., green-brindled crescent, Allophyes oxyacanthae L., small angle shades, Euplexia lucipara 1., rustic shoulder-knot, Apameasordens Hufn., burnished brass, Diachrysia chrysitis L., spectacle, Abrostola triplasia L. and many more, none of which have varied significantly either at Hopton or in the more general survey. As with the less common species, the fluctuations that there have been can be almost directly related to differences in weather conditions, and in particular in temperature, between recording nights.

It is well known that the population of a moth species can, and does, fluctuate at a given site. Much of the variation in numbers of these generally distributed species probably reflects changes due to many factors such as predation, weather conditions during hibernation, the habitat itself, and the influence of man, as well as weather conditions at flight time. When the project has run for the 10 years, it may be possible to comment on the more unusual cyclical changes typified by the slow build-up, or decline, of a species over long periods, the irregular contraction and expansion of range in some species noted for such a phenomenon, and on species which have not yet been recorded at Hopton. There have been 'good years' for some species 1988 was particularly good for the marbled white-spot, Lithacodia pygarga Hufn. and 1989 for the yellow horned, Achlyaflavicornis galbanus Tutt - and poor years' for others - the lesser yellow underwing, Noctua comes Hb. in 1989 - but this has always been the case. It will need another year like 1986, when conditions for moth flight were good for much of the summer, in the not too distant future before there can be any tentative conclusions as to the permanency of the declines recorded in 1988 and 1989.

It has become apparent that single site records of moth numbers (even if taken on a more regular basis than at Hopton) must be considered in conjunction with several fators before firm conclusions can be drawn as to general changes in the moth fauna. These factors are differences in weather conditions between years, the effect of such differences on individual moth species, changes in the habitat - frequently due to man's activities, and

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whether any variations in populations are site-specific or are more widespread. After only four years recording at Hopton the only definite conclusions that can be drawn from the data so far available, are that there are less moths to be seen in colder conditions, and that there are still 'new' species to be recorded at the trap site. It is hoped that after 10 years data on both moths and weather from the Fen Street site at Hopton will, in conjunction with more widespread records from the Norfolk and Suffolk moth surveys, show what is happening to our moths. The indications are that, like many other groups of plants and animals, they are declining quite quickly.

Nomenclature is from A Recorder's Log Book or Label List British Butterflies and Moths by J. D. Bradley and D. S. Fletcher, London: Curwen Books 1979.

Acknowledgements I am most grateful for comment and advice from G. M. Haggett. I thank J. L. Fenn, G. M. Haggett and A. Watchman for valued assistance with identification, Dr A. G. Irwin for allowing access to the Norwich Castle Museum collections and G. M. Haggett and S. R. Ward for their expertise and company on the many nights we have been gathering moth records throughout Norfolk and Suffolk. I also thank Hilary Vaughan for allowing access to the site. I am also most grateful to Roger and Hilary Huckel for generously allowing the use of their electricity, and particularly to Ray Mitchell for so readily and ably stepping in to provide continuing help with the practicalities of the project after the death of Harry Caswell in April 1989.

References Bowden, J. (1982). An analysis of factors affecting catches of insects in light traps. Bull. ent. Res. 72, 535.

Bradley, J. D., Tremewan, W. G., & Smith Arthur (1973 & 1979). British Tortricoid Moths Vol. 1 & 2. London: The Ray Society.

Goater, B. (1986). British Pyralid Moths. Colchester: Harley Books.

Skinner, B. (1984). Colour Identification Guide to the Moths of the British Isles. Middlesex: Viking.

Watchman, A. (1989). Notes on some Suffolk Moths in 1988. Trans. Suffolk Nat. Soc. 25,31.

M. R. Hall, 'Hopefield', Norwich Road, Scole, Diss, Norfolk, IP214DY.

Trans. Suffolk Nat. Soc. 26 (1990)