Shaw, Mark R

English

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Shaw, Mark R. (2013) Moth populations and bad weather – four speculative 
observations. Entomologist's Record and Journal of Variation , 125 (1). pp. 33-37.  
 
 
 
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Deposited on: 19 October 2013 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Moth populations and bad weather – four speculative observations (published in 
Entomologist’s Record and Journal of Variation (2013) 125: 33-37. Minor style/format 
corrections may have been made by the Editor) 
 
Mark R. Shaw, 
Honorary Research Associate, 
National Museums of Scotland, 
Chambers Street, 
Edinburgh EH1 1JF 
 
There is no doubt in my mind that fifty years ago substantial defoliation of more 
than just spindle and bird cherry trees was not really unusual; that the regular cleaning of 
car headlamps and even radiator grills was necessary in summer; that garden buddleia 
and valerian were always plastered with butterflies and other insects practically the whole 
summer long; and that an open lavatory window with the light left on made a reliably 
high-yielding moth trap. These things seemed fairly constant, though I suppose it is 
possible that my memory has dumped the bad years. If I never again experience the (quite 
noisy!) steady rain of caterpillar faeces falling from oak woodland canopy in spring, or 
myriad moths fluttering in the car headlights down every country lane, it is because 
various long term trends over the past half century (unrelated to global warming per se, 
albeit with similar roots) have progressively made Britain unsuitable for high populations 
of most species of Lepidoptera. However, there are also shorter-term effects (that is, ones 
that may not persist) that result simply from runs of unsuitable weather. Whether or not it 
is right to categorise recent events as aspects of real climate change (and even then, 
whether or not as a consequence of global warming), such runs are not new and are to be 
expected simply on a stochastic basis. So optimists can say they may not last – though 
there is no denying that in the past the British environment has undoubtedly presented 
much more favourable conditions for moth and other insect populations to recover than at 
present.  
My main interest in Lepidoptera is as hosts for parasitic wasps, so I habitually try 
to collect caterpillars, leaf mines, etc. in bulk in order to rear these fascinating and little-
studied insects. Often, in order to obtain livestock of particular parasitoids for 
experiments, I have collected the same species of common Lepidoptera caterpillars over 
several years, sometimes witnessing declines and even local extinctions of species 
apparently as a result of abnormal runs of unsuitable weather. This has led to a few 
observations and ideas that might be worth sharing, although unfortunately I had not 
usually collected the kind of quantitative data that would raise my recollections and 
comments above being merely anecdotal and speculative. 
 
(1) Effect of cold springs on overwintered adults.  
 
In the early part of summer 1980, a few weeks after coming to live in Scotland, I 
was able to collect larvae of the choreutid moth Choreutis pariana (Clerck) in the 
southern Edinburgh suburbs in sufficient numbers to assess not only its parasitoid 
complex, but also its distribution across the various rosaceous trees and shrubs present (it 
was found on six species in four genera, and was used by several species of parasitoid: 
Shaw, 1984). This day-flying moth overwinters as an adult in both sexes, and depends on 
spring warmth for the crucial activities of feeding, mating, and maturing and laying its  
eggs. The population had been known in the area for many years (E. C. Pelham-Clinton, 
pers. comm. in 1980), and there are published records of it from as early as 1857 (Logan, 
1958) traceable via the Scottish Insect Records Index (cf. Shaw, 1987) maintained at the 
National Museums of Scotland (NMS), as well as Edinburgh specimens in the NMS 
collection from 1950 and 1956 (A. B. MacNicol), and probably also from various 
occasions in the first half of the 20
th
 century (R. F. Logan, K. J. Morton) but these lacking 
clear data. 
Following my survey there was a run of particularly miserable springs in the mid 
1980s, which were particularly short of sunshine, and by 1987 (when I next looked) it 
was not possible to find any evidence of the larvae of C. pariana even on its most 
favoured tree (Malus). This situation has persisted to the present (I still search briefly for 
it every year), and it is clear that a local extinction took place, almost certainly coincident 
with the consecutive run of adverse springs. Perhaps recolonisation will eventually occur, 
or perhaps there is no longer a nearby population able to access the Edinburgh suburbs. 
The butterfly Gonepteryx rhamni (Linnaeus) has a life cycle broadly similar to 
that of C. pariana, and during the same run of cold and sunless springs its population in 
the places that I regularly visit in the Morecambe Bay area of S. Cumbria and N. 
Lancashire seemed also to suffer badly, though fortunately without being extinguished. 
 
(2) Effect of cold springs on common early-feeding arboreal caterpillars. 
 
For several years, sporadically between 1989 and 2007, I used to beat various 
hedges in May, mainly concentrating on blackthorn and to a lesser extent hawthorn, in S. 
Cumbria/N. Lancashire in order to collect caterpillars of (especially) the geometrids 
Operophtera brumata (Linnaeus ) and Theria primaria (Haworth ), both of which 
supported parasitoids that I needed for a succession of experiments that lasted several 
years. These moths have flightless, non-feeding females and do not depend on good 
spring weather for mating and oviposition, as that precedes bud-burst. However, female 
flightlessness does mean that local movements and hence replenishment of populations 
tend to be sluggish.  
During the years from 1989 to about 1994 there was no difficulty in finding good 
numbers of these (and other) caterpillars at the right time in May, but from about 1995 
onwards (and the phenomenon persists to the present time in that area) the populations in 
exposed hedges declined so strongly that by 2003 it became commonplace to beat for up 
to ten minutes or even longer between finds of any kind of caterpillar. (No, I wasn’t 
always beating in the same place!) Although some of my sites were field hedges, the 
general area in which I was collecting was almost devoid of arable crops (and some sites 
were on Nature Reserves), so there seemed little reason to blame agrochemicals for the 
decline; and, indeed, populations of later-feeding caterpillars that I was also collecting, 
such as the geometrid Lomaspilis marginata (Linnaeus) feeding on Salix in the same sites 
in July and August, seemed relatively unaffected.  
It was only when in 2003 I beat along a hedge adjoining the busy A6 road on one 
side with a bustling factory on the other that I felt I understood the phenomenon.  Here 
the populations of the various early-feeding caterpillars, including O. brumata and T. 
primaria, had not suffered the same catastrophic decline, and in fact were as high as I had 
remembered their being of old, in strong contrast to quieter hedges. A logical explanation 
seemed to be reduced bird predation compared with elsewhere, as a result of continuous 
daytime disturbance. There had been a recent and more or less continuous run of cold 
springs, and the time needed for the caterpillars to become fully grown and leave the 
birds’ foraging arena would have been prolonged by that, such that the consistent 
predation pressure from (endothermic) birds would have had an enhanced impact on the 
(exothermic) insect populations in areas in which the birds were not deterred. After a run 
of such springs, a really major decline was apparent where birds could be fully active. (It 
is also possible that the hedge between the road and the factory was made significantly 
warmer, allowing faster larval development and a better escape from predation in that 
way too). 
 Indeed, this sort of effect seems likely to be a major factor in the climatic limits 
of distribution for many insects that need to feed in situations in which they are 
vulnerable to predation by vertebrates – if it isn’t warm enough to feed up quickly, 
predation levels may simply be too high for a population to persist. In this respect bird 
predation might also have contributed to the declines mentioned in (1) above. 
 
(3) The effect of wet foliage on particularly small adults? 
 
During the early and mid 1970s I was part of a project based at Manchester 
University that surveyed the parasitoids of arboreal leaf-mining Phyllonorycter 
(Gracillariidae) species (Askew & Shaw, 1974). For this, moderately large samples of the 
host mines were collected from various trees at many sites, largely in N. W. England, and 
it was generally found possible to collect large samples, at least of the commoner species, 
just about everywhere we tried. Indeed, one sampling programme involved collecting at 
least 90 Phyllonorycter mines from each of oak and birch at the same site twice a week 
(from mid-June to mid-October), which – even in the sparser first generation – was easily 
accomplished within a couple of person-hours, and we simply moved our pitches on a 
few yards on successive occasions, with no real worry that our sampling was heavy 
enough in relative terms to distort the populations of the insects we were studying 
(Askew & Shaw, 1979).  
It so happened that in 2012 I made a promise to others to try to collect and rapidly 
preserve several individuals of the exothecine braconid wasp Colastes braconius Haliday, 
which is a regular parasitoid of Phyllonorycter (among a wide range of other leaf-mining 
hosts). From the 1970s I knew that well-grown Phyllonorycter species on alder and 
sycamore were attractive to this parasitoid (Shaw & Askew, 1976), so I attempted to 
collect more or less fully-formed mines from these trees in Fife (July 13-15).  However, 
even with the help of Keith Bland, I could collect only half a dozen mines on alder and 
two on sycamore in a total of about 3 person-hours of searching at several sites. 
Subsequently in S. Cumbria (August 4-5) I failed to find a single mine on alder at the 
alder-dominated Borrodale Wood near Tebay in two hours, and found only 14 mines on 
sycamore at sites around Witherslack in another two hours. (Brief examination of several 
different trees suggested that other Phyllonorycter species were doing no better.)  
Even for the usually relatively less numerous first generation, this is pretty 
abysmal – and, although unfortunately I have no idea of the size of the relevant 
populations in the autumn of last year, it is hard to believe that the atrocious weather in 
the first half of the 2012 summer was not playing a significant part. These Phyllonorycter 
species overwinter as pupae, the adults then needing to feed, mate and oviposit in early 
summer. The abnormally wet and miserable May and June this year wouldn’t have made 
that easy (though perhaps nowhere near as bad as for species overwintering as adults, 
with additional needs in spring), but it might have been especially hazardous for females 
of such small and frail moths to negotiate the persistently sodden tree leaves onto which 
they would easily bedraggle – even though actual oviposition is on the leaf underside (in 
all but one of the species being sought).  
Of the mines collected, only two were parasitised (by eulophid chalcidoids, not by 
C. braconius), this low level (cf. Askew & Shaw, 1979) perhaps suggesting that these 
tiny parasitoids were also finding it difficult to deal with persistently wet leaves. 
Interestingly, although avian predation might have been expected to be higher in a cold 
summer (see above), only two of the very conspicuous mines on sycamore seen at 
Witherslack had been predated by birds, possibly because their very low density hadn’t 
promoted an effective search image. 
 
(4) Horses for courses... 
 
A brighter note in conclusion is that at Angle Park, Fife, in mid July 2012 Keith 
Bland and I found very large numbers of two nymphuline pyralids, Elophila nymphaeata 
(Lnnaeus) and Nymphula stagnata (Donovan) flying around a shallow lochan with good 
growths of Potomogeton and Sparganium. So perhaps not all moths were suffering! 
 
 
References 
 
Askew, R. R. & Shaw, M. R. (1974). An account of the Chalcidoidea (Hymenoptera) 
parasitising leaf-mining insects of deciduous trees in Britain. Biological Journal of the 
Linnean Society 6: 289-335. 
 
Askew, R. R. & Shaw, M. R. (1979). Mortality factors affecting the leaf-mining stages of 
Phyllonorycter (Lepidoptera: Gracillariidae) on oak and birch. 1 Analysis of the mortality 
factors. Zoological Journal of the Linnean Society 67: 31-49. 
 
Logan, R.F. (1858). Entomological notes for 1857. I Lepidoptera. Proceedings of the 
Royal Physical Society of Edinburgh 1: 406-410.  
 
Shaw, M. R. (1984). Foodplants and parasites (Hymenoptera: Ichneumonoidea) of 
Choreutis pariana Clerck) (Lepidoptera: Choreutidae) in Edinburgh. Entomologist’s 
Gazette 35: 41-44. 
 
Shaw, M. R. (1987). Scottish insect records. Entomologist’s Record and Journal of 
Variation 99: 37-38. 
 
Shaw, M. R. & Askew, R. R. (1976). Ichneumonoidea (Hymenoptera) parasitic upon 
leaf-mining insects of the orders Lepidoptera, Hymenoptera and Coleoptera. Ecological 
Entomology 1: 127-133. 
 


An account of the Chalcidoidea (Hymenoptera) parasitising leaf-mining insects of deciduous trees in Britain.

Entomological notes for 1857. I Lepidoptera.

Foodplants and parasites (Hymenoptera:

Ichneumonoidea (Hymenoptera) parasitic upon leaf-mining insects of the orders Lepidoptera, Hymenoptera and Coleoptera.

Mortality factors affecting the leaf-mining stages of Phyllonorycter (Lepidoptera: Gracillariidae) on oak and birch. 1 Analysis of the mortality factors.

Scottish insect records. Entomologist’s Record and

Moth populations and bad weather – four speculative observations

Moth populations and bad weather – four speculative observations

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