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Journal of the Lepidopterists' Society 47(1), 1993, 17-21
BISTON BETULARIA (GEOMETRIDAE), THE PEPPERED
MOTH, IN WIRRAL, ENGLAND: AN EXPERIMENT
IN ASSEMBLING
Cyril A. Clarke
Department of Genetics and Microbiology, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, England
Frieda M. M. Clarke
43 Caldy Road, West Kirby, Wirral L48 2HF, England AND
Bruce Grant
Department of Biology, The College of William and Mary, Williamsburg, Virginia 23185
ABSTRACT. The melanic form of the peppered moth, Biston betularia form "car-bonaria," has continued to decline in frequency, comprising only 25.8% of a sample of 933 moths trapped on the Wirral peninsula near Liverpool, England, in 1991. The large sample was made possible, in part, by the use of two assembling traps and a mercury vapor trap. The assembling traps held either females of the North American subspecies, Biston betularia cognataria, or native British B. betularia females, thus allowing a comparison of the relative effectiveness of the two forms in attracting local males. Our results indicate that British B. betularia males do not discriminate between the mating phero-mones released by the females of the two races.
Additional key words: "carbonaria," "cognataria," pheromone, industrial melanism.
Previous papers by Clarke et al. (1985, 1990) document the fall in frequency of the melanic form of the peppered moth, Biston betularia f. "carbonaria," at Caldy Common, West Kirby, on the Wirral peninsula near Liverpool, England, during the years 1959 through 1989. Figure 1 extends the census showing a slight "hiccup" in 1990 with "carbonaria" up from the previous year from 29.5% to 33.1%. The 1990 sample, however, was limited to only 154 moths, including 51 "carbonaria," 6 intermediates (=f. "insularia"), and 97 pale typicals. In 1991 "carbonaria" dropped precipitously to 25.8%, the lowest figure so far recorded.
The continued decline evidently reflects major habitat modification resulting from reduced industrial pollution accompanying the Clean Air Acts begun in the 1960's although it remains unclear what ecological factors are involved. The typical form of the moth, once widely thought to gain protection from predators by its resemblance to gray foliose lichens, is rapidly replacing "carbonaria" as the common form in the virtual absence of such lichens (Clarke et al. 1985, Grant & Howlett 1988). Clarke et al. (1985) also noted a gradual lightening of the trees in the absence of industrial soot, and Grant and Howlett (1988) have
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Journal of the Lepidopterists' Society
total catchM 283 431 994 478 394 610 207 318 269 490 631 407 689 860 444 219 933 I 225 | 839 I 982 I 299 | 477 I 848 I 276 313 I 446 I 355 I 637 I 122 I 352 I 805 | 170 1154 I
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85H \/ \.
80—J ^
65—j \
60 I \
55— V
50 —j \
40—] •-«
35—J \ A
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25 J •
20~l ' I ' I ' I ' I ' I ' I T I ' I ' I ' I ' I ' I ' I ' I ' I ■ | ' 1 59 61 '63 '65 *67 '69 '71 '73 '75 '77 '79 '81 '83 '85 '87 '89 '91
year FlG. 1. The decline in the proportion of the melanic form of the peppered moth, Biston betularia f. "carbonaria," in West Kirby, Wirral, England. "Carbonaria" is expressed as a percentage of a total in which typicals and "insularia" are combined. The total number of B. betularia trapped in the 33 year period was 15,969 of which only 86 were female (0.54%).
further suggested that the comeback of birch trees (Betula pendula) following the Clean Air Acts may afford suitable hiding places for both the typicals and the melanics. Atmospheric sulfur dioxide concentrations at West Kirby have dropped markedly during this same period, and with minor fluctuations have remained low in recent years (see fig. 2 in Clarke et al. 1990). The mean winter S02 levels (jug per cubic meter) were 20.17 in 1988, 25.25 in 1989, 31.89 in 1990 and the mean for January and February 1991 was 44.39. Whatever the cause, the decline in the frequency of the melanic variant of this species at this location continues.
A second component of our study involved the method of trapping. All 1990 moths were trapped by M V light (mercury vapor lamp) because we had no female Biston betularia available that year for use in an assembling trap (AT). The AT is a cage containing virgin females to which local males are attracted by airborne pheromones released by the captive females, i.e., the female odor serves to lure males to the trap. In 1991 we used both MV and AT methods, and we also had the
Volume 47, Number 1
19
opportunity of altering the assembling techniques by using virgin female Biston betularia cognataria (Guenee) deriving from a large number of pupae brought by one of us (BG) from Virginia, USA.
Biston betularia cognataria is the North American equivalent of Biston betularia (L.). It has typical and melanic (f. "swettaria") forms, as well as intermediates grouped together as "insularia" (see West 1977). However, typical cognataria generally are darker in appearance than British typicals, and, in fact, have been described as resembling intermediate-grade British "insularia" (see Kettlewell 1973:plate 9.1). Kettle well (1973) regarded cognataria and betularia as distinct species because of differences in color and in behavior in the early stages, there often being bivoltinism in the former but never in the latter. Rindge (1975), on the other hand, concluded that the evidence indicates that cognataria and betularia are members of the same biological species. They can be fully interbred and there is no disturbance of the sex ratio in the hybrids, and both male and female genitalia are identical or nearly so, as is the structure of the male antennae.
We used cognataria typicals from Virginia (about 20 at a time in the core of the trap and frequently replenished with fresh females as they emerged) to attract British B. betularia males throughout June 1991. Towards the end of this month the cognataria emergences were tailing off and the females old. Then our British B. betularia virgins (deriving from 20 pupae kindly supplied by Tony Liebert) began to eclose, and we used them in a different trap during July. Our original intention was to produce a quantitative bio-assay of relative pheromone effectiveness, but as we were unsuccessful in coordinating eclosions of the two kinds of females, we were unable to run both assembling traps simultaneously throughout both months. While the period of overlap when both cognataria and betularia females were available as lures in separate traps extended into July, we must emphasize that the number of individuals present in the cores, and their ages, unfortunately, were not controlled.
Nevertheless, the qualitative evidence that the cognataria pheromone is a powerful attractant to betularia males is compelling (Table 1). In June, when only cognataria females were releasing pheromone from the traps, 329 betularia males were drawn in as compared to only 109 males captured by the MV during that same period. During July, when both kinds of females were "calling" simultaneously from separate assembling traps, the numbers of males lured to the traps did not differ significantly by chi-square (144 to the cognataria-AT versus 171 to the betularia-AT, x2 = 2.31, df = 1, 0.10 < P < 0.25). To our knowledge such data on comparative assembling have not been published previously, and, as we have no reason to conclude that the mating pheromones
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Journal of the Lepidopterists' Society
Table 1. Biston betularia catches at West Kirby, Wirral, England, 1990 and 1991. Only the MV trap was used in 1990 as no virgin females were available. The 1991 combined catch figures also are subdivided to show numbers taken by each of the three traps used: a) MV trap; b) assembling trap using betularia females; c) assembling trap using cognataria females.
|
Year |
Trap used |
Total catch |
Carbonaria |
Insularia |
Typical |
% carbonaria |
|
1990 |
MV only |
154 |
51 |
6 |
97 |
33.1 |
|
1991 |
All traps combined |
933 |
241 |
37 |
655 |
25.8 |
|
1991a |
MV trap |
289 |
78 |
15 |
196 |
26.9 |
|
1991b |
betularia- AT |
171 |
41 |
4 |
126 |
23.9 |
|
1991c |
cognataria- AT |
473 |
122 |
18 |
333 |
25.8 |
of cognataria and betularia are different, our findings are consistent with Rindge's (1975) assessment that cognataria and betularia are con-specific.
It is true that pheromones occasionally are not entirely species specific, because Clarke (1979) showed that Orgyia thyellina Butler (Ly-mantriidae) females from Japan regularly assembled Orgyia antiqua (L.) males in the Wirral, though this is probably an exception to the general rule. The point, however, must be made that Priesner (1975) thought the pheromones of all Orgyia species were similar, but the findings were made by antennogram techniques which are not particularly sensitive. If the pheromones in the genus Orgyia were alike there would be mating chaos where species fly together, and we know from observation that O. recens Hiibner and O. antiqua do not attract each other (Greenberg et al. 1982).
A final point in the 1991 series relates to the placement of the three traps. Jones, Majerus, and Timmins (unpublished) have produced evidence that in five polymorphic moth species in which melanism is thought to be of ancient origin, great sampling differences in morph frequencies over very short distances occur depending on the local environments in which traps are placed. They suggest that such habitat selection is not likely to be present where the melanism is of recent origin, as in Biston betularia, and our 1991 data support this view. Specifically, our MV trap was completely in the open, the cognataria-AT under a very thick, old oak tree, and the betularia-AT in a relatively exposed position near a small birch tree. Yet, all three morph frequencies were proportionately represented in all three trapping locations. In fact, the three data sets are remarkably homogeneous by G-test (G = 3.28, df = 4, 0.5 < P < 0.75).
In summary, the 1991 sample of 933 Biston betularia at West Kirby shows a return to the lowering of the proportion of f. "carbonaria," and we report that much of the assembling data resulted from using the American subspecies, B. b. cognataria.
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21
Acknowledgments
We are grateful to Tony Liebert for providing B. hetularia pupae; Sally Thompson for technical assistance throughout the study; R. J. Agass, Director of Housing and Environmental Protection, Metropolitan Borough of Wirral, for the S02 levels; M. E. N. Majerus for sending us a draft of his submitted manuscript; and the Nuffield Foundation for support.
Literature Cited
Clarke, C. A. 1979. Some observations on Orgyia thyellina Butler and Orgyia antiqua
(L.). Entomol. Rec. 91:315-316. Clarke, C. A., G. S. Mani & G. Wynne. 1985. Evolution in reverse: Clean air and the
peppered moth. Biol. J. Linn. Soc. 26:189-199. Clarke, C. A., F. M. M. Clarke & H. C. Dawkins. 1990. Biston hetularia (the peppered
moth) in West Kirby, Wirral, 1959-1989: Updating the decline in f. carbonaria. Biol.
J. Linn. Soc. 39:323-326. Grant, B. & R. J. Howlett. 1988. Background selection by the peppered moth (Biston
hetularia Linn.): Individual differences. Biol. J. Linn. Soc. 33:217-232. Greenberg, S., A. H. Wright & C. A. Clarke. 1982. Contrasting results in assembling
experiments using Orgyia thyellina Butler, O. recens Hiibner and O. antiqua (L.).
Entomol. Rec. 94:25-27. Kettlewell, B. 1973. The evolution of melanism. Clarendon Press, Oxford. 423 pp. Priesner, E. 1975. Electroantennogram responses to female sex pheromones in five
genera of Lymantriidae (Lepidoptera). Z. Naturforsch. 30:676-679. Rindge, F. H. 1975. A revision of the New World Bistonini (Lepidoptera: Geometridae).
Bull. Am. Mus. Nat. Hist. 156:69-155. West, D. A. 1977. Melanism in Biston (Lepidoptera: Geometridae) in the rural Central
Appalachians. Heredity 39:75-81.
Received for publication 17 March 1992; revised and accepted 31 August 1992.