Peppered moth (Biston betularia)

KingdomAnimalia
PhylumArthropoda
ClassInsecta
OrderLepidoptera
FamilyGeometridae
GenusBiston (1)
SizeAdult wingspan: 45-62 mm (2)
Caterpillar length: up to 60 mm (3)

Common and widespread (2).

In its typical form, the Peppered Moth has pepper-and-salt camouflage pattern. In some areas it also has a sooty black or ‘melanic’ form known as carbonaria (2) (4). The long, stick-like caterpillar may be various shades of brown or green, with small warts and projections that resemble bark. The head is deeply notched (3).

This species is widespread throughout most of the British Isles and often fairly numerous (2). The melanic form is most frequent in the industrial areas of central Scotland, northern England, the Midlands and London, but is currently declining (5).

Found in woodland, hedgerows, parks and gardens, even in urban areas (6).

This species is single-brooded, with a protracted emergence. Adults are on the wing from May into August. Females lay their eggs in large batches, but the newly hatched caterpillars soon disperse. They spin silk threads and float off downwind until they land again by chance. Fortunately they can eat a very wide range of deciduous trees and shrubs. Caterpillars feed only at night, and are full-grown in September. The pupal stage overwinters in the soil and adults emerge the following spring (6).

The Peppered Moth has been widely used as a textbook example of evolution by natural selection. During the industrial revolution, sooty deposits darkened much of the habitat. The melanic form of the moth was first recorded in Manchester in 1848. Within 50 years it had almost replaced the typical form both there and in other industrial areas (5) Classic experiments carried out by Ketterwell during the 1950s suggested that bird predation was the crucial factor. The melanic form was better camouflaged when resting on sooty tree trunks and branches than was the paler typical form, hence it survived better. Conversely, the typical form was at an advantage in unpolluted areas where the tree bark was covered in lichens.

In recent years, various doubts have been cast on this simplistic explanation, and less fairly on the quality of Keterwell’s work (5) (7). While his pioneering experiments were undoubtedly flawed when judged by modern standards, his basic premise is still accepted by most evolutionary biologists. Although the full picture may well have been more complicated.

Following the Clean Air Acts, introduced from 1964 onwards, smoke pollution and soot deposition have been greatly reduced. The melanic form of the Peppered Moth has now lost its advantage and undergone a dramatic decline in frequency. It is now scarce in areas where it previously dominated and may soon disappear completely. Whatever the finer details of its rise and fall, the carbonaria story is a fascinating one.

Not currently threatened.

No conservation action has been targeted at this species.

Enjoying Moths by Roy Leverton (Poyser).

Hooper, J. (2002) Of Moths and Men: Intrigue, tragedy and the Peppered Moth. Fourth Estate, London.

Information supplied and authenticated by Roy Leverton with the support of the British Ecological Society:
http://www.britishecologicalsociety.org/

  1. National Biodiversity Network Species Dictionary (March 2003): http://www.nhm.ac.uk/nbn/
  2. Skinner, B (1984) Colour identification guide to moths of the British Isles. Viking, Middlesex.
  3. Carter, D.J. & Hargreaves, B. (1986) A field guide to caterpillars of butterflies and moths in Britain and Europe. William Collins sons and Co. Ltd., London.
  4. Leverton, R. (2001) Enjoying moths. T & AD Poyser, Ltd., London.
  5. Marjerus, M.E.N. (1998) Melanism: Evolution in Action. Oxford University Press, Oxford.
  6. Leverton, R. (2004). Pers. comm
  7. Hooper, J. (2002) Of Moths and Men: Intrigue, tragedy and the Peppered Moth. Fourth Estate, London.