Melanism in Adalia bipunctata L. (Col., Coccinellidae) and its Relationship to Atmospheric Pollution

Abstract

The use of lichens as biological indicators of air pollution is well established (Hawksworth & Rose 1970; Gilbert 1970, 1974). In the British Isles the melanic form of the peppered moth (Biston betularia L.) also seems to be a good indicator of air pollution and the same may be true of a number of other species of Lepidoptera (Kettlewell 1973). The relationship between melanism in B. betularia and air pollution does not appear to be a simple one as this melanism is related to the amount of lichen cover on tree trunks. Thus the proportion of the melanic form of this moth in the population cannot be used as an independent measure of air pollution. The discovery of other organisms that are influenced by air pollution but not by the condition of tree trunks would be invaluable. The two-spot ladybird (Adalia bipunctata L.) in common with many other Coccinellids (Hodek 1973) is a highly polymorphic species. The various morphs are under the control of a number of alleles at a single locus, with the black morphs being dominant to the red morphs (Lus 1928). In Britain these morphs can be conveniently classified into two groups, the typicals with red elytra and two black spots and the melanics with either four (quadrimaculata) or six (sexpustulata) red spots on black elytra. Intermediate forms, which are usually heterozygous, also occur, but are uncommon. Frazer & Rothschild (1960) have shown that A. bipunctata is distasteful, has a distinctive scent and is avoided by vertebrate predators. Both shiny black and red appear to be warning colours (Kettlewell 1973), so there can be little doubt that all the morphs are warningly coloured. Birds such as rooks and tits take occasional specimens of ladybirds, but the redstart (Phoenicurus phoenicurus L.) appears to be the only bird which has been recorded as regularly eating them (Buxton 1950). In Great Britain the Redstart is a summer visitor and it is doubtful whether it is abundant enough to be an important predator. Thus it would appear that selective predation of the morphs that blend less well with their surroundings, as has been shown for Biston betularia, would not operate on Adalia bipunctata and an alternative explanation for the existence of this polymorphism must be sought. Creed (1966, 1971a) postulated that melanism in A. bipunctata is directly related to air pollution and has produced evidence to link melanic frequency with smoke levels. He suggested that some unidentified component of the polluted atmosphere was less toxic to the melanic morphs than to the typicals (Creed 1971 a) and has proposed that the species may be used as an indicator of atmospheric smoke levels (Creed 1971b, 1974). We have challenged this view (Benham, Lonsdale & Muggleton 1974) and have produced evidence