VARIATION IN TWO POLYMORPHIC SPECIES OF LADYBIRD BEETLES

Variation In Two Polymorphic Species Of Ladybird Beetles

Variation In Two Polymorphic Species Of Ladybird Beetles

Hypothesis

Colour polymorphism in coccinellid beetles has attracted the attention of researchers regarding the maintenance of equilibrium polymorphism (In Adalia bipunctata Linnaeus, widespread in Europe, extensive research has been focused on postwinter copulation, and evidence has been obtained that the polymorphism is influenced by non-random mating involving a larger contribution by melanic phenotypes to the mating group

A factor that seems of major importance is the more efficient absorption of solar energy by melanics, probably leading to relatively greater activity (Benham, Lonsdale & Muggleton 1974; Creed 1975; Muggleton, Lonsdale & Benham 1975; Brakefield 1984a,b; Brakefield & Willmer 1985; Steward & Dixon 1989). On the other hand, the mate choice for melanic morphs is also supposed to cause non-random mating (O'Donald & Muggleton 1979; Majerus et al. 1982; O'Donald et al. 1984).

For the present species, Harmonia axyridis Pallas, Komai (1956) reported a geographical cline in morph frequencies and suggested the presence of climatic factors affecting colour polymorphism. Recently, Osawa & Nishida (1992) revealed non-random mating in a natural population and female mating preference for non-melanic males.

While most of the previous studies have focused on the comparison of the colour morph frequencies of mating individuals with those of solitary individuals (Creed 1975; Muggleton 1979; Majerus et al. 1982; Brakefield 1984c; O'Donald et al. 1984), factors other than the colour morph were relatively disregarded. Body size is an important character affecting mating success (Osawa & Nishida 1992), but has not been mentioned in regard to colour polymorphism. Because thermal absorption rate depends not only on the colour morph but also on the body size of the individuals (Brakefield & Willmer 1985; Stewart & Dixon 1989), if the thermal factor is involved in mating success, selection on body size may be affected by the difference in thermal absorption. The present study focused on the mating pattern with reference to body size as well as to colour morph and therefore estimated selection on both quantitative and qualitative characters. The particular aspects are (1) whether body size is associated with mating success and, if so, how, and (2) whether non-random mating is observed, thus whether there is mating advantage with respect to colour morph.

A study area was chosen in the campus of Gifu University, near Gifu City, central Japan. Beetles overwinter at the adult stage and copulate in early May of the next spring in the studied population. Reproduction occurs near the place of copulation. Most of the beetles prey upon aphids on the plants Vicia sativa and Pittosporum tobira. First generation offspring emerge and emigrate before copulation unlike those of the Kyoto population where Osawa & Nishida (1992) reported copulation before emigration.

Adult beetles of the overwintered generation were collected in May 1992 and 1993, regardless of their mating status. Beetles were measured for body length under the microscope to 0·01 mm. Differences in body size between mating and solitary individuals were scored as standardized selection differentials ...