Ecological Studies on the Polymorphic Ladybird Adalia bipunctata in the Netherlands. II. Population Dynamics, Differential Timing of Reproduction and Thermal Melanism

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Abstract
Evidence was sought for the relevance of the theory of thermal melanism to the dynamics of polymorphism in A. bipunctata in the Netherlands. A mark-release-recapture experiment performed from April-Oct. 1980 found no difference in length of dispersal or rate of loss between melanics and non-melanics consistent with an influence of thermal melanism on activity. Comparisons of melanic frequencies in 5 populations indicated that melanics tend to disperse earlier in May from shrubs to adjacent trees than non-melanics. Inspection of all data suggests that the mean dispersal length of .apprx. 20 m is a considerable underestimate of the mean migration length of offspring about parent''s birthplace. Sequential samples from 12 sites with differing melanic frequencies showed that melanics tend to emerge earlier from pupae than non-melanics. The time lag (.+-. 95% C.L.) between the estimated peaks in emergence was 1.79 .+-. 0.74 days in 1980 and 0.76 .+-. 0.71 days in 1981 with an emergence period of .apprx. 31 days. Melanics in post-hibernation populations tend to mate, oviposit and die earlier than non-melanics. This study supports the hypothesis that thermal melanism through an influence on activity results in an earlier reproduction of melanics. It is suggested that changes in relative fitness relationships associated with clines within the study area involve interactions between climate, thermal melanism and life cycle parameters of both the ladybird and its aphid prey. Selective predation may also influence the polymorphism. The differential timing of adult eclosion may contribute to the maintenance of the polymorphism.