Abstract
Two colour morphs of Chrysophtharta agricola (Chapuis) (Coleoptera: Chrysomelidae: Paropsini) adults are present in the field in Tasmania and mainland Australia. Tasmanian populations were sampled to assess the frequency of each morph at several sites. The black form represented less than 3 % of beetles (n = 1724) with the proportion not varying between sites. Collection records showed that it has been present in populations for at least the last 125 years. The black morph is also present at low numbers in mainland Australia. Crossing tests with the two colour morphs of C. agricola were conducted in the laboratory, using laboratory-mated and field-mated females. Despite its low frequency, the black form was found to be genetically dominant over the common form. Measured fitness parameters, including fecundity, longevity, adult size, and egg hatch rate showed no significant differences between morphs. Furthermore, field sampling of mating pairs and rearing of the offspring of field collected females found no evidence of non-random mating. Modelling the population over 100 generations confirmed that the black trait is controlled by a dominant but neutral allele, and, thus, is maintained at a constant low level in the population.