Trade-offs between melanisation and life-history traits in Helicoverpa armigera

1. Previously we established a homozygous melanic strain (JBM) with 16 black pupae spontaneously occurring within a laboratory population (JBW) of Helicoverpa armigera and demonstrated that the melanisation is controlled by a single recessive autosomal gene. 2. Data obtained indicate that the melanisation is globally expressed in the pupal and adult stages (except for the body hairs of adults) but not in the egg and larval stages. No differences in body colour can be found between the melanic JBM and the wild-type JBW strains before the metamorphic pupation moult. After pupation, the JBM pupae gradually blacken, whereas the wild-type JBW pupae gradually turn brown, indicating that the biosynthetic steps leading to brown pigments are shut off in the JBM strain. In the adult stage, wings are darker and hairs on the abdomen and tergum are lighter in the melanic moths than in the wild-type individuals. 3. Life-table experiments reveal that the melanism is associated with slower development in all life stages, smaller body weight, lower mating rate and fecundity, less mating time, and accordingly, lower net reproduction rate and population trend index. 4. Single pair inbreeding and reciprocal crosses show that the mating rate is much lower in the inter-strain crosses than in the intra-strain crosses, indicating the presence of mating preference for its own colour morph and the presence to some degree of reproductive isolation between the two colour morphs.