Allochronic separation versus mate choice: nonrandom patterns of mating between fall armyworm host strains.

Assortative mating may result from intrinsic individual mating preferences or from assortment traits not requiring expression of preferences. Assortment traits are phenotypes expressed in both sexes that enhance the probability of encountering individuals possessing similar trait values. In the noctuid moth Spodoptera frugiperda, it has been suggested that nonrandom mating between two host strains is caused by a temporal assortment trait-that is, differential timing of calling and copulation during the night. By experimental manipulation of this trait in controlled mate-choice experiments, we investigated whether mating by same-strain individuals is enhanced mainly by the allochronic shift of mating activity or is also affected by time-independent intrinsic mating preferences. The observed patterns suggest that nonrandom mating between the two host strains in the laboratory is shaped by an interaction of both effects that is dominated by mating preferences during the first encounter night. This interaction changes over time as the preferences become weaker on subsequent nights. Males were less restricted than females with regard to both the time shift in mating activity and mate preferences. Although the nature of the mate-preference mechanism remains elusive, its restriction to females suggests that male-produced close-range pheromones emitted during courtship play a role.