Pollination ecology of interspecific hybridization in Verbena

An association of Verbena stricta, V. bracteata, V. halei, V. urticifolia and derived interspecific hybrids was studied in southern Oklahoma. All hybrids but one involved V. stricta parentage. Of the four species, V. stricta produces the largest and most aggregated blossoms, the most accessible pollen, and the greatest quantity of nectar. This species is visited by the broadest spectrum of insects including Hymenoptera, Diptera, and Lepidoptera. Pollen-carrying visitors were noted in common on each of the remaining three species. Insect exclusion and artificial pollination experiments indicate that V. stricta is an outcrossing species with low seed-set through self-fertilization. Verbena halei and V. urticifolia are highly autogamous and V. bracteata less so. The likelihood that V. stricta will enter interspecific hybridization is greater than that for the other species. Number, frequency, and pollen-carrying ability of interspecific pollen vectors favor crosses with V. stricta involving V. urticifolia and V. halei.