Differential Flower Herbivory and Gall Formation on Males and Females of Neea psychotrioides, a Dioecious Tree

The goal of this study was to determine whether there exists intersexual differences in the degree of herbivore attack on a tropical, dioecious tree. Neea pychotrioides is a Costa Rican tree that experiences at least two types of attack on its reproductive structures: flower herbivory and gall formation on inflorescences. Although female trees were more common than males in the two study populations (overall sex ratio = 1.5 females/male), male flowers were more likely to be eaten by lepidopteran larvae. Similarly, inflorescence-borne, fly-induced galls were more frequent on males. Ten families of insects from three orders (Diptera, Hymenoptera, Lepidoptera) occupied the galls with varying levels of parasitism and hyperparasitism occurring within. Although the sexes were not significantly different in vegetative size, males had larger floral displays (more inflorescences and larger flowers) that may have acted as attractants to herbivores and gallmakers. The results are consistent with the hypothesis that flower damage and the energetic costs of producing galls exert a greater expense to males, ultimately resulting in death, and over time, a skewing of the sex ratio.