Temporal variation in flower visiting insects of Calotropis gigantea in the Southern Province of Sri Lanka

Abstract

Calotropis gigantea (Giant milkweed) is a medicinal plant native to a few Asian countries including Sri Lanka. However, the plant is considered invasive in countries such as Australia, the USA and Brazil. It produces flowers throughout the year leading to massive fruit production, and pollination may directly influence the reproductive output of the plant. Knowledge of the insect flower visitors of C. gigantea is essential to cultivate them for experimental purposes, to conserve them in their native range, or to eradicate them in their introduced range. Though the plant is commonly distributed in Sri Lanka, information on its flower visitors is rare. Therefore, the objective of this study was to identify the flower-visiting insects of C. gigantea and their temporal abundance. Monthly field visits were made to eleven sites in Southern Province from August 2015 to August 2016. During sampling, flower visitor species, their abundance, and the number of Calotropis fruits in the selected plants were recorded. Danaus chrysippus, Xylocopa caerulea, Xylocopa fenestrate and Apis cerana were identified as flower visitors of C. gigantea. The most common was Xylocopa spp. and it was recorded in all sites, and during all studied months while Danaus chrysippus was uncommon. The abundance of Xylocopa spp. had a positive correlation with the abundance of C. gigantea fruits (r = 0.094, p<0.001) suggesting its likely role in the pollination of the latter. Monthly rainfall had a strong correlation (r = 0.806, p = 0.002) with the flower-visiting insect abundance of C. gigantea in the Matara district. Mean flower-visitor abundance was comparatively high in sites having the least anthropological impact.