The Effects of Caging to Prevent Herbivory on the Pollination Ecology of the Endangered Plant Astragalus bibullatus

Abstract

Astragalus bibullatus is an endangered perennial herb endemic to the limestone cedar glades of Rutherford County, Tennessee. The species has been introduced into new sites; however, herbivory by mammals has led to loss of biomass of A. bibullatus after reintroduction. To prevent herbivory, 1.2-m-tall open-topped metal cages with mesh openings of 0.5 cm2 (hereafter, referred to as small mesh cages) were placed around the immature plants. Initial observations from 2021 showed that pollinators would not enter the cages, either through the mesh or from the top. The goal of my study was to examine whether increasing the mesh size would increase pollination success by allowing access to insects and preventing herbivory by exclusion of mammals. A field experiment was conducted from 2022-2023 that utilized two different sizes of mesh for caging plants to compare to no caging as a control. The size of the mesh in large cages was selected based on the average wingspan of insects visiting uncaged plants. Pollinator visitation and fruit and seed production were recorded, and trail cameras captured images of herbivores visiting plants. We observed eight species of insects visiting flowers in 2022 and the large mesh size allowed access to plants for all pollinators. Only one fruit and a single seed was produced from plants in small mesh cages during both seasons; however, plants in large mesh cages and uncaged plants produced similar numbers of fruits and seeds. Signs of herbivory observed on the uncaged plants were likely from common herbivores such as white-tailed deer (Odocoileus virginianus) and eastern cottontail rabbits (Sylvilagus floridanus). Based on the effects of caging on pollination and herbivory, I recommend that large mesh cages be used in the reintroduction of A. bibullatus to allow effective pollination and prevent herbivory.