If plants and herbivorous insects are mutual evolutionary forces, we should be able to document this interaction in ecological time if the plant is consumed by few herbivore species and the herbivores are monophagous. This study investigated the impact of peduncle-boring insects on the reproductive success of Hesperoyucca whipplei by measuring plant size, peduncle height, the percent of flowers producing capsules, the number and size of fruit capsules, germination success and relating these variables to peduncle damage caused by peduncle-boring insects. Since peduncle-boring larvae tunnel through the peduncle they affect the flow of nutrients to the flowers and developing seeds. I found that peduncle damage caused by peduncle boring moth larvae negatively influenced germination success. Peduncle damage caused by weevil larvae reduced peduncle height and the number of pedicels. The effect of peduncle damage on seed set (number of seeds) was not significant in this study.