When a seed-feeding beetle is a predator and also increases the speed of seed germination: an intriguing interaction with an invasive plant

Abstract

Bruchine beetles are usually considered seed predators, particularly because these beetles consume the seed embryo and kill the seeds. Previous studies suggest that under certain conditions, these insects do not kill the embryo. In this study, we asked whether the germination speed of seeds of the invasive tree Leucaena leucocephala is enhanced by the seed-feeding beetle Acanthoscelides macrophthalmus (Coleoptera: Chrysomelidae: Bruchinae). We also tested whether differences in germination between attacked and intact seeds could be related to seed size. We finally examined whether the number of larvae per seed affected seed germination and whether seedling emergence patterns were similar to those exhibited by germination. We found that compared to intact seeds, proportionally faster germination occurred in seeds attacked by A. macrophthalmus in 14 of the 26 populations studied. In addition, those populations that presented greater germination speed in the attacked seeds during the evaluation time also had the largest seeds. Similar to the germination experiments, seedling emergence was faster in the attacked compared to the intact seeds. Our findings show that the germination speed of L. leucocephala seeds can be enhanced by A. macrophthalmus. However, this effect is much more intense when a single larva develops inside the seed. In evolutionary terms, it is possible that the selection for larger seeds is favoured, increasing the speed of seed germination and contributing to predator satiation at the seed level. However, this effect could be minimized by the production of many seeds by L. leucocephala plants, causing predator satiation by “masting” effects, which would favour the selection for higher seed numbers.