Mechanisms of Seed Discrimination and Selective Seed Foraging in Carabid Weed Seed Predators

Abstract

Ground (carabid) beetles (Coleoptera: Carabidae) are among the most prevalent biological agents in temperate agroecosystems, with numerous species functioning as predators of both insect pests and weed seeds. Several carabid species are omnivorous, and the diversity and complexity of their feeding habits makes it difficult to predict the magnitude of population suppression they impose on pest and weed species. Thus, it remains unclear why omnivorous carabid beetles choose to feed on seed species when alternative food sources are available. It was first thought that seed feeding in carabids was driven by chance alone and hence, take place upon random encounter. Recent lines of evidence show that carabid predators can be quite choosy or selective about the seeds they consume when seeds of different species are offered in laboratory or field feeding experiments. Seed feeding habits in carabids thus seem driven by active seed discrimination and selection behaviors. Yet, little is known about the sensory, behavioral, nutritional, and biophysical mechanisms that underlie seed preferences in carabid seed predators. In this thesis I explore the feeding ecology of omnivorous carabid species and describe aspects of the sensory, behavioral, nutritional, and biophysical ecology involved in their seed feeding habits. Multiple-choice feeding bioassays coupled with sensory manipulations of carabid predators showed that carabids rely on their olfactory system to detect seeds of different species and identify the suitable seed species among them. Seed choice was driven by the perception of long chain volatile chemicals derived from the epicuticular lipids located on the seed coat surface. Seed surface volatiles seemed to encode information about the nutritional quality of seed species, especially their fatty acid content. Experiments with synthetic diets showed that omnivorous carabids potentially seek seed consumption to overcome the scarcity of some lipids in their diets. Carabids most likely choose seed species based on desirable lipid content if the physical properties of seed species in the environment pose no challenges to efficient seed handling. However, if physical seed traits such as mass or size are highly variable among seed species, seed handling costs are likely to vary widely among seed species and thus, constrain the active selection of nutritious seeds. Carabids would more preferably select seed species that are easier to handle in such a case, irrespective of their nutritional quality. The findings of this thesis bridge important knowledge gaps in the seed feeding ecology of carabid species as the sensory basis of seed perception and discrimination is elucidated, and some of the key factors that render seed of certain species more vulnerable to elevated carabid attacks are identified.