Seed choice in ground beetles is driven by surface-derived hydrocarbons

Ground beetles (Coleoptera: Carabidae) are among the most prevalent biological agents in temperate agroecosystems. Numerous species function as omnivorous predators, feeding on both pests and weed seeds, yet the sensory ecology of seed perception in omnivorous carabids remains poorly understood. Here, we explore the sensory mechanisms of seed detection and discrimination in four species of omnivorous carabids: Poecilus corvus, Pterostichus melanarius, Harpalus amputatus, and Amara littoralis. Sensory manipulations and multiple-choice seed feeding bioassays showed olfactory perception of seed volatiles as the primary mechanism used by omnivorous carabids to detect and distinguish among seeds of Brassica napus, Sinapis arvensis, and Thlaspi arvense (Brassicaceae). Seed preferences differed among carabid species tested, but the choice of desirable seed species was generally guided by the olfactory perception of long chain hydrocarbons derived from the seed coat surface. These olfactory seed cues were essential for seed detection and discrimination processes to unfold. Disabling the olfactory appendages (antennae and palps) of carabid beetles by ablation left them unable to make accurate seed choices compared to intact beetles.

Ground beetles are generalist predators of various arable weed seeds. Sensory manipulations and multiple-choice feeding bioassays show that seed choice is stimulated by volatile chemicals derived from the epicuticular lipids on the seed coat.

The biology of seed discrimination and its role in shaping the foraging ecology of carabids: A review

Species of carabid (ground) beetles are among the most important postdispersal weed seed predators in temperate arable lands. Field studies have shown that carabid beetles can remove upwards of 65%-90% of specific weed seeds shed in arable fields each year. Such data do not explain how and why carabid predators go after weed seeds, however. It remains to be proven that weed seed predation by carabids is a genuine ecological interaction driven by certain ecological factors or functional traits that determine interaction strength and power predation dynamics, bringing about therefore a natural regulation of weed populations. Along these lines, this review ties together the lines of evidence around weed seed predation by carabid predators. Chemoperception rather than vision seems to be the primary sensory mechanism guiding seed detection and seed selection decisions in carabid weed seed predators. Selection of weed seeds by carabid seed predators appears directed rather than random. Yet, the nature of the chemical cues mediating detection of different seed species and identification of the suitable seed type among them remains unknown. Selection of certain types of weed seeds cannot be predicted based on seed chemistry per se in all cases, however. Rather, seed selection decisions are ruled by sophisticated behavioral mechanisms comprising the assessment of both chemical and physical characteristics of the seed. The ultimate selection of certain weed seed types is determined by how the chemical and physical properties of the seed match with the functional traits of the predator in terms of seed handling ability. Seed density, in addition to chemical and physical seed traits, is also an important factor that is likely to shape seed selection decisions in carabid weed seed predators. Carabid responses to seed density are rather complex as they are influenced not only by seed numbers but also by trait-based suitability ranks of the different seed types available in the environment.

Inferring species interactions from ecological survey data: A mechanistic approach to predict quantitative food webs of seed feeding by carabid beetles

Ecological networks are valuable for ecosystem analysis but their use is often limited by a lack of data because many types of ecological interaction, for example, predation, are short-lived and difficult to observe or detect. While there are different methods for inferring the presence of interactions, they have rarely been used to predict the interaction strengths that are required to construct weighted, or quantitative, ecological networks.Here, we develop a trait-based approach suitable for inferring weighted networks, that is, with varying interaction strengths. We developed the method for seed-feeding carabid ground beetles (Coleoptera: Carabidae) although the principles can be applied to other species and types of interaction.Using existing literature data from experimental seed-feeding trials, we predicted a per-individual interaction cost index based on carabid and seed size. This was scaled up to the population level to create inferred weighted networks using the abundance of carabids and seeds from empirical samples and energetic intake rates of carabids from the literature. From these weighted networks, we also derived a novel measure of expected predation pressure per seed type per network.This method was applied to existing ecological survey data from 255 arable fields with carabid data from pitfall traps and plant seeds from seed rain traps. Analysis of these inferred networks led to testable hypotheses about how network structure and predation pressure varied among fields.Inferred networks are valuable because (a) they provide null models for the structuring of food webs to test against empirical species interaction data, for example, DNA analysis of carabid gut regurgitates and (b) they allow weighted networks to be constructed whenever we can estimate interactions between species and have ecological census data available. This permits ecological network analysis even at times and in places when interactions were not directly assessed.

Seed Predation of Interseeded Cover Crops and Resulting Impacts on Ground Beetles

Interseeding cover crops into standing grains can promote both agronomic and environmental benefits within agroecosystems. Producers must decide which cover crops are the best fit for their goals, and whether diverse cover crop mixtures provide benefits that are worth the increased seed cost. Broadcast seeding is an accessible strategy to try interseeding but can lead to patchy establishment; it is unknown how much seed loss is due to seed predators. In a two-year study, six cover crop species-planted as either single species or mixtures-were interseeded into standing corn. We evaluated seed predation at the time of seeding, agronomic impact through cover crop, and weedy biomass at the end of the season, and conservation impact through activity-density of ground beetles (Coleoptera: Carabidae). Cover crop seeds were vulnerable to seed predation, primarily by vertebrate seed predators, and seed loss varied across cover crop species. Cover crop biomass did not differ according to cover crop diversity and weedy biomass was not affected by cover crop presence or species. Cover crop diversity effects on carabid activity-density were inconsistent: carabids were higher in diverse mixtures in 1 year of the study, but only predicted by vegetative cover, not by cover crop, in the second year. Interseeding cover crops into corn has potential benefits for ground beetles, although the value of mixtures must be further explored.

Do properties and species of weed seeds affect their consumption by carabid beetles?

Seed predators are an integral part of agroecosystems, where they can reduce the populations of weeds. The preference of predators for seeds and the observed predation rate may be affected by the properties of seeds (e.g. taxonomy, chemical composition, physical defence). In this work, we focused on seed consumption of Taraxacum officinale Web. and Stellaria media (L.) Vill., from France and the Czech Republic, by three species of ground beetle that are seed predators (Coleoptera: Carabidae): Poecilus cupreus (Linnaeus), Pterostichus melanarius (Illiger) and Anchomenus dorsalis (Pontoppidan). The seed species were offered in arenas, simultaneously, under three different experimental manipulations of moisture and seed coat conditions: dry and intact, water-imbibed and intact, and water-imbibed with a damaged seed coat. Seed consumption was checked after 0.5, 1, 2, 24, and 48 hours of exposure. Anchomenus dorsalis largely refused to feed on seeds. Taraxacum officinale seeds with damaged coats were most preferred by the remaining two species of carabids. The consumption by P. cupreus of T. officinale seeds with damaged coats increased from 0.18 % after 0.5 hours to 83.83 % after 48 hours, and by P. melanarius from 13.76 % after 0.5 hours to 76.77 % after 48 hours. Seeds of S. media were consumed less. There was a significant difference in consumption rates due to the country of origin of the seeds, but there were no differences between the carabid sexes. That carabids preferred water-imbibed and damaged seeds may suggest an involvement of olfactory clues in the seed selection process, and/or shorter seed-handling times.

The resilience of weed seedbank regulation by carabid beetles, at continental scales, to alternative prey

Carabids are generalist predators that contribute to the agricultural ecosystem service of seedbank regulation via weed seed predation. To facilitate adoption of this ecosystem services by farmers, knowledge of weed seed predation and the resilience of seedbank regulation with co-varying availability of alternative prey is crucial. Using assessments of the seedbank and predation on seed cards in 57 cereal fields across Europe, we demonstrate a regulatory effect on the soil seedbank, at a continental scale, by groups formed of omnivore, seed-eating (granivore + omnivore) and all species of carabids just prior to the crop-harvest. Regulation was associated with a positive relationship between the activity-density of carabids and seed predation, as measured on seed cards. We found that per capita seed consumption on the cards co-varied negatively with the biomass of alternative prey, i.e. Aphididae, Collembola and total alternative prey biomass. Our results underline the importance of weed seedbank regulation by carabids, across geographically significant scales, and indicate that the effectiveness of this biocontrol may depend on the availability of alternative prey that disrupt the weed seed predation.

Which Seed Properties Determine the Preferences of Carabid Beetle Seed Predators?

Simple Summary

The carabid beetles are well known for the consumption of weed seeds in arable land, but how they choose the seeds is poorly known. In this work, we try to explain the patterns in preferences of 37 species of carabids based on eight seed properties of 28 species of seeds. Surprisingly, chemical properties of the seeds did not affect the preferences. Instead, preferences were driven mainly by seed structural properties. The importance of particular seed properties was also affected by the degree of predator specialization.

Abstract

Ground beetles are important invertebrate seed predators in temperate agro-ecosystems. However, there is a lack of information regarding which seed properties are important to carabids when they select seeds for consumption. Therefore, seed properties, such as size, shape, morphological defence, and chemical composition, were measured, and in addition to seed taxonomy and ecology, these data were used to explain carabid preferences. Carabid preferences were assessed using a multi-choice experiment with 28 species of weed seeds presented to 37 species of Carabidae. Multiple regression on distance matrices (MRM) was used to determine the importance of particular sets of seed properties for carabids. The analysis was conducted for the full set of carabids (37 species) as well as for subsets of species belonging to the tribes of Harpalini or Zabrini. For the complete set of species, seed dimensions, seed mass, taxonomy, plant strategy, and seed coat properties significantly explained carabid preferences (proportion of explained variance, R² = 0.465). The model for Harpalini fit the data comparably well (R² = 0.477), and seed dimensions, seed mass and seed coat properties were significant. In comparison to that for Harpalini, the model for Zabrini had much lower explanatory power (R² = 0.248), and the properties that significantly affected the preferences were seed dimensions, seed mass, taxonomy, plant strategy, and seed coat properties. This result suggests that the seed traits that carabids respond to may be specific to taxonomic and likely relate to the degree of specialisation for seeds. This study contributes to understanding the mechanisms that determine the preferences of carabid beetles for seeds.