Hungry hungry beetles
Carabids are ubiquitous in many landscapes, but can they lend a helping hand in agriculture? Do their eating habits serve a role?
The lesser known but essential part of the farm are the uncropped areas in the field. These can be hedgerows bordering the land, field margins (on the edges of the crop) or unfarmed patches (found anywhere on the farm). These areas serve as biodiversity pockets on farmland, as such, they can help to manage soil quality, increase biodiversity and ultimately enhance ecosystem services for the farm.
In agroecology, we explore ways that farms can benefit from harnessing the power of biodiversity. Part of this is increasing the incidence of beneficials on the farm, like carabid beetles (also known as ground beetles). Given that deliberate introduction of species has proven to have dire consequences to ecology in the past, our focus is on manipulating existing natural ecosystems to conserve biodiversity, and encourage beneficial organisms that provide services useful to agriculture, by creating suitable habitat in in uncropped areas and managing cropped areas appropriately.
Uncropped areas can simply be fallow fields, or they can be managed more deliberately by planting seed treatments to manage for general biodiversity, birds or pollinators. Carabids are a diverse family of beetles that are ubiquitous in farmland, consisting of many omnivorous species which have the ability to eat both weed seeds and feed on economically damaging invertebrate pests. They sustain high levels of species richness on farmland, making them ideal for study.
A significant challenge in farms is to control the growth of weeds to ensure crop yields are maximised. Unfortunately, controlling weeds with herbicide has complications like herbicide resistance. To address this, we aim to find ways to diversify our strategy, so farmers have a broad range of management options available to them. In order to find more strategies to employ, we need to investigate the widescale effectiveness of biological control systems, to ensure that promotion of natural pest control by beneficial organisms is reliable for farmers across the UK.
To understand the habitats and farm management strategies in relation to carabid beetles, Rothamsted scientists analysed data from the largest field ecology experiment to date, the Farm Scale Evaluations (FSE) of genetically modified herbicide-tolerant (GMHT) crops, which covered 257 conventional farms in Great Britain (GB). The dataset consists of a nation-wide network in which data was collected on crop, soil type, arable plants, management intensity and invertebrate taxa.
It’s raining seeds
We collected 120 carabid beetle species as well as 201 taxa of plant seeds in the FSE. These data have been used to look at the role carabids play in depleting the weed seed bank. This group of beetles is known to eat seeds as well as invertebrates, which can be quite beneficial for mitigating pests, only the specifics on when they eat, what they feed on, in which fields (and if this has an impact on weeds) was not entirely clear.
Science Methods: Seed Rain Collection
Seed rain collection is used in the field to monitor population assemblages during a period of time. Invertebrates simply tumble into the trap, and entomologists can then identify them and interpret the data they have gathered. The trap itself is shielded from rain, and prevents escapees.
We found that overall; the total weed seedbank is regulated by carabid beetles in two out of the four crops studied across the whole of GB – this probably means that they eat enough seeds to have an effect on weeds in many fields. However, this effect was even more pronounced for monocotyledon (grass) seeds which were consistently reduced in the seedbanks of all of the crops studied across GB.
Many carabids are renowned in ecology for their generalist (omnivorous) feeding habits. Many eat seeds when they become available, usually from mid-summer to the onset of winter, whereas from springtime onwards all carabids utilise invertebrates for food. However, like many generalists, these flexible omnivores still have dietary preferences. In the field for example, we found associations between the common carabid Pterostichus melanarius and large-seeds from common, spring germinating weeds even when they were less abundant in fields than other types of seed. This would suggest that there may be nutrients contained in these seeds that are essential for this species, and other carabids, or perhaps that other seeds lead to greater toxicity in the beetles bodies Behavioural studies in the lab have also demonstrated preferences like this, but more work must be done to understand the biology/biochemistry behind this behaviour to fully understand what dynamics are at play.
Evidence like this makes conserving and boosting carabid populations on farms very appealing. This can be done through increasing their habitats both for overwintering and breeding. Typically these essential life stages occur in hedgerows and field margins. At present, we are looking to develop and maximise field margins to enhance how their ecosystem function, particularly in ways that will increase any beneficial services to agriculture, such as pollination or pest and weed control. For example, seed mixtures designed to benefit pollinators, and general biodiversity exist currently, however, we aim to create the next step of seed mixes which serve to enhance ecosystem services more broadly.
1. Bohan, D.A., Bourseult, A., Brooks, D, R., Petit, S. (2011). National-scale regulation of the weed seedbank by carabid predators