Guest blog by Melanie Gibbs, Ivy Ng’iru, Leah Griffiths, Ellie Grove and Stephen Short from the UK Centre for Ecology and Hydrology
Field margins are excellent for providing habitats for a wide variety of different plants and animals, and for increasing farm biodiversity. They offer habitats to beneficial insects such as pollinators and natural pest controllers, which contribute essential ecosystem services. Some insects like moths and butterflies, along with invertebrates like spiders, are also an invaluable source of food for insectivorous birds. In addition to these ecological benefits, wide field margins act as important barriers that prevent pesticide spray drift from entering the wider environment, particularly water courses. While pesticides are vital for crop protection, their unintended consequences can significantly impact the biodiversity of farm ecosystems. This makes understanding the role of field margins even more critical.
Image: Field margin at RSPB Hope Farm, Campbridgeshire (c) rspb-images.com
We are a team of research scientists from the UK Centre for Ecology and Hydrology working with the RSPB at Hope Farm to try to find out whether moths, butterflies, and spiders that live in field margin habitats are exposed to pesticide spray drift at high enough levels to cause harm – by harm we mean having a negative effect on their growth or health without killing them (called sub-lethal effects). More specifically we would like to understand which types of field margin (e.g., grass versus pollen and nectar) support a higher number of species of moths, butterflies, and spiders, whilst also providing better protection against pesticide spray drift. To do this we are trialling DNA methodologies and approaches in three different ways:
1. To identify moth, butterfly, and spider species most at risk of exposure to pesticide spray drift.
We have been surveying different types of field margins at Hope Farm to determine which species of moths, butterflies and spiders are resident and breeding in field margins. To do this we have been surveying the relatively immobile juvenile stages (moth and butterfly caterpillars, and spiderlings) present and actively feeding in the field margins during key months when pesticides are being applied to arable crops (May and June).
While it is possible to visually identify species at their juvenile stages, the process is quite challenging. It demands advanced taxonomic skills and a good understanding of each species' natural history—both of which take considerable time and expertise to master. To get around this we are using a molecular biology technique called DNA barcoding. This is a type of genetic analysis where short, standardised sections of DNA (called a barcode) are used to tell different species apart from each other. Each species has its own DNA barcode, just in the same way that every person has their own fingerprint. Reference libraries of different DNA barcodes can be used to identify the species of each organism.
Image: Mullein moth, RSPB Hope Farm (c) rspb-images.com
By using this technique, we can gain accurate information of which species are resident in the field margin, and which of these species are most at risk of exposure to pesticide spray drift. It is known that the section of field margin that receives most of the spray drift is within the first three metres from the crop edge. We have been mapping and comparing different field margins to work out which margin types contain the highest number of species, and to determine what percentage of these species are living within the first three metres of field margin. This will give us a much more accurate picture of which species are most vulnerable to exposure to pesticide spray drift. With this information, we will work with the RSPB to try and provide evidence to help design field margins that maximise both habitat quality (for beneficial species) and protection against pesticide spray drift.
2. To predict which moth, butterfly, and spider species are most likely to be harmed by exposure to pesticide spray drift.
Every animal needs to eat, so that it grows, develops and can reproduce. However, many plant species do not take the onslaught of herbivory without a fight. They produce defensive compounds to deter herbivores, such as moth and butterfly caterpillars, from feeding on them. In response, moth and butterfly species have evolved detoxification pathways that allow them to eat the plants without being harmed. These pathways are regulated by a large suite of different metabolic genes, which can also be involved in breaking down some kinds of man-made chemicals, like pesticides. There is variation across different species in the number and type of metabolic genes that they have in their genome.
This means that in theory, different types of moths, butterflies and spiders will vary in how well they can deal with exposure to man-made chemicals like pesticides. In our work we are testing this for the species of moth, butterfly and spider we have found to be resident and breeding in field margins at Hope Farm. We are doing this by counting the number of metabolic genes in each of the large metabolic gene families that each species has within its genome. We predict that species that have lower numbers of metabolic genes in their genome will have a lower capacity to cope with exposure to pesticides, and are most likely to be harmed by pesticide spray drift. By combining this information, with the information about which species are at the highest risk of exposure to pesticide spray drift (from 1, above), we hope to be able to provide much more accurate information about which species living in field margins are most likely to be caused harm by pesticide spray drift. This information may help to further inform design of margins, for example by refining habitat structure within the first three metres to discourage use by species sensitive to pesticide spray drift.
Image: Ivy Ng'iru working at Hope Farm (c) Melanie Gibbs
3. To measure the amount of pesticide spray drift moth species are exposed to, and to quantify the amount of harm they experience.
Using laboratory experiments with our model moth species, the Cabbage moth Mamestra brassicae, we have compared and scored the number and types of metabolic genes expressed by caterpillars in response to sub-lethal pesticide exposures. At these concentrations, growth of the caterpillars is negatively affected. This could have longer-term effects and impact how well they cope with stressful conditions in nature. From our initial experiments we have found distinct genetic responses depending on the type of chemical pesticide the caterpillar is exposed to i.e., different numbers and types of genes are activated to regulate the breakdown of different types of pesticide. We are hoping that with further work, we may be able to use these types of genetic responses to develop biomarkers (specific molecules that can be used as a sure sign of pesticide interaction) across a broader range of chemicals and moth/butterfly species. If assessed just after spraying, these biomarkers could be used to directly measure the amount of harm each type of pesticide might have caused the caterpillar due to spray drift into the field margin. This could be a very valuable tool which could improve post-licensing monitoring of new pesticide products at the field(margin)-scale, and facilitate the measurement of longer-term effects that low concentrations of pesticides might potentially be having on beneficial species.
Image: Small tortoiseshell butterfly in field margin (c) rspb-images.com
By using advanced DNA tools and innovative approaches, we hope to develop effective strategies for enhancing biodiversity on farms, ensuring that field margins continue to serve as valuable habitats while offering better defence against pesticide drift. We are incredibly grateful for the support of the RSPB, Butterfly Conservation, PARC and SYBERAC for funding this important research. With their help, we hope to make a meaningful impact in reducing the unintended consequences of pesticide use while promoting sustainable farming practices.