Case Study: Carabid beetles for natural-enemy pest control
Author: Kelly Jowett:
If there was an agent acting within your crops that could reduce eggs and first instar larvae of the cabbage root fly by up to 90%; reduce emerging wheat blossom midge by 81%; and reduce seed stock of crop weeds in the range of 65-90%, then I’m sure you’d want to keep it in there. Well, with a little effort you can have this voracious destroyer of pests working across your farm. Carabid beetles, sometimes known as ground beetles, eat ALL major crop pests, and weed seeds too, and are present in all farm habitats across the UK. This makes them ideal agents of natural enemy pest control.
Natural enemy pest control is a bit of a mouthful but is just what it describes: the eating of crop pests by the predators that would control them in a natural system. The problem for natural enemy pest control is that we are not dealing with a fully natural system. Since the post-war agricultural revolution, farmland has been subject to tremendous change. Over time this has caused substantial decline in the populations of carabid beetles and other natural enemies. Unfortunately, pest species are adapted to bounce back quicker - especially since large crop areas are ideally suited to support population booms. Whilst pesticide sprays are necessary in this system, they compound the problem - hitting the predators as well as the pests, added to associated resistance building.
The importance of biodiversity
Carabid beetles are incredibly variable. Of the 350 species in the UK, 30 of these are common in farmland. They range in size from 2mm to 3cm in length; some can fly long distances, some run up to 16cm per second; some breed in the autumn, and some breed in the spring. Each has their own habitat preferences, meaning that they are active at different times, tolerate different weather and farm management practices, and are better at eating different crop pests. Therefore, having a diverse range of carabid species will mean that you have the best chance of effective pest predation.
So what can we do to help boost carabid numbers and diversity?
Luckily carabid beetles are well studied and we already know a lot about what they need to thrive. We can break this down into areas to feed, areas to breed, and areas to shelter.
Areas to Feed
Of course we want them to feed in the crop or pasture area when pests are there- but pests are not there all the time (luckily for us!) to support the carabid beetles needs. So semi natural areas are essential to provide the invertebrate and plant resources for all year round food. Conservation headlands, grass/ flower margins, beetle banks and taking field corners out of management, can all provide these close to productive areas, so carabids can move quickly into the crop to eat the pests they prefer, when they appear.
Areas to Breed
Carabids lay their eggs in the soil, hatching into larvae- which take around 6 months to grow, before they pupate and emerge into adult beetles. The larvae are actually predatory on crop pests too, and eat more than adults as they need a lot of protein to grow. Some larvae also eat weed seeds, which is particularly useful as they live in the top soil levels in the crop area. To help larvae grow undisturbed in crop areas, minimum tillage systems may be useful.
Areas to Shelter
Carabids have two main periods when they need shelter: hibernation, and aestivation, which is a summer rest period when conditions are too hot and arid. At these times, the beetles take advantage of permanent habit with structure that creates a protective microclimate- such as hedges, ditches, banks, and at a small scale the tussocks created by some grasses.
Another important aspect is the shelter from machinery and chemical applications. These can cause direct mortality, or affect the carabids ability to feed and breed, so safe areas near to crops allow some beetles to thrive and repopulate when such management is necessary.
Linking up farm habitats
To preserve or enhance the diversity of carabids on your farm it is important to have a range of habitats, but also for them to be linked up. Firstly populations need those seasonal and foraging movements, but as conditions change, different species can immigrate into your farm from surrounding areas to boost your assemblage of species. Carabids move in the landscape in a variety of ways, flying species will use habitats as stepping stones, whilst running species may need ‘corridors’ of hedges and margins to encourage them to move around the farm.
Farm habitats for beetles: a) grass and flower margins, along with hedges provide food resources year round, and connect other habitats; b) beetle banks create stable resources in the centre of fields, to encourage beetle presence in crop areas; c) taking corners out of management encourages scrub and tussocky grasses- ideal for sheltering carabids.
But there is still work to be done
Although scientists have done a lot of research on carabids, there is still a lot we don’t know. My recent work has shown that different species are associated with different landscape features such as field boundary habitats, crops, soils, and management such as tillage regimes. If we can better understand which habitats are most beneficial for different beetles, it will become possible to tailor your farm habitats to get a good selection of predatory species that suit a farms particular pests.
Pitfall trapping to monitor farm habitats
Pitfall trapping is a quick and easy way to see what carabids you have on your farm, and if you do pitfall trapping periodically, you can track how the populations vary over time relative to your farm management interventions. For details see my factsheet:
How to pitfall trap on your farm: Factsheet
Feed-back to scientists, especially me!
The main knowledge gap is how these habitats work in practice, over time. This is where we need to work with farmers closely, monitoring farm habitats, seeing what works, where, and why. But also, crucially, we need to know which interventions fit well into your farm business. Even if something works for beetles, if it’s going to be difficult for farmers to apply, it will not be widely applied!
Which leads me on to my plea: I need farmers to take my survey!
Watch my animation: Here.
Take a short introduction to ID quiz: Link to ID Quiz
Then take my beneficial beetles survey to tell me your opinions on carabids, management that may be useful for them, and monitoring. Link to survey
If you are motivated after reading this article to carry out pitfall trapping or set up monitoring on your farm, I would be happy to help with advice, and support with verification of carabid identification.
Email : Kelly.jowett@rothamsted.ac.uk or contact me on twitter: @kelly_jowett