Trap crop and companion cropping for insecticide-free oilseed rape

Georgie Bray, Hope Farm Manager shares her experiences from the farm. 

Trials and Tribulations at Hope Farm – Insecticide-free oilseed rape
In recent years, we at Hope Farm have been hit by the same problems as most oilseed rape (OSR) growers in the country. With climate change, come increasingly difficult growing seasons, and a history of increased area grown in the country has led to huge increases in pests like cabbage stem flea beetle. For the most part, farming OSR has relied on pyrethroid insecticides to keep the no. 1 pest of cabbage stem flea beetle at bay, but now we are faced with an 80% resistance in most cases, whilst these pesticides still cause harm to our beneficial insects. The loss of neonicitinoid seed dressings have been blamed for losing the crop. For us however, that doesn’t reasonably stack up. Droughts during establishment are becoming increasingly frequent, and we know this is bad news for OSR. Equally, wet winters don’t do the crop any favours either. We know that relying on a pesticide that decimates beneficial insects and other wildlife isn’t a reasonable answer. Despite all these challenges – we’re finding that there are other alternatives to get this crop off the ground that can work sustainably for nature and farm economics.

Oilseed rape is traditionally a boom or bust crop, and increasingly, farmers across the country have accepted that it doesn’t pay to continuously lose money on it – so why are we bothering? Well – a big part of growing a sustainable farming rotation is to have a wide rotation of different crops, putting different strains on the soil, enhancing the soil in different ways, and relying on different control strategies for pests and disease. Losing a crop out of the rotation is only going to put a heavier strain on other crops in terms of pests and disease susceptibility, so we want to keep our farmed ecosystem as far away from a monoculture as we can. OSR is also a great resource for wildlife. It’s a pollinator resource, is fantastic in the winter (particularly direct drilled) for partridge, snipe, woodcock, hare, skylark, meadow pipit, and we’ve seen quite a few yellowhammers and reed bunting using the OSR this year with the companion crop grown amongst it. Finally – if you get it right, OSR can contribute nicely to the farm budget at the end of the year, and if you don’t put all your eggs in one basket to get the crop off the ground, the potential to lose money on it isn’t too high either.

The importance of soil health
At the end of last year (2021 harvest), we were at a point where we hadn’t lost a lot of money in OSR – but we also hadn’t grown a lot of it either! We had a tough reassessment of our soil health in about 2018, noticing areas of the farm that needed some serious TLC. It was hard for our new contractor, Martin Lines, but working with them and their expertise from the bottom up to build back a decent soil structure, increase our organic matter, soil biology, and general soil health has paid dividends for the farm. Our cover crop and compost trial that had been running for 2015 gave us a glimmer of hope as well, demonstrating what a difference regenerative farming practices can make – even if we were learning how to do this along the way. In OSR, this made the difference in one year between 0.5t/ha and 2.5-3.5t/ha. It was a nod to nature being the solution rather than more inputs, and so we’ve carried on along that trajectory this year. We have more on the cover crop and compost trial, in a write up from our webinars last year…

Whilst our soil health had improved over the last few years, in 2020 and 2021 harvests, we didn’t manage to get any OSR to harvest. Thankfully, we didn’t lose much money in the process as we treated the failing OSR as a cover crop instead. This wasn’t really a satisfactory end goal for us though. For 2022, we put everything into getting a good OSR crop off the ground, but looking for long term investment, feeding the soil to feed the crop – so no significant spend was lost if the OSR failed. This included the introduction of a trial, with the help of Rothamsted Research. We underlined some core principles for growing OSR in the future, plus a trial to look at trap cropping of turnip stubble strips as well. Those principles were:

• Looking after soil biology, with research coming out that shows how important our predatory and parasitoid insects after for cabbage stem flea beetle control, alongside the improved ability for crops to enhance nutrients
• Organic matter added as a basal food source to the soil food web, whilst improving soil water cycling and other benefits
• Cover crops set to further improve soil biology and structure, and nutrient cycling
• Fitting in right place in rotation after barley, so we have enough time to get the drill in the ground ahead of Cabbage Stem Flea Beetle Migration
• Drill timing when the soil is moist, as soon as possible after harvest
• Direct drilling to keep the soil ecosystem intact (going back to the first point on soil biology)
• Companion cropping to hopefully disguise the OSR growing from cabbage stem flea beetle, and further improve soil structure and nutrient cycling with a diversity of roots in the ground.

Trap crops have been experimented by Rothamsted since 2005, but back then, with the use of insecticides. Theory behind this crop is that it grows quicker than OSR, and so is more attractive to the flea beetle, whilst being more resilient to damage at establishment phase. With the added camouflage of the OSR with the companion crops, we hoped this would increase the trap crops potential. We have not sprayed insecticides since 2018 and hope farm and do not intend to, so instead have mown and then cultivated the trap crop just recently in February to control the cabbage stem flea beetle larvae and prevent them reaching pupation where they may produce more adult pests in our next establishing OSR crop.
We drilled thankfully into perfect conditions on 8th August 2021, after baling the barley straw to avoid added risk to slug damage. Back in early autumn, we had a pain at harvest with the moist conditions to get our other crops harvested, but that pain was relieved partially by an OSR crop that was romping away. The trap crop was growing very well but with seemingly more holes in the leaves than the OSR. Things continued to improve overwinter in this field with the cover crop of buckwheat, phacelia and tabor berseem clover romping away and providing an additional late season pollinator resource. Wildlife has been using the field all winter and a good diversity too. The OSR has looked strong, and it’s interesting that pigeons aren’t that interested in the field. We didn’t use any gas guns, but we think the taller phacelia in the field may deter pigeons landing that may want a shorter vegetation structure to land and feed.

Image (c) G Bray  Hope Farm

It’s exciting to get some data back already from Rothamsted’s sampling. Although only one field and one year, it’s interesting to see that the trap crop had an average of 40 cabbage stem flea beetle larvae per plant, whilst 5m from the trap crop in the OSR there were less than 10 larvae. As you moved to 20m away that increased to about 15 larvae per plant. We have a control field of sorts with a neighbouring OSR crop drilled on the same day but without compost, companion crops and the trap crop. It’s not growing so well, with a lot of pigeons using the field, but time will tell come harvest on what stacks up as a financially sustainable strategy for OSR this year.