With neonicotinoid insecticides banned and most CSFB populations showing at least some resistance to pyrethroids, there is an urgent need to find effective and sustainable replacements to control the pests.

That is according to Claire Hoarau, who is in the third year of a PhD project* trying to discover such replacements at Harper Adams University, part-funded by AHDB.

The project is examining the impact of biopesticides, such as insect-killing (entomopathogenic) fungi, nematodes, and bacteria.

Ms Hoarau says: Im also looking at plant extracts like azadirachtin, a feeding deterrent found in neem oil, and insecticidal fatty acids.

We could use them singly, or in combination with pyrethroids.

CSFB are serious pests, she adds.

Severe adult damage can kill seedlings in the field as they emerge in the autumn, leading to complete crop failure.

The larvae, active from October to April, feed in the petioles and main stems, threatening plant survival and making plants susceptible to diseases as well as water damage.

Screening The initial work involved screening the various options in a laboratory experiment.

This required maintaining a CSFB population in a controlled environment.

I collected several thousand adult beetles in the summers of 2019, 2020 and 2021, and to supplement this population I gathered oilseed rape plants full of larvae from the field to raise the larvae to the adult stage in the laboratory.

Bioassays of all the potential controls were conducted against adult beetles and the most successful were chosen and tested in the field last year from September to November.

Laboratory bioassays consisted of spraying the beetles with a range of biopesticide concentrations field rate and half and double that.

The insects mortality was then assessed to evaluate their susceptibility, including to products already commercialised for completely different insect species.

In the field, the selected products were applied via a knapsack sprayer at field rate in 300 litres/hectares of water, except for nematodes which require a much higher volume about 1,000 litres/ha to survive.

Field assessment consisted of examining the amount of leaf damage caused by the adult beetles and the density of larvae inside the plants.

A final aspect of my project will be to evaluate the cost effectiveness of the methods tested, given that they may eventually be applied in the field under commercial conditions, says Ms Hoarau.

In the laboratory, the nematode species heterorhabditis bacteriophora and steinernema feltiae, and Bayers fatty acid-based product FLiPPER gave the best results on CSFB mortality, followed by the fungus beauveria bassiana strain GHA, which was developed by Certis and is sold as Botanigard WP.

Azadirachtin and the bacteria didnt lead to mortality significantly different from the control, adds Ms Hoarau.

In the field, the results were not as positive as expected, she admits.

Improve Ive identified several factors which may improve the results and make them better reflect the true potential of these biopesticides.

These include ultra-violet radiation, humidity, temperature, and rainfall.

I couldnt start the field treatments until the end of September, and the autumn weather made weekly spraying sessions difficult indeed frequent rainfall prevented spraying altogether, and it was becoming too cold for the various pathogens to thrive.

I was also constrained to spraying during the day, even though living organisms such as entomopathogenic nematodes and fungi are sensitive to UV radiation.

In future, I am planning to spray in the evening and aim to start the treatments towards the end of August/early September.

I have more bioassays planned with a fatty acids-based product sent by my funder Certis.

Im also testing various adjuvants to improve the persistence and leaf coverage of the selected biopesticides.

Adrian Joynt, who manages the 1,060-hectare mixed arable and beef Apley Farms near Shifnal, Shropshire, welcomes the new research.

He grows OSR in a five-year rotation on 600 hectares of combinable crops on the farms medium to light soils.

He says he had no significant problem with CSFB until 2018 when 22ha of the 120ha crop had to be treated with insecticide.

In 2019 the crop was unsprayed because tests showed the beetles were 90% resistant to pyrethroids and the damage was more extensive.

The final yield was definitely affected, he says.

Drilling date trial In 2020 we hosted a fieldscale drilling date trial for ADAS and lost 21ha overall, 15 of which were in the trial.

Last season, however, there was noticeably less damage from the pest as the crop established.

And there were fewer beetles in the store at harvest.

Weve tried several companion crops, but my observations are that they have very little effect on limiting grazing damage.

Oilseed rape is a high value crop at current prices, and theres no other reliable combinable break that gives a consistent return.

The new products have shown potential in the lab, but more work is needed to assess their efficacy in the field, in particular the relationship between drilling date, application timing and the weather and soil conditions around the critical establishment period.

Any new product coming into the market also needs to be cost-effective and practical to use, he adds.

• *AHDB project 21510042 Novel approaches to control cabbage stem flea beetle (CSFB) (PhD)
• September 2019 November 2022
• Project leader: Harper Adams University study under supervision of Dr Tom Pope (director of studies) and Dr Heather Campbell (supervisor), with collaboration of Dr Dave Chandler, of University of Warwick
• Funding: AHDB and AgriFood Charities Partnership £36,150 each, plus £6,000 from Certis UK