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: “I’m 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 Bayer’s 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 didn’t 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 “I’ve 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 couldn’t 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.
I’m also testing various adjuvants to improve the persistence and leaf coverage of the selected biopesticides.”