Cultural controls, varietal vigour, companion cropping and boosting beneficials are all non-chemical methods of attack showing merit in tackling cabbage stem flea beetle (CSFB).
But use of novel biopesticides to complement these is something the industry is expecting to see more of as chemical solutions become fewer Harper Adams University PhD student Claire Hoarau is conducting a number of experiments to determine the scope for biopesticides in CSFB control, including the first lab studies to evaluate the effect of nematodes on adult CSFB mortality.
She is also exploring the potential for entomopathogenic fungi, bacteria and natural products, such as botanical insecticides.
Four species of nematodes in commercial formulations were screened in the lab against adult CSFB – steinernema feltiae, steinernema carpocapsae, steinernema kraussei and heterorhabditis bacteriophora – and cumulative mortality was recorded every two days, until eight days post-inoculation.
Mortality Ms Hoarau says: “The species heterorhabditis bacteriophora worked the best with more than 75% mortality after six days at the middle dose.
Others were good but slower to kill, reaching high mortality after four days.
The species that did not work so well was steinernema kraussei, but it was still better than the control, so these are encouraging results.”
Nematodes work by entering the beetle’s body and releasing a fatal bacteria.
The nematodes then feed on the beetle and multiply.
Ms Hoarau’s next steps will be to test the product in the field during flea beetle migration.
“The nematodes come as a paste to be dissolved in water and they would be applied by spraying with the same equipment as other pesticides.
Biocontrols “However, with biocontrols we have to maximise their effect by applying them at the correct moment.
“Biopesticides based on entomopathogens are living organisms, so when applied in the field they’re exposed to the environment, so the sun, UV radiation, temperature, humidity and all these factors can be detrimental to these biopesticides.
“I am going to spray in the evenings because nematodes are sensitive to UV and humidity.
Beetles are nocturnal and feed mostly at night.
How long the nematodes last depends on humidity – if it’s dry they might just last 24 hours.
I’m planning to apply them several times, with at least four applications every seven days.”
However, not all results were as promising.
Several lab bioassays were carried out with the entomopathogenic bacteria bacillus thuringiensis tenebrionis, which infects the host insects by ingestion and causes them to die of septicaemia within a few days.
Formulations Three formulations were tested at field rate and mortality of adult beetles was monitored for 12 days post-treatment.
However, no mortality above 40% after 12 days was seen in any of the treatments and Ms Hoarau decided not to investigate entomopathogenic bacteria any further.
The plant-based compound azadirachtin, which is derived from the neem tree and currently used in horticulture as an insecticide, also did not have high mortality in the lab, but studies in the USA where it is combined with entomopathogens (microorganisms pathogenic to insects) showed significant results.
“It sounded like azadirachtin enhanced the effect of the entomopathogen so I’m going to try this combined with a fungi.
“The fungi would be applied by dissolving spores in water and spraying.
It’s a contact pesticide, so spores will germinate on the beetle’s cuticle and they’ll produce toxins and break into the insect and grow into it.
Usually after a few days you can see the fungi developing on the beetle.
“It can also contaminate other CSFB. It would take a few days to take effect from application, but being a living organism too, it is going to depend on the temperature and humidity.” says Ms Hoarau.