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Exploring novel ways to tackle cabbage stem flea beetle

While most oilseed rape growers are striving to kill cabbage stem flea beetles (CSFB), a programme of rearing their number one pest is going ahead using AHDB funds.

What might seem an irresponsible move, namely breeding a species which has wiped out many crops, is fully justifiable, according to Claire Hoarau of Harper Adams University.

 

In her PhD project Miss Hoarau is examining new approaches to controlling CSFB.

The research involves screening various biopesticides in a controlled laboratory environment, she explains.

 

“That requires maintaining a good CSFB population.”

 

Several thousand adult beetles were collected from fields last summer and are currently kept on pots of oilseed rape plants at the university.

 

“The programme of rearing beetles is underway, which means eggs, larvae, pupae and adults are available for experiments throughout the year. It means the research can progress faster than would otherwise be possible.

 

“In recent years, most UK CSFB populations have shown at least some resistance to pyrethroid insecticides, the only class of pesticides which farmers are allowed to spray against this insect.”


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Control

 

Resistance to pyrethroid insecticides has made controlling the pest even more difficult as it follows the withdrawal of neonicotinoid seed treatments, she adds.

 

“There’s an urgent need to find effective and sustainable replacements.

 

“This project is focusing on biopesticides, such as fungi, nematodes and bacteria, as well as plant extracts such as azadirachtin, a feeding deterrent found in neem oil. We could use them singly or in combinations with pyrethroids.”

 

The work is very much in its infancy, she acknowledges.

 

“In December, I completed two preliminary experiments in the laboratory. One involved exposing some CSFB pupae to nematodes, the other applied Azatin (azadirachtin) to leaves for adults to feed on.

 

“Azatin is registered in the UK against thrips. The nematodes I used are commercially available species of steinernema and heterorhabditis, recommended for use against some pest species of moths, weevils and flies.”

Leaves

 

Azatin was applied at various concentrations by dipping oilseed rape leaves in the product and leaving the beetles to feed on them, and the area of leaf consumed was measured at the end of the experiment.

 

The nematodes treatment followed established methods, with single nematodes in water placed onto individual pupae.

 

“I then counted the number of dead pupae at the end of the experiment.

 

“For both experiments I incorporated controls, dipping the leaves in water only for the Azatin bioassay and applying only water to pupae in the nematode bioassay.”

 

Treatment with nematodes gave no relevant results.

 

“The methods need further refinement.

 

“The azadirachtin assay, however, indicated that this active ingredient may indeed have feeding deterrent activity against CSFB.

 

“I will need to confirm these results with further experiments, again with improved methods and higher numbers of tested insects for a more robust statistical analysis of the data.”

Fungi

 

Miss Hoarau also plans to test the impact entomopathogenic fungi.

 

“They will be isolates of metarhizium and beauveria provided by the University of Warwick.

 

“In previous studies some of them have proved to successfully infest the pest in laboratory settings.”

 

Also on the new approaches menu are strains of bacteria known to be effective against other beetle species.

 

“However, because it’s still early days in my project, some organisms or strains and their sources have not yet been confirmed.”

 

Selected biopesticides will be tested in different combinations to pinpoint any antagonistic, additive and/or synergistic effects – firstly in the laboratory and then the field.

 

Another aim, by testing the use of biopesticides at different times of the day, will be to see whether some application periods are more effective than others against the nocturnal insect.

 

“A final aspect of my project will be to evaluate the costs versus benefits of the methods being tested. That’s because they’ll eventually be applied in the field under commercial conditions,” says Miss Hoarau.

In the field – Adrian Joynt, Shropshire

Adrian Joynt grows 100 hectares of winter oilseed rape at Apley Estate, Bridgnorth, Shropshire, which he has managed for nine years.

 

He last used neonicotinoid-treated seed in 2013 and fears that without effective flea beetle control he may have to abandon the crop.

 

He says: “In autumn 2017 we sprayed 20% of the crop once with the pyrethroid Lambdastar (lambda-cyhalothrin) at full rate and didn’t see any larval damage in the following spring.

 

“The next autumn we sprayed the whole crop once with full rate Lambdastar and had some larval damage in the spring, but no crop loss.

 

“Last autumn we treated all the crop in the same way and about 20% is likely to be written off this spring.

 

“The level of damage is increasing annually, and it soon may not be feasible to grow what is our most profitable combinable break crop.

 

“I had some beetles we captured on the farm last autumn tested for pyrethroid resistance and the level was high. So we sprayed only once because I felt the remaining population would be almost completely resistant.

 

“Looking to the future, spraying a broad spectrum insecticide is unsustainable because of damage to non-target species and beneficials.”

 

A feeding deterrent might be useful, he believes.

 

“But as a pesticide it would probably need multiple applications during the growing season. A lot of the feeding damage occurs when the crop is at the cotyledon stage, so I think a genetic deterrent produced by plant breeding would be more effective.

 

“The research into new controls at Harper Adams is very welcome, but I would hesitate to use products based on fungi or bacteria until I was 100% sure that there would be no damage to naturally occurring fungi and bacteria in the soil.”

The CSFB challenge

Cabbage stem flea beetle (CSFB) adults graze on young oilseed rape plants. This can result in plant death.

 

The larvae also burrow within petioles and stems of surviving plants.

 

Following the withdrawal of neonicotinoid seed treatments and the emergence of populations with resistance to pyrethroid insecticides, the pest has increased in importance.

 

Integrated pest management (IPM) of CSFB requires innovation in crop protection and post-pesticide management systems. A wide range of controls is required, including novel approaches, such as the use of biopesticides.

 

Biopesticides offer an attractive pest control option because of their minimal impact on the environment, specificity to target pest and potential in resistance management. Despite this, their use to control CSFB has received little attention.

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