Cabbage stem flea beetle (CSFB) bashed by biologicals

With pyrethroid power increasingly rendered ineffective by resistance, cultural control feels like the last line of defence against cabbage stem flea beetle (CSFB). To shore up the crumbling walls, innovation is urgently required and one AHDB-funded PhD student is working on biological solutions.

In horticultural crop production, especially in protected systems, biologicals increasingly provide a solid foundation for the control of an array of pests. The need to find viable alternatives to synthetic pesticides in arable crops has seen Harper Adams University PhD student Claire Hoarau turn to naturally occurring nematodes, bacteria, fungi and toxins in a quest to beat the beetle.

Biological basics

• A biopesticide (bioprotectant) is any biological crop-protection agent based on living microorganisms, botanicals or semiochemicals
• A biocontrol agent is any living organism used to control another (e.g. predators and nematodes)
• Biological approaches are attractive, as the best ones are not only effective but also have minimal environmental impact, high target-pest specificity and provide breadth to crop protection programmes

Claire’s work investigated four nematode species, with the current front-runner Heterorhabditis bacteriophora (Hb). Hb has a proven track record. For example, it is already approved in the control of a major horticultural pest beetle – the black vine weevil. Her bioassays have assessed three nematode concentrations (4,000, 10,000 and 40,000 nematodes/ml), with each treatment targeted at ten adult CSFB. So far, the results are variable, especially at lower doses. However, the highest dose of Hb resulted in complete mortality in just two days.

The tests also found Steinernema feltiae to be effective, providing up to 100% mortality under controlled laboratory conditions.

A fungus, Beauveria bassiana (strain GHA), produced by Certis under the name Botanigard WP, also worked well in the lab and is being tested under field conditions.

For hundreds of years, soap has been used to keep insect populations in check. Such solutions work on direct contact with the pests. Often, it is the fatty acid component that matters most. In some protected crops, a broad-spectrum contact biopesticide is authorised for UK use – marketed as FLiPPER. This fatty-acid-based product penetrates the pest target, disrupts metabolic processes and causes reduced feeding and death. In lab bioassays, Claire found that a field-rate-equivalent application (10 ml/l) resulted in 85% beetle mortality just a day after application. Other physically acting products have also shown potential, although not as high as FLiPPER.

The soil-dwelling bacterium Bacillus thuringiensis (Bt) is commonly used as a biopesticide. Three formulations of B. thuringiensis subspecies tenebrionis (Btt) have been used in CSFB screens too. This saw beetles fed leaf material that had been dipped in a Btt solution. Unfortunately, this approach resulted in relatively low levels of beetle mortality.

Finishing in autumn 2022, there is still plenty left to do. One of the most exciting elements is taking the promising approaches identified in the controlled lab experiments and testing them under field conditions. This aspect will include assessments of the impact of these biologicals on non-target organisms, as well as economic assessments. It will also look at the potential of using biological approaches in combination with other control options.

Industry partners: AgriFood Charities Partnership (AFCP) and Certis Europe.