Novel approaches to control cabbage stem flea beetle (CSFB) (PhD)

Following the neonicotinoid seed treatment ban in 2013 and the development of resistance in CSFB to pyrethroid sprays, alternative options (not based on synthetic chemistry) are urgently needed.

Chapter 1: Biological control agents for use against CSFB

Relatively few studies have investigated biological control agents against CSFB. However, more research has been published on two closely related chrysomelid pests of brassica crops that have similar life cycles, namely the crucifer flea beetle and the striped flea beetle, which enable us to extrapolate reasonably across to CSFB.

The biological control agents investigated include entomopathogenic fungi (EPF) such as Metarhizium anisopliae and Beauveria bassiana, entomopathogenic nematodes (EPN) such as Steinernema feltiae and Steinernema carpocapsae, parasitoids such as Microctonus brassicae and predators such as the ground beetle Trechus quadristriatus.

Results vary depending on the setting (laboratory versus field), but several biological control agents investigated resulted in CSFB mortality greater than 50% under laboratory conditions. The biological control of the CSFB shows potential as an alternative to the use of conventional synthetic insecticides. Nonetheless, many research gaps remain, as current research has focused largely on crucifer flea beetle and striped flea beetle, with comparatively few studies investigating the potential of biological controls against CSFB. The research published on CSFB has been limited to a small number of species of EPN and EPF with comparatively little work investigating the potential of parasitoids and predators. More field studies using EPF are required while. In contrast, laboratory studies are underrepresented for EPN. Further research is required, testing existing and new strains of fungi and nematodes, exploring the potential of endophytic fungi, enhancing the formulation and application of biological control for use in inundative strategies, and investigating the potential of conservation biological control. Effective biological control agents should ultimately be combined with cultural control methods in integrated pest management (IPM) systems.

Chapter 2: Rearing and maintaining CSFB

Cultures of CSFB need to be maintained under laboratory conditions before use in many efficacy experiments. This work evaluated laboratory rearing techniques, as well as the efficacy of methods, to maintain field-collected CSFB adults. Each technique had its pros and cons.

Collecting CSFB adults at oilseed rape harvest (July or early August) allowed for thousands of CSFB to be collected quickly. However, a small number of adults in each collection was often parasitised. This often led to a rapid decline in populations, as many CSFB adults became parasitised under laboratory conditions.

Rearing CSFB adults from eggs laid by field-collected adults under laboratory conditions ensured that the adults were clean of parasitoids. However, the process was time-consuming, and the number of adult beetles obtained low.  

Field-collecting CSFB larvae and rearing them through to adults under laboratory conditions was more effective than attempting to rear adults from eggs.

It was concluded that combining the collection of CSFB adults in the summer and the collection of plants infested with third instar larvae in the later winter/early spring is the most effective way to ensure a continuous supply of CSFB adults and larvae for use in laboratory experiments.

Chapter 3: Assessing the potential of biopesticides to control CSFB

We tested an entomopathogenic fungus, three entomopathogenic bacteria isolates, two fatty acids and azadirachtin against CSFB adults under laboratory conditions. We also tested the efficacy of the pyrethroid insecticide lambda-cyhalothrin.

Fatty acids were effective, with up to 100% CSFB mortality after 24 hours. The entomopathogenic fungus Beauveria bassiana resulted in up to 56% mortality 14 days after treatment. Entomopathogenic bacteria formulations and azadirachtin were not effective (< 50% and <40% mortality, respectively). Results from a bioassay using lambda cyhalothrin indicated that the CSFB used in this study were resistant to this insecticide.

Entomopathogenic fungi and fatty acids could potentially be used to control CSFB as part of an IPM programme. This study is the first to investigate the efficacy of different biopesticides to control CSFB under laboratory conditions. As such, these biopesticides require further testing to optimise formulation, application methods and to assess impact on non-target organisms. Finally, efficacy under field conditions must be determined to understand the influence of environmental variables.

Chapter 4: Potential of entomopathogenic nematodes to control CSFB

Four entomopathogenic nematode (EPN) species were tested against CSFB adults under laboratory conditions. In addition, a bioassay was completed to test for EPN compatibility with a range of adjuvants (glycerin, xanthan gum and flame retardant) to protect EPNs from UV radiation and desiccation.

EPNs have the potential to control CSFB adults under laboratory conditions. Heterorhabditis bacteriophora caused 75% CSFB mortality at a concentration of 4000 nematodes/ml after six days, Steinernema feltiae caused 80% CSFB mortality when applied at a concentration of 40,000 nematodes/ml after two days, Steinernema carpocapsae caused 85% mortality at a concentration of 10,000 nematodes/ml after six days, and Steinernema kraussei caused no more than 70% CSFB mortality overall compared to the water control, which led to 23% mortality. S. feltiae and H. bacteriophora survival was 100% when exposed to adjuvants, except S. feltiae with glycerin and H. bacteriophora with flame retardant. Further research to evaluate the efficacy of EPN and adjuvants under field conditions is necessary.

Chapter 5: Field assessment of biopesticides to control CSFB

Selected biopesticides (screened under laboratory conditions in Chapter 3) were tested in a commercial oilseed rape crop. Biopesticides tested included physically acting products, entomopathogenic fungi, entomopathogenic nematodes and plant extracts.

Each product was tested using the field rate indicated on the product label or rates recommended by the manufacturers. Efficacy of biopesticide products was compared with the synthetic pyrethroid insecticide lambda-cyhalothrin (Hallmark with Zeon technology) as a positive control, water as a negative control and untreated plots. Product efficacy was determined by recording foliar damage caused by CSFB adults and numbers of CSFB larvae inside the plants.

Only the physically acting product FLiPPER, when combined with Hallmark, significantly reduced leaf damage compared to the water treatment and untreated plots. Larval density did not vary significantly among the treatments tested.

Several limitations of this study were identified, and changing the methods used may improve the quality of results. In particular, assessment methods could be improved, as well as the timing of treatment application and the formulation of these biopesticides, particularly those based on entomopathogens, which may be important to protect these organisms from negative abiotic factors.

Chapter 6: Sentiment analysis of the farming press about CSFB management

In this study, a sentiment analysis of a sample of the UK farming press between 2010 and 2022 was completed to record changes in the opinion of agricultural professionals to the ban of neonicotinoid insecticides and its impact on CSFB control.

Results showed an increased interest in topics related to oilseed rape and CSFB control after the initial moratorium of neonicotinoids in 2013, with an increasing number of articles published on this topic between 2013 and 2019, when the permanent ban of neonicotinoids came into effect.

Neonicotinoids were frequently mentioned when the ban was first introduced, as well as pyrethroid insecticides, as farmers were concerned with the future of CSFB control without neonicotinoid seed treatments. An important impact of the neonicotinoid ban has been that many farmers have increased their reliance on pyrethroid foliar sprays.

The sentiment analysis results with the automated analysis completed using R (version 4.3.1) showed positive sentiment increasing in the articles over the whole study period. However, the manual analysis completed by reading 10% of the articles did not show a clear trend over time. Words associated with negative emotions such as anger, disgust and fear increased at the time of the neonicotinoid ban. However, future work is necessary to improve the methods and confirm this study’s findings.

Chapter 7:  Conclusion

This study shows that biopesticides have the potential to effectively control this pest, although more work is needed to ensure their efficacy under field conditions. Several ideas were suggested to improve the results presented.

Biopesticides are not silver bullets. They will need to be implemented into an IPM programme. Many other studies are being continuously published that focus on other control methods that could potentially be implemented along with biopesticides in the future.

Chapter 7 (see page 111) in the final report includes a detailed general discussion.

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Student: Claire Hoarau, Harper Adams University

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