Is there a biological solution to oilseed rape’s cabbage stem flea beetle woes?

Using biopesticides and biocontrol agents to tackle cabbage stem flea beetle (CSFB) in oilseed rape (OSR) is the subject of a new PhD report. Jason Pole examines the results, which reveal a long road ahead for field-grown crops.

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.

AHDB research is making strides in stacking cultural control approaches (based on companion crops, organic amendments, stubble lengths, seed rates and cultivation intensity).

However, it is still likely that cultural controls will benefit from something that comes ‘out of a can’, but it is in the strategic interests of farming to look beyond conventional chemistry.

This is why Claire Price focused exclusively on biological solutions in her recently completed AHDB-funded PhD project at Harper Adams University.

‘Bio’ definitions

Biopesticide (bioprotectant): any crop-protection agent based on botanicals, semiochemicals or living microorganisms (such as fungi).

Biocontrol agent: any crop-protection agent based on larger organisms (i.e. larger than microorganisms), such as nematodes.

High-value horticultural crops, especially those grown under protection, frequently drive innovation in biological control solutions.

Claire’s challenge was to assess the potential for biologicals to work against CSFB in the relatively chaotic environments associated with UK field crops.

Numerous biological options were examined.

Fatty acids

Fatty acids help remove the waxy layer that protects an insect’s exoskeleton (cuticle), before damaging the insect’s cells.

With potential to reduce feeding and cause death, products based on fatty acids are already used to control soft-bodied insects, such as whitefly and aphids, on protected crops in the UK.

Claire’s work was the first to show that fatty-acid-based products affect CSFB under laboratory conditions.

The scanning electron microscope images (below) show highly magnified (x2000) CSFB cuticles after treatment with fatty acids (left) and water (right), with differences evident.

Magnified CSFB cuticles

Fatty acids (applied in isolation) did not reduce adult feeding damage or lower larval numbers under field conditions. However, when combined with a pyrethroid insecticide, the treatment resulted in the least amount of feeding damage observed.

Although the approach has potential, efficacy needs to be enhanced for it to provide a realistic control option against CSFB in the field. This could include testing adjuvants (to improve leaf coverage) or making applications at night (to keep products wetter for longer).

Entomopathogenic nematodes (EPNs)

EPNs are biological control agents, which are commonly used in some cropping situations. Several studies have examined their potential to control flea beetles, including the crucifer flea beetle and the striped flea beetle.

This PhD tested EPNs and showed several had efficacy against adult CSFB under laboratory conditions, with Steinernema feltiae proving to be the most effective.

Unfortunately, field applications did not reduce leaf damage or larval numbers.

Claire identified ways to potentially increase the efficacy of the approach, such as the use of adjuvants, optimum application temperatures, larval targeting (instead of adults) and screening more nematode species and strains.

Entomopathogenic fungus and botanical biopesticides

Numerous fungal species infect insects, and some (fungal species) are exploited for commercial biopesticides. An example is Beauveria bassiana strain GHA, which is used for the control of whitefly and thrips in various protected crops in the UK.

Claire tested this fungal species but knew it was going to be a tough test; so, the fungus was given a helping hand in one experiment.

This support came from the addition of a well-researched botanical biopesticide known to have mixing potential: azadirachtin (produced by the neem tree).

Disappointingly, the approach was not effective at controlling CSFB in the field (in terms of adult feeding or adult/larval mortality).

However, the fungus showed some promise under laboratory conditions, but only when the recommended field rate for soft-bodied insects was doubled.

Once again, field-level efficacy needs boosting. For example, by finding ways to counteract the negative effect of UV radiation, heat and humidity on the fungus.

Unfortunately, spraying in the evening/night is unlikely to be effective for this organism, as it does not perform well in cool temperatures (below 15°C). As a result, new species and strains, selected for UK arable crop conditions, need to be explored.

Entomopathogenic bacteria

Bacillus thuringiensis (Bt) is the most commonly used biopesticide in the world. The bacterium produces toxins that disrupt the gut’s cells in target pests.

Research into the use of Bt to control flea beetles is limited. This research used Bt-based products in laboratory screens. These were all found to be ineffective against CSFB adults. However, Bt may be effective against larval stages (but this was not tested).

Conclusion

Through this PhD project, Claire has acquired the skills to help provide evidence-based solutions for agriculture.

The study encountered many challenges with biologicals, far greater than seen in the relatively controlled environments offered by protected horticulture. The work was also hampered by COVID-19 lockdowns that reduced the level of field experimentation.

The field results were disappointing but reflected low pest pressure at the experimental sites. The low pest pressure meant that, in many cases, the biologicals examined did not differ from water or where no treatment was applied.

Although evidence of laboratory efficacy was encouraging, it is likely to be a while before biopesticides are optimised for field-level control of CFSB.

Even when available, products are likely to need more frequent applications at relatively high doses. Without the development of products for use in arable crops, it would currently cost 20 times more to use a fatty-acid product compared with an effective pyrethroid product.

However, a straight swap – conventional chemistry for biopesticide – is unlikely. More likely is the integration of biopesticides in an IPM programme to layer techniques and build control.

Access the final project report for this PhD