The twin challenges of increasing CSFB resistance to pyrethoids and the negative impact of these insecticides on natural insect predators of the pest came under the spotlight at the AICC conference.
Encouraging a long-forgotten parasitic wasp in oilseed rape growing areas could provide some reduction in cabbage stem flea beetle pressure, according to an ongoing PhD project.
With the one class of insecticides available to control the pest breaking down to resistance, alternatives are desperately needed to negate CSFB’s impact on the national oilseed rape crop.
Rothamsted Research PhD student Patricia Ortega-Ramos has been looking into potential integrated pest management (IPM) techniques and, more specifically, the role of beneficial parasitic insects in cabbage stem flea beetle mortality.
There are seven known parasitoids of cabbage stem flea beetle, with six affecting the pest at the larval stage and one during its adult stage.
Of the six affecting the larvae, only one is important in the UK – the parasitic wasp Tersilochus microgaster.
Work in the late 1950s showed that its parasitism rate – or the proportion of hosts parasitised at the susceptible stage in a given population – was about 60%. However, more recent studies in the 1990s have reported a more modest rate of 7.7%.
But perhaps of greater interest is Microtonus brassicae, which could be important in the context of current struggles on-farm because it parasitises adult cabbage stem flea beetles.
It was first detected at Rothamsted in 1997 and was largely forgotten until it was found infesting Miss Ortega-Ramos’ laboratory cultures of beetles and it is now the focus of her study looking at distribution, life cycle and biocontrol potential.
The parasitic wasp female attacks cabbage stem flea beetle by laying an egg inside the adult beetle. After hatching, the larva then feeds inside the beetle before exiting the pest, killing it in the process.
It is then suspected the parasitoid larva falls to the soil where it spins a cocoon, in which it pupates, but more work is needed to confirm this. The adult parasitoid exits the cocoon and starts the lifecycle again.
Utilising the huge numbers of samples sent in by growers and agronomists to Rothamsted after a call for samples ahead of autumn 2019, Miss Ortega-Ramos tested captured beetles for presence of the parasitoid.
“We found Microctonus brassicae was present in 70% of the fields studied so far and the parasitism rate was about 7%,” she said.
While this sounds modest, encouraging numbers to increase parasitism rates with changes in agronomy practices, such as reduced tillage and implementation of flowering field margins, might be possible, according to Miss Ortega-Ramos.
However, an assessment of the impact of pyrethroid insecticides on the beneficial wasp shows recommended and low doses of lambda-cyhalothrin results in high mortality rates.
Combined with evidence that pyrethroid resistance is rife in cabbage stem flea beetle populations across England, the use of these insecticides in autumn is questionable if growers are to encourage natural predation.
Continuing her work next season, Miss Ortega-Ramos is aiming to improve understanding of cabbage stem flea beetle and parasitoid population dynamics and interactions and how to further increase the numbers of beneficial parasitoids in OSR fields.
“Hopefully the research being done now will enable us to lower the pest pressure and keep oilseed rape as part of our landscape,” she added.
Results from an AICC survey on the impact of cabbage stem flea beetle on oilseed rape crops in 2019-2020 point to the smallest harvest of the UK’s number one break crop since 1985.
Data was captured in the first week of November 2019 from 106 AICC members covering 79,040 hectares of the UK crop. In England, respondents accounted for 73,130ha, or 16% of the total OSR area.
AICC agronomists were asked to reveal the hectarage of oilseed rape grown for harvest 2019, along with the area in the ground for harvest 2020. In addition, flea beetle damage assessments and cropping intentions for 2021 were also requested.
Presenting the results, AICC Academy member David Boulton revealed there had been a 20% fall in OSR plantings across the surveyed area between harvest years 2019 and 2020.
There were significant regional differences in that figure, with the North East seeing an increase in area, while the East Central and South East regions experienced sharp drops.
“These results essentially reflect the difficulty in controlling cabbage stem flea beetle,” said Mr Boulton.
This was backed up by assessments of damage caused by the pest, which increased across all regions except Yorkshire in 2019 when compared with the previous autumn, he added.
“Overall, there has been a 27% increase in damage on the last cropping year.
“The results show OSR will continue to feature in rotations in the north areas of England and Scotland but, particularly in the Midlands, it looks like it will be dropped and replaced a by a less risky alternative,” said Mr Boulton.