A hybrid oilseed rape that has been bred with resilience against cabbage stem flea beetle could make the difference between crop success and failure when deployed as part of an integrated approach.

OSR varieties have advanced in recent years with new traits aimed at tackling challenges such as turnip yellows virus and sclerotinia, along with continuous improvement in yield.

But these benefits have been overshadowed by the biggest challenge faced by growers – cabbage stem flea beetle (CSFB).

Efforts by Limagrain to create plants that can better withstand the pest are now starting to yield results.

See also: Long-term spring OSR grower reveals his approach

Speaking to Farmers Weekly, Limagrain OSR product manager Florentina Petrescu explains that it takes about nine years from initial crosses to a commercially available variety.

“We are now seeing varieties bred to mitigate the risk of flea beetle coming out of the breeding programme.” 

Called CSFB Resilience, she says it is a novel genetic approach that tackles the pest at key crop stages.

Florentina explains that varieties with the unique growth habit should be used as part of an effective integrated pest management (IPM) strategy. “It’s not a solution on its own.”

The first variety to be commercially launched with CSFB Resilience is Avenger, which joined the AHDB Recommended List last December with a yield of 104% of control.

The variety has pod shatter resistance, which is useful with the variable maturity resulting from larval damage, and also has sclerotinia tolerance, which helps mitigate the higher risk with extended flowering in affected plants.

Avenger also has turnip yellows virus resistance and a light leaf spot score of 7. Its good stem health also helps against secondary fungal infection from the larval damage.

Finally, it has a high oil content of 46% – again, low oil can be a problem in affected plants due to the greater number of immature seeds.

In conclusion, Florentina says growing Avenger with its unique growth habit can help mitigate against cabbage stem flea beetle when used as part of a IPM approach (see “The cabbage stem flea beetle challenge”).

The variety copes better with the two key risk periods in summer and winter [adult and larvae].

“Farmers can get a little help from the variety and adding this to an integrated approach could make the difference between success and failure,” she says.

The breeding programme

Limagrain first had to determine what the desired characteristics for a variety that can withstand flea beetle attacks are. This starts by knowing the enemy and looking closely at its lifecycle.

There are two key periods for flea beetle damage, says Florentina. “It’s the same pest, but with two modes of attack.”

1. Adults

The first mode of attack is by the adults in summer. They feed on cotyledons and the first leaves when the crop is establishing itself.

This can lead to significant leaf area loss up to destruction of the whole plant. “So in extreme cases, there is total crop loss,” she says.

2. Larvae

In the following late winter/spring, the second mode of attack is by larvae, which mine the petioles and stems. “You can get significant plant damage, which impairs the supply of nutrients to the flowers and subsequent seed.”

The resulting yield loss has been estimated by the AHDB to be 50-70kg/ha for each single larvae. So, for example, if you get 30 larvae in a plant, that could result in up to 1-1.5t/ha in lost yield, she says.

The French equivalent to the AHDB (Terres Inovia) puts the loss at up to 1.3t/ha (based on a 4.5t/ha average yield). But even if there are few larvae, the damaged stems and petioles are at higher risk from fungal infections such as cylindrosporium and phoma.

The damaged areas are access points for the fungus to infect the plant. The damage also results in heterogenous plant development, with the side branches flowering and maturing later than the main stem.

“The result is bushy plants that seem to be flowering all the time. “This [prolonged flowering] in turn increases the risk of pollen beetle damage and sclerotinia.”

This variability means side branches are not as mature at harvest, so increasing the number of red seeds and a lower oil content. 

If farmers delay harvest, there is increased risk of losing seed and is why pod shatter resistance is gaining importance.

What do the CSFB Resilience varieties bring?

Varieties with CSFB Resilience have the genetics to mitigate risk from the pest at key crop stages and plants have been selected using four measures:

• Autumn dynamic growth – grow rapidly from adult threat
• Spring stem elongation – grow rapidly from larval threat
• Number of larvae – this is a measure of the ability for larvae to live in stems
• Larvae damage severity – cope better with less damage.

Florentina explains that plants with a strong dynamic growth in autumn can better withstand adult attacks.

“It’s an escape mechanism against early flea beetle attacks before winter.” This is measured at three key growth stages: 11-14, 14-17 and 17-20.

Second is spring stem elongation as fast development after winter (rapid stem elongation) leads to improved plant robustness and a higher resilience to larvae.

Third, the number of larvae is measured using the Berlese test, as this shows the capacity to limit the number of larvae in the stems. They are looking for varieties that are less susceptible.

Finally, varieties are evaluated on CSFB damage severity. She highlights that this factor is directly linked with yield. “Highly damaged plants can’t achieve full yield potential.”

These varietal differences can be big and she recalls seeing plots with bushy, stunted plants just 1ft away from plots with much better-looking plants.

Ratings

These assessments were carried out 2022, 2023 and 2024 and this has led to the first variety being commercially launched with the CSFB Resilience.

Avenger had fewer larvae, less damage from larvae than a sensitive variety (see table below).

In France, Terres Inovia has done its own independent evaluation looking at larval numbers and damage and the results mirror the breeder’s ratings with Avenger scoring 7.2 compared with 4.5 for a susceptible control, on a 1-9 scale with 9 being the best.