How drill date affects CSFB management in oilseed rape

Now the risk from cabbage stem flea beetle (CSFB) has passed for this year’s oilseed rape (OSR) crop, it is time to take stock and plan ahead. In particular, it is essential to consider the drilling window and how this affects pest pressure – of both CSFB adults and larvae.

Last autumn began positively, with many crops suffering less damage, compared with previous years.  Unfortunately, the good news did not last, with many crops eventually infested with high numbers of CSFB larvae during the winter and spring. Nevertheless, overall crop performance is better this season. In March/April 2021, growers expected to take 89% OSR to harvest, compared with 61% in 2020*.

CSFB has developed resistance to pyrethroid insecticides to various degrees (Figure 1), so it is essential to target – even miss – sprays as much as possible. This is where integrated pest management (IPM) comes in – to help crops avoid the pest and/or mitigate against its potential to cause damage.

Figure 1. Pyrethroid resistance in CSFB in 2019. The coloured scale shows the proportion of resistant individuals in each sample tested. Adapted from Willis et al. (2020).

AHDB research has identified the key components of a CSFB IPM strategy and used a traffic-light code to indicate the current reliability for control. Effective management relies on a combination of these components.

Of these, sow date is critical – determining the level of pressure the crop experiences and the ability of the crop to tolerate damage. In fact, the drilling window selected should be the basis around which all other management options are chosen.

Top tip: Adjust sowing dates to help optimise moisture levels (e.g. wait until the seedbed has sufficient moisture and/or rain is imminent).

Early sowing (before mid-August) increases the likelihood that the crop will emerge and establish sufficiently before CSFB migration begins. Late sowing (second half of September) may result in the crop emerging after the migration peak, reducing the threat from adult beetle feeding. Late-sown crops are also associated with reduced larval pressures. This is because egg laying and development is slowed by cooler conditions. Trial results suggest a three-week delay in sowing may result in a tenfold reduction in larval load.

Crops drilled between mid-August and mid-September tend to be at greatest risk from adult CSFB, especially those drilled from the end of August to early September, due to alignment between beetle migration peak and the most susceptible crop growth stages.

IPM requires the layering of techniques – each adding to the potential control level. However, the most suitable management options depend on the drilling date – which changes the focus, from the management of adult beetles to the management of larvae (see table, below).

The table cites the management actions – the ‘what’. Our dedicated CSFB IPM web page details the impact of these actions – the ‘why’.

When formulating a plan of attack, bear in mind that the evidence is now fairly strong for some of these options but less so for others. Also, all interventions have knock-on effects for other pests, weeds, diseases and crop development – and these must be considered too.

The management options listed have been developed as part of a programme of work, funded by AHDB and the wider industry. The latest project is ‘Reducing the impact of cabbage stem flea beetle (CSFB) on oilseed rape in the UK’ (2020–23). It unites entomologists, crop physiologists, plant breeders, agronomists, distributors, farming cooperatives, agrochemical and fertiliser manufacturers and machinery specialists. The project aims to improve understanding of the pest’s biology and investigate management options.

Sites required

We need OSR field sites to host 2021–22 season trials on:

• Companion crops (crop sown between mid-August and early September)
• Stubble lengths (crop sown between mid-August and early September)
• Organic amendments (crop sown between mid-August and early September)
• Seed rates (crop sown in early August or July)

This article was prepared based on information provided by Sacha White (ADAS) and published at ahdb.org.uk/csfb

*Harvest 2021 data is from the Bayer Crop Science National Cabbage Stem Flea Beetle Management Study, run in conjunction with ADAS and NIAB.