Integrated pest management (IPM) of cabbage stem flea beetle (CSFB)

Our research has identified the key components of a cabbage stem flea beetle (CSFB) integrated pest management (IPM) strategy and used a traffic-light code to indicate the current reliability for control in oilseed rape.

Effective chemistry can stop pest targets in their tracks. Without it, it is essential to get to grips with pest life cycles to find the chinks in their armour. As no single non-chemical (or chemical) approach is completely reliable, control requires a combination of techniques to deliver multiple hammer blows and suppress CSFB populations. Our research team has mapped out the key factors across a growing season and used a traffic-light system to indicate those most likely to affect control.

Typically, more research is required for 'uncertain control', followed by 'moderate control'.

It is important to account for natural pest enemies. Pesticide applications (especially broad-spectrum pyrethroids) and intensive tillage are likely to have a negative impact on natural enemy populations, such as parasitic wasps and ground beetles. 

Top tips: Flower strips provide nectar resources for parasitic wasps and beetle banks provide habitats for ground beetles.  Minimising cultivations may also help by reducing mortality of soil dwelling natural enemies (e.g. ground beetles and pupae of parasitic wasps).

Although this may become part of the solution, the impact of potential approaches is currently far from clear.

Oilseed rape volunteers can act as a trap crop and divert CSFB away from adjacent cash crops. When trap crops are destroyed, CSFB eggs or larvae die. In trials, the approach reduced adult CSFB infestation (by up to 88%) and damage (by up to 76%) in the oilseed rape cash crop. It also resulted in higher plant populations (by up to 56%) and reduced larval populations (by up to 69%). However, benefits were variable and not always observed. It is likely that relatively large areas of adjacent trap crops (at least 2 ha) are more likely to deliver benefits, especially if left in place until after the bulk of CSFB migration has occurred (i.e. late September). It is also believed that volunteers at an early growth stage at the end of August are most attractive to CSFB. Sown trap crops (e.g. turnip rape) and some cover crops may also be effective.

Top tip: Use cultivations to encourage a fresh flush of volunteers or broadcast home-saved seed to produce a cheap, sacrificial trap crop.

When available, these are likely to provide moderate control and cost more than pyrethroids.

There is no clear evidence to suggest that varieties differ in susceptibility or attractiveness to CSFB. However, a vigorous variety will reach the four-true-leaf stage quicker, making it more tolerant to adult feeding damage, and better able to outgrow larval feeding damage too.

Note: If a crop is drilled early, consider a slower-developing variety, so it does not reach stem extension too early.  If the crop is drilled late, rapid establishment is beneficial.

Fields in which oilseed rape has not been grown recently or nearby are likely to have relatively low CSFB starting pressures. The presence of cereal stubble/straw could make it harder for the pest to locate emerging oilseed rape, but robust trial data is sparse. It may also help to conserve soil moisture. Trials suggest that wheat straw may be better than barley straw. Spiders also use stubble to support their webs. It is possible that longer stubble offers greater benefits.

Sufficient soil moisture levels during crop emergence are particularly critical. Adjust sowing dates (however, see points on sow date, below) 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 (however, see points on soil moisture, above). Late sowing (second half of September) may result in the crop emerging after the migration peak, reducing the threat from adult beetle feeding – such 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 ten-fold 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. 

Generally, in terms of yield, increasing the seed rate beyond that needed to achieve optimal plant populations – 25–40 plants/m2 – results in little benefit, unless conditions are dry during establishment. Interestingly, higher seed rates are associated with greater larval numbers per hectare, which could build pest populations across seasons.

• Increasing seed rate may reduce damage from adults by diluting their feeding
• Decreasing seed rate may produce larger plants more tolerant of larval feeding in the spring

Anecdotal reports suggest several companion crop species (e.g. buckwheat and legumes, such as clovers and fenugreek) could play a role in CSFB management, through attracting the pest, improving soil, masking the crop from CSFB or offering a shelter for natural enemies). Others, such as mustards, act as sacrificial plants that are eaten in preference. Limited research has detected significantly less damage in crops (two-leaf and five-leaf stage) companion-cropped with either a berseem/clover mix or mustard.

Top tips: Choose a companion crop that won’t outcompete oilseed rape and can be removed easily (e.g. killed by frost or taken out with a herbicide) and sow it about a week before.

Low-soil-disturbance-establishment methods are associated with more moisture-retentive soils (helping to improve establishment, particularly in a dry summer) and higher levels of natural pest enemies. The pest may also be attracted to disturbed soil. There is little robust evidence to show that this translates to a definitive reduction in CSFB damage, but anecdotal evidence and limited trial results are encouraging.

The addition of organic amendments around establishment could reduce CSFB damage, but reports are largely anecdotal and supporting trial data is sparse. Effects observed could be due to improvements in crop growth or the reduction of CSFB infestation (e.g. by masking the crop or deterring the pest).

CSFB larvae are far more likely to be present in leaf petioles than in the stem. Managed defoliation reduced larval numbers significantly in trials (by 23–55%), with late defoliation, before stem extension, most effective. Linked on-farm trials found that sheep grazing and topping/flailing reduced larval numbers by 51% and 25%, respectively. However, researchers did not detect significant/consistent yield increases in crops with reduced larval populations. Interestingly, defoliation also decreased the severity of phoma stem canker to levels equivalent to a fungicide spray.

This approach is best suited to early drilled crops which will have the highest larval numbers but also be best able to recover from the defoliation. 

Top tips: Defoliation is likely to be most beneficial between November and December, as this gives the crop more time to recover. Later defoliation is more risky – crop recovery is strongly affected by spring weather. Do not defoliate after stem extension.

Grazed cereals and oilseed rape (grown for grain and seed, respectively) are also treated as fodder crops, which restricts the use of plant protection products. Authorisations must include appropriate qualifiers, such as ‘for animal fodder’, otherwise treatment is not permitted

Crop management that enhances crop resilience (e.g. through nutrition and plant growth regulation) could make the crop more tolerant to larval damage. However, robust evidence is lacking.