Reducing cabbage stem flea beetle larvae by companion cropping
Information
It is becoming increasingly difficult to control the cabbage stem flea beetle (CSFB). New trial results from Deutsche Saatveredelung AG (DSV) show that companion cropping can be an effective strategy for managing the CSFB. The TerraLife®-BrassicaPro oilseed rape companion planting mix reduced the number of CSFB larvae by up to 42%.
Damage caused by the CSFB in oilseed rape
Oilseed rape is an economically attractive crop in arable farming. At the same time, the CSFB poses an ever-growing challenge to oilseed rape cultivation due to increasing infestations and a worsening situation regarding resistance to insecticides. The pest can cause significant damage to the crop at two stages during the growing year. Even the adult beetle can significantly impair the early development of oilseed rape plants by feeding holes in the cotyledons. Once eggs have been laid, the damage takes its course: after hatching, the larvae bore into the petioles of the outer oilseed rape leaves, where they initially cause mining damage. They can then migrate as far as the shoot tip of a stem, where their feeding can significantly impair the development of the oilseed rape plant. A severe larval infestation weakens the plants considerably and, just like extensive beetle feeding in autumn, can lead to the death of individual oilseed rape plants or to more extensive damage.
Resistance Makes Control More Difficult
he chemical control of the CSFB is becoming increasingly difficult due to resistance to synthetic insecticides. One important form of resistance is KDR (knock-down resistance). In this case, changes to the voltage-gated sodium channel – the site of action of pyrethroids (a class of synthetic insecticides) – mean that the CSFB is less sensitive to these active substances. The rapid paralysis (knock-down) of the pest, which is typical of pyrethroids, either fails to occur or is significantly weaker.
An even stronger form is sKDR resistance (super knock-down resistance). Here, additional or combined changes at the site of action result in an even higher level of resistance. In such populations, pyrethroids can only exert their effect to a very limited extent. However, mutations have also been found where, due to very high resistance factors, a complete loss of efficacy in the field must be assumed.
If active substances from the same group are used repeatedly, selection pressure increases and resistant beetles can proliferate more readily. At the same time, a prolonged influx of beetles and the fact that egg-laying can continue for several weeks make it difficult to determine the optimal time for treatment. Insecticides should therefore only be used following monitoring and once the applicable control threshold has been exceeded, in accordance with the current authorisation. Preventative and non-chemical measures are therefore becoming increasingly important.
Why use an oilseed Rape companion crop mix?
A companion crop mixture can help to reduce infestation of oilseed rape plants by CSFB. DSV has been investigating this effect for several years at its breeding sites in Terminiers (France, approximately 120 km south of Paris) and Asendorf (Lower Saxony, Germany) using the TerraLife®-BrassicaPro companion crop mixture. The positive trends observed in previous years were statistically confirmed in this growing season. Compared with an oilseed rape crop without a companion seed mixture, the number of CSFB larvae at the trial site in Terminiers fell by up to 42 %. This corresponds to a reduction from 110 to 64 larvae per plant – a significant decline, given the very high initial infestation level. In Asendorf, the number of larvae – under far lower pressure – was significantly reduced by 20 %. Based on current knowledge, it can be said that the importance of a companion crop in supporting the control of CSFB larvae increases as larval numbers rise, and that the reduction in CSFB larvae is attributable to a diversionary effect of the companion crop.
How was the effect of the companion planting assessed?
The assessment was carried out using the Berlèse method, a tried-and-tested standard for monitoring Cabbage Stem Flea Beetle larvae. The survey was conducted by colleagues at our trial sites. The method is straightforward to implement and can also be used by farmers to determine the level of infestation on their own land. The end of February has proven to be a suitable time for the survey. However, sampling should always take place before temperatures rise significantly, as the larvae become mobile as temperatures rise and may begin to migrate out of the plants.
To this end, a specified number of representative plants are collected from several locations within the area for each variant. The plants are placed individually or in defined groups on a grid or in a funnel above a collection vessel. In the traditional set-up, a lamp provides light and heat from above; in the passive method, the material dries in a warm room. The mobile larvae leave the plants, move downwards and fall into water to which a drop of washing-up liquid or a preservative has been added. The containers are checked regularly and the larvae are counted. The result is expressed as the number of larvae per plant. Comparable results require the same number of plants, sampling dates, temperatures and emergence times. For reliable experimental comparisons, replicates and standardised procedures are also important.
Next steps:
rials are planned at three sites next year. The Berlèse test will continue to be used as a tried-and-tested method for counting larvae. The focus will remain on how to effectively reduce infestation by Cabbage Stem Flea Beetle larvae. In parallel, a fertilisation rate trial will be carried out at three sites to investigate the impact of different fertilisation rates.
Conclusion: TerraLife®-BrassicaPro Reduces Cabbage Stem Flea Beetle Larvae.
The results show that TerraLife®-BrassicaPro can make a valuable contribution to integrated pest management. In addition to its insect-repellent effect, the targeted combination of different plant species improves soil structure, promotes soil life, optimises nutrient dynamics and protects the soil from erosion. Against the backdrop of KDR and sKDR, the combination of monitoring, preventative cultivation measures, objective assessment of infestation using the Berlèse test, and the targeted use of insecticides is becoming increasingly important.
Source: Own surveys from the internal 2025/26 rapeseed companion crop trials at two sites (Asendorf and Terminiers), with three replicates per site.
