Cabbage stem flea beetle: advice for autumn 2019
As thoughts turn towards planting of the new oilseed rape crop, we asked ADAS expert, Steve Ellis for his advice on mitigating risk from grower enemy number 1, cabbage stem flea beetle (csfb). Here is what he said:
When is the best time to drill?
Drilling so that seeds have good seed to soil contact and some moisture remains key. Drilling early can increase the ability of the crop to tolerate adult damage as it establishes before adult migration begins. However, drilling early is also likely to result in higher larval pressure. Late drilling (into September) will result in lower larval pressure and possibly avoid adult damage by establishing after the bulk of adult migration has occurred.
What seed rates should be used?
Our trials (comparing 10, 20, 40, 80 and 120 seeds per m²) found few benefits of increasing seed rate. There was no significant difference in the numbers of larvae per plant between seed rates particularly where establishment was good. Where plots were drilled into a dry seedbed and establishment was poor the highest seed rates produced significantly higher yields. Larval numbers did not differ between plants so increasing seed rates resulted in higher numbers of larvae per unit area. High seed rates could therefore result in higher pest return for the following season. On balance, we recommend using a seed rate of no more than 40 per m², unless drilling conditions are poor.
Does method of cultivation/direct drilling make a difference?
The current project has generated no strong evidence that establishment method or stubble management effects csfb. However, other work suggests that direct drilling and leaving long straw stubble may reduce pressure although this needs to be confirmed in randomised, replicated trials.
Is there any evidence that varieties differ in the susceptibility to csfb?
Comparison of recommended list varieties showed no obvious differences in susceptibility to csfb. Crop vigour is now attracting some attention to select varieties suited to particular cropping scenarios. For example, if you opt for a September sowing you may choose a variety that shows good autumn vigour to achieve rapid establishment and growth.
What alternative control methods are being trialled?
Leaving OSR volunteers in situ until late September acts a trap crop to attract migrating adult csfb away from drilled OSR. Trial work has shown that this significantly reduces adult numbers and damage and significantly increases plant populations in nearby drilled OSR. Data (as yet unanalysed) also shows large reductions in larval numbers. Any csfb eggs and/or larvae in the volunteers will also be killed when these are destroyed.
Defoliating OSR in the winter to control CSFB larvae has also been investigated. Plot trial work has shown that defoliation significantly reduced larval populations by up to 55%. Defoliation did not reduced yield unless it was after stem extension. Trials on-farm showed that defoliation using a topper or grazing with sheep resulted in an average of 39% less larvae. In some cases, larval numbers were reduced by as much as 77% where crops were defoliated.
What are the main risk factors for the pest?
An analysis of csfb data from more than 1600 sites across 14 years found that weather conditions and drill date has the strongest effect on risk from the pest. High adult csfb damage was associated with hot weather in June and July, a dry August and September drilling. High larval populations in the autumn tended to occur after low rainfall in March, April, June and July, a warm September and August drilling. High populations of larvae in the spring was associated with August drilling and a warm November and January. Many other factors were found not to be associated with increased CSFB pressure, including soil type, seed rate, drill depth, row width and the level of CSFB pressure the previous year.