Winter defoliation of oilseed rape to control cabbage stem flea beetle larvae 2019/20

Summary

A group of four farmers are exploring the impact of winter defoliation on oilseed rape (OSR) to manage cabbage stem flea beetle (CSFB) populations.

Cabbage stem flea beetle has been the most important insect pest of winter OSR in the UK for several years.  It is estimated to affect 67% of the total winter OSR area. 

Building on the 18/19 trial, this field lab will assess the impact of defoliating OSR on larval populations of CSFB and on yield at harvest. Defoliation of OSR could be done using several methods, most likely using a topper or livestock. 

The aim of the defoliation is to kill larvae physically (e.g. ingestion by livestock or action of topper) or exposure to inclement weather or natural enemies (e.g. by removing leaves and breaking open petioles). 

This Field Lab will provide a better understanding of:

• the optimal effect of timing & severity of defoliation
• effect of weather on OSR recovery following defoliation
• best management of defoliated crops for increased recovery
• varietal differences

How it works

Foliar pyrethroids are the only insecticides registered for use against CSFB. However pyrethroid resistance is widespread in the UK. This has resulted in:

• many areas where farmers have no effective control options for the pest,
• an increase in pyrethroid usage of 292% between 2010 and 2016 as farmers make repeated applications in an attempt to control CSFB,
• significant CSFB damage in recent years, and this year it is estimated that considerably more than 11% of the national crop will be lost to the pest.

If successful this Field Lab will encourage wider uptake of this non-chemical control method and reduce reliance on broad spectrum pyrethroids on farms.  It aims to show:

• reduced larval populations
• improved yields
• that the method is practical and economical

Trial design

Each farm will select a field and will carry out at least two treatments: a defoliation treatment and an untreated (undefoliated) control treatment (UTC). 

The triallists will carry out the defoliation, using a method that is chosen by them and is most appropriate to their resources. The defoliation will occur on at least one tramline width, the rest of the field will be the UTC. 

Assessments:

• ADAS research staff will visit each site in late February/early March to take GPS coordinates of the treatment areas and collect 30 plants from each treatment area for larval assessments.
• Triallists will take photographs of the treated and untreated areas to track the impact on crop growth, make pollen beetle assessments, measure crop height and comment on any other impacts of the defoliation.

• Yield will be assessed by analysing yield map data.