Cabbage stem flea beetle – Risk mitigation strategies: Part 2
3 Sep 2018
Getting winter oilseed rape (WOSR) established can be difficult but adult feeding damage is only part of the problem when it comes to the destruction which cabbage stem flea beetle (csfb) can cause; damage from their larvae can also devastate WOSR crops.
Dr Steve Ellis and Dr Sacha White, entomology experts at ADAS, discuss the implications of some early findings from an ongoing AHDB project on csfb in the following blog.
Sacha explained, “In this AHDB project we have 14 years of data from over 1600 sites across England and Scotland on csfb adult damage and larval populations, as well as other data such as temperature, drill date and variety. We are modelling this data to see if any of factors can be identified as having a significant impact on csfb with a view to developing an integrated pest management programme (IPM) programme.
If large numbers of larvae are in the plant from October through to March/ April they can inhibit growth and development and potentially destroy the crop. Our modelling has given us the initial results on factors affecting spring larval populations. We are currently working on identifying risk factors for adult damage and autumn larval populations.
Factors to consider this summer (and beyond)
We didn’t find any clear effect of drill date on adult damage; however, in the modelling for the spring larvae we found that drill date was a really important factor.”
Steve said, “The later you drill the lower the subsequent larval population. This is probably because the numbers of beetles migrating decreases as the season progresses so less eggs are laid and as a result less larvae establish in the plants; so potentially larval pressure can be reduced by not sowing too early.”
Sacha added, “The data show that there is a really clear gradual reduction in spring larval populations, with the highest associated with early August drilling and the lowest associated with late September drilling. Temperature is also likely to have a role in this effect, as csfb in late drilled crops will experience lower temperatures and we know that egg-laying and development are slowed by cool temperatures”
‘’There are both pros and cons to increasing seed rate. On the plus side high plant populations might dilute the larval population per plant. On the negative side the more plants you establish the more plant material there is for the csfb to feed on and as a result more larvae might succeed in finding a host. There is also a physiological trade off; very high plant populations have over large canopies that are more susceptible to yield loss due to lodging,” said Steve.
Steve explained, “Rather than sowing something specifically to lure the csfb away from the WOSR, we are asking can we make use of WOSR volunteers that aren’t going to cost us anything?
We have found that leaving volunteer WOSR in a field adjacent to one of newly sown WOSR led to a reduction in adult csfb in the emerging crop as well as higher plant populations and lower levels of adult csfb damage. It appeared that the volunteers were acting as some sort of trap crop; however, we need to carry out more trials on this and we will be repeating this experiment again this year.”
Mowing or grazing WOSR
Sacha said, “There is very little that growers can use to control larvae; there is resistance to pyrethroid insecticides in the csfb population and where there is not, it is very difficult to target larvae. We carried out an experiment looking at the effect on larval population and yield of mowing the WOSR in the winter.
We found there were significant reductions in the larval populations where we mowed in December or January and also an increase in yield compared to the unmown control. We will be looking at this again this year.”