Latest national monitoring of pest damage in oilseed rape plants has highlighted an increase in numbers of cabbage stem flea beetle larvae found in northern parts of England.
The Food and Environment Research Agency (Fera) carries out oilseed rape pest assessments in both the autumn and spring for cabbage stem flea beetle and rape winter stem weevil larvae.
In autumn, 25 plants are taken from 100 sites around the country and destructively sampled for petiole scarring caused by flea beetle larvae and also numbers of the larvae inside the plants.
Similar destructive sampling is carried out in the spring from 25 plants across 50 sites and numbers of larvae are the only data recorded.
Results from spring 2016 can be seen in the graph below, with the mean number of larvae found per plant in the North significantly higher than the Eastern “hotspot” area.
In 2015 the North had just 0.51 larvae/plant, but the 2016 assessment uncovered a huge jump to and average of 5.34 larvae/plant.
There is also a trend of significantly increasing numbers in most other areas of England, particularly in the South East.
It is the second full season growers have not had widespread access to neonicotinoid seed treatments, which help prevent early adult feeding damage to seedlings.
This is combined with rising resistance to pyrethroid insecticides, which have been the most effective way of controlling adult flea beetles, but are becoming less effective due to over-reliance on the chemical group.
It should be noted that growers in hotspot areas in the eastern counties of Hertfordshire, Bedfordshire, Cambridgeshire and Suffolk used a finite amount of neonicotinoid seed treatment through an emergency use derogation this season.
Mean CSFB larvae per plant after spring assessment (2010-2016)
You can see the full report on the CropMonitor website.