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The autumn and spring pest surveys have been completed and the results compiled. These are publicly available on the CropMonitor™ website:
Mean numbers of CSFB larvae per plant for this survey and the preceeding six years are shown in Figure 1. Eighty sites were surveyed. The results show an increase in numbers across all regions with samples from the East region again showing the highest numbers of larvae per plant.The percentage of plants with scarring has also increased again this year (Tables 1-3).
The five larvae per plant spray threshold for CSFB has been exceeded at more sites than in any previous year (Table 4). Prior to 2014, the last time this threshold was exceeded at a surveyed site was in Autumn 1999. Numbers of sites exceeding this threshold fell steadily through the 1990s and the five larvae per plant threshold was not exceeded again at any site surveyed until 2014. Table 5 shows the equivalent data for the now no longer recommended threshold of two larvae per plant which was in effect if the insecticide was being applied as a tank mix with another product. These data show a similar picture i.e. an increase in the number of surveyed sites exceeding the threshold in the autumns of 2014 and 2015.
The mean numbers of CSFB per plant by county are shown in Figure 2. The four counties from which the most infested samples were returned were: Hertfordshire, Cambridgeshire, Bedfordshire and Essex. These all had more than four larvae per plant on average with levels in Hertfordshire exceeding 5 per plant. Samples from Suffolk, Oxfordshire, North Yorkshire and Wiltshire contained more than two larvae per plant on average. The four counties for which emergency use authorisation for neonicotinoid seed treatments was granted were Hertfordshire, Cambridgeshire, Bedfordshire and Suffolk and these counties co-incide with all but one of the areas of highest infestation.
Numbers of another pest, Rape Winter Stem Weevil (RWSW), have also increased by a very large amount (Tables 1-3). Again, this increase is most pronounced in the East region where the number detected increased from 1 last year to 44 this year. Given that the number of plants surveyed in this region in 2015 was 15% lower than in the previous year, the true comparative increase would have been even greater. The numbers of RWSW found in the Midlands & West region also increased this year to double the numbers found in autumn 2014.
Mean numbers of CSFB larvae per plant for this survey and the preceeding six years are shown in Figure 3. Forty-one sites were surveyed. The results show an increase in numbers across all regions with samples from the North, East and Sout-east regions showing the highest numbers of larvae per plant.
The five larvae per plant spray threshold for CSFB has been exceeded at 25% of the sites surveyed (Table 6). Table 7 shows the equivalent data for the now no longer recommended threshold of two larvae per plant which was in effect if the insecticide was being applied as a tank mix with another product. These data show a similar picture i.e. an increase in the number of surveyed sites exceeding the threshold in the springs of 2015 and 2016.
The mean numbers of CSFB per plant by county are shown in Figure 4. The four counties from which the most infested samples were returned were: North Yorkshire, Norfolk, Cambridgeshire and Oxfordshire. These all had more than six larvae per plant on average with levels in North Yorkshire and Norfolk exceeding 9 per plant. Samples from half of the counties from which samples were taken contained more than two larvae per plant on average. Numbers of RWSW found in spring have also increased again, although overall numbers remain low (Table 8).
Although CSFB numbers have increased across the regions surveyed, there was clearly some factor affecting CSFB and RWSW numbers in the East region which resulted in a particularly large increase in numbers over previous years in autumn. This is the region in which most CSFB control failures due to pyrethroid resistance have been recorded. It has become evident from the results of the spring survey that there has been a large increase in CSFB larva numbers over previous years across all regions, with very high numbers now present in the North and the South-East as well as in the East.
The increase in RWSW numbers in the same region over the other regions surveyed raises the question as to the pyrethroid resistance status of this pest also. It remains to be seen whether the more general increase in CSFB numbers is due to particularly warm winters or to the withdrawal of neonictonoid seed treatments. The Met Office Temperature Anomaly maps (Figure 5) clearly show that the mean daily temperature this autumn and winter have been well above average in each month for all of the regions covered by these surveys, with the exceptions of the South West and West Midlands in November and February. These regions were also the two regions where the lowest overall CSFB larva numbers were recorded during the spring survey.
Ongoing work, which is currently unfunded, is progressing slowly in collaboration with Reading University and Rothamsted Research to attempt to tease out the relationships between CSFB numbers, weather data and insecticide application data from both this survey and the Pesticide Usage Survey. We would also recommend an analysis to compare CSFB numbers in neonicotinoid seed treated and untreated crops from the data gathered during this survey in areas where neonicotinoid seed treatment has been authorised this year.