National Survey of Cabbage Stem Flea Beetle Larvae in Winter Oilseed Rape Plants in Autumn 2016 and Spring 2017

Abstract

The outputs of the spring 2017 and autumn 2016 national surveys of CSFB larvae in winter oilseed rape maintain the consistency of surveillance of cabbage stem flea beetle (Psylliodes chrysocephala; CSFB) larvae as part of a long-term data set. The data are broken down by region and county for both spring and autumn surveys. These data will be used to assess changes in CSFB prevalence and to monitor trends in agronomic practice for pest management. They will also be used to provide data for current and future research projects on the integrated pest management of CSFB. Eighty winter oilseed rape fields were selected at random from a list of farms in England. Winter oilseed rape crops were assessed for the presence of CSFB. Plants from all 79 fields were assessed in autumn and a subset of plants from 39 of the same fields was assessed the following spring. 

The numbers of larvae found were lower in autumn 2016 compared with autumn 2015. This pattern was observed across all regions and it was most notable in the ‘East’ region. When the same fields were surveyed in spring, this reduction in numbers was still evident for the ‘East’, ‘North’ and ‘South East’ regions; however the reduction was not as substantial as might have been expected following the low autumn numbers. Although the numbers of larvae found during the autumn survey were lower in 2016 than in 2015, the numbers found in the spring survey were higher than in the previous spring for one region: ‘South West’. The threshold of five larvae per plant was exceeded at one site (in Kent in the South-East region) during the autumn 2016 survey. There were 10 sites in the South-East in total and 6 of these were in Kent. Nationally, the threshold of five larvae per plant was exceeded at six sites during the spring 2017 survey. The six sites were in: Cambridgeshire (2 of 5 sites), Essex (1 of 2 sites), Lincolnshire (1 of 2 sites) and North Yorkshire (2 of 4 sites).

It is possible that smaller numbers of larvae were initially detected in some regions in autumn 2016 than in previous years due to a decrease in the adult beetle population: A lowered population of adult beetles was reflected in yellow water trap catches of this life stage in autumn 2016 (unpublished Fera Science Ltd. Data). The winter weather was warmer than usual, according to the Met Office Temperature Anomaly Map, and this allowed the adult beetles to remain active throughout winter. Oviposition may have continued during winter and the conditions for survival of the larvae were better than in most previous winters, in that the mean temperature was higher. This may explain the differences between the numbers of larvae found during the autumn 2016 and spring 2017 surveys in some regions.

The implication for the OSR industry is that it is possible that even with small recorded numbers of adults and larvae in autumn that a large population of CSFB larvae will be present in OSR plants in spring. Modelling work would be required to ascertain the effects of mean temperature on 2 population size and on changes in population size. Future research should work to produce predictions of spring CSFB populations based on varying temperature anomalies during the winter months.