Maximising control of cabbage stem flea beetles (CSFB) without neonicotinoid seed treatments

Abstract

Cabbage stem flea beetles (CSFB, Psylliodes chrysocephala) migrate into establishing oilseed rape (OSR) and chew holes in cotyledons and early true leaves causing ‘shot-holing’ symptoms. This results in stunting and poor plant vigour and occasionally total crop failure. The 2013/14 cropping season was the last before the suspension of use of neonicotinoid seed treatments in oilseed rape took effect. The aim of this project was to conduct a one year study to investigate establishing oilseed rape without neonicotinoids and to identify topics for further research. The main objectives of this project were to:

1. Identify levels of CSFB control achievable without the use of neonicotinoids
2. Assess tolerance of oilseed rape seedlings to simulated ‘shot hole’ damage at a range of growth stages

A field experiment was established at Settrington, North Yorkshire on a site with a previous history of CSFB damage. There were eight replicates of three experimental treatments, untreated seed, Cruiser OSR (fludioxinil, metalaxyl-M, thiamethoxam) treated seed and untreated seed plus a spray of Hallmark (lamda-cyhalothrin) at the 1–2 leaf stage. Numbers of adult CSFB were monitored with yellow water traps. Plant populations, numbers of shot holes and yield were also assessed. A pot experiment located in a glasshouse at ADAS High Mowthorpe also investigated the impact of defoliation to the cotyledons, leaf 1 and leaf 2 on green leaf area and dry matter yield. A total of sixteen treatments were compared in which between 0 and 100% of leaf area was removed. A hole punch was used to simulate shot holing by CSFB and scissors were used to remove entire leaves.

Numbers of CSFB in the field experiment were relatively low. Cruiser-treated seed improved plant populations compared with the untreated control but all treatments had sufficient plants to achieve potential yield. Cruiser and Hallmark both reduced the numbers of shot holes but levels of damage were less than the 25% leaf area lost at the 1–2 leaf stage to justify insecticide treatment. There was no statistically significant effect of treatment on crop yield.

In the pot experiment, there was surprisingly little impact of defoliation on green area or dry matter yield. In 13 out of 15 defoliation treatments there was no significant difference in green area between defoliated plants and the untreated control and in the remaining two treatments (including removal of both cotyledons) green leaf area was significantly increased. Overall, only three of 15 defoliation treatments significantly reduced dry matter yield compared with the untreated control, nine had no effect and, where both cotyledons were removed, dry matter yield was increased.

In summary, results of glasshouse studies under ideal growing conditions showed that once above ground oilseed rape has significant inherent ability to compensate for loss of leaf area. These results now require further validation in the field. Crop damage does not necessarily equate to loss of yield and results from the pot experiment suggest that defoliation thresholds may be too conservative. Tolerance to loss of green area has the potential to limit unnecessary applications of insecticides. Recommendations are provided for further research to help improve our understanding of the interaction between CSFB and oilseed rape and contribute to the development of a rational approach to pest control minimising reliance on insecticide sprays.