Abstract/Summary

Current European Union and UK legislation prohibits the use of neonicotinoid insecticidal seed treatments in oilseed rape (OSR, Brassica napus). This ban, and the reduction in efficacy of pyrethroid insecticide sprays due to resistance, has exacerbated pest pressure from the cabbage stem flea beetle (Psylliodes chrysocephala) in winter OSR. We quantified the direct impact of P. chrysocephala injury on the productivity of OSR. Leaf area was removed from young plants to simulate differing intensities of adult feeding injury alone or in combination with varying larval infestation levels. OSR can compensate for up to 90% leaf area loss at early growth stages, with no meaningful effect on yield. Significant impacts were observed with high infestations of more than five larvae per plant; plants were shorter, producing fewer flowers and pods, with less seeds per pod which had lower oil content and higher glucosinolate content. Such effects were not recorded when five larvae or fewer were present. These data confirm the yield-limiting potential of the larval stages of P. chrysocephala yet suggest that the current action thresholds which trigger insecticide application for both adult and larval stages (25% leaf area loss and five larvae/plant, respectively) are potentially too low as they are below the physiological injury level where plants can fully compensate for damage. Further research in field conditions is needed to define physiological thresholds more accurately as disparity may result in insecticide applications that are unnecessary to protect yield and may in turn exacerbate the development and spread of insecticide resistance in P. chrysocephala.