Quantifying the impact of Psylliodes chrysocephala injury on the productivity of oilseed rape

BACKGROUND: Current European Union and United Kingdom legislation prohibits the use of neonicotinoid insecticidal seedtreatments in oilseed rape (OSR,Brassica napus). This ban, and the reduction in efficacy of pyrethroid insecticide sprays dueto resistance, has exacerbated pest pressure from the cabbage stemflea beetle (Psylliodes chrysocephala) in winter OSR. Wequantified the direct impact of P. chrysocephalainjury on the productivity of OSR. Leaf area was removed from young plantsto simulate differing intensities of adult feeding injury alone or in combination with varying larval infestation levels.
RESULTS: 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 thanfive larvae per plant; plants were shorter, produced fewerflowers and pods, with fewer seeds per pod which had lower oil content and higher glucosinolate content. Such effects werenot recorded whenfive larvae or fewer were present.
CONCLUSION: These data confirm the yield-limiting potential of the larval stages ofP. chrysocephalabut suggest that the current action thresholds which trigger insecticide application for both adult and larval stages (25% leaf area loss andfive larvae/plant, respectively) are potentially too low as they are below the physiological injury level where plants can fully compensatefor damage. Further research infield conditions is needed to define physiological thresholds more accurately as disparity mayresult in insecticide applications that are unnecessary to protect yield and may in turn exacerbate the development and spread of insecticide resistance in P. chrysocephala.