Early plant injury as an indicator of infestation level of the cabbage stem flea beetle?

Abstract: Forecasting abundance of the cabbage stem flea beetle, Psylliodes chrysocephala, is widely based on monitoring of adult beetles with yellow water traps but there is not always a direct correlation between trap catches and larval density. The objective of this study was to investigate assessment of early plant injury as a monitoring method. This was done in a field cage experiment, by testing the relationship between beetle density and immediate and subsequent plant injury from adults feeding on plants and from larvae mining plant stems. Overall feeding of adults was low. A statistically significant relationship between beetle density and plant injury was not found when plant injury was expressed as damaged plants nor feeding holes. However the experiment revealed a small but significant increase in the number of damaged leaves: 3.3% per beetle at increasing density of beetles. The observed mean number of damaged leaves per cage of 24 plants was 11.6, 13.8, 13.2 and 19.0 at 2, 4, 8 and 16 beetles, respectively, four weeks after the beetles were released into the cages at plant growth stage BBCH 18. There was a statistically significant relationship between beetle density and larval density per plant. The larval density was low and the observed mean numbers of larvae per plant were 0.15, 0.38, 0.87 and 1.42 at 2, 4, 8 and 16 beetles. Overall the present study demonstrates a correlation between adult beetle density and subsequent number of larvae per plant. However, early plant injury was not a satisfactory indicator of infestation level. The possible causes of low levels of feeding and larval density per plant are discussed.