Relationships between the lucerne flea, Sminthurus viridis (L.) (Collembola: Sminthuridae), and damage to lucerne

The effect of lucerne flea, Sminthurus viridis, on plant growth (stem length and leaf number) and damage (percentage of leaves damaged and the percentage reduction in leaf area in the top, mid and lower thirds of stems) was considered at 2‐weekly intervals in three consecutive lucerne growth cycles, each with a different mode of attack. The data were analysed to progressively determine relationships leading to a proposal for plant‐based thresholds for lucerne flea control. Lucerne flea affected each factor measured, with damage considered more appropriate for making control decisions than plant growth. Damage increased from the base of plants upwards and its effect on yield was carried over into subsequent growth cycles. Damage was significantly related to lucerne flea numbers (P < 0.001) in two growth cycles and was higher when large nymphs or adults were dominant in one of these cycles. Yield was closely and best related to the percentage of the total number of leaves damaged (R2 = 81.3%). The percentage of leaves damaged was also related to loss of final yield estimated at 2 (R2 = 81.3%), 4 (R2 = 78.9%) and 6 (R2 = 66.6%) weeks into the first growth cycle only. Direct damage in the other two cycles did not result in yield loss. The percentage of leaves damaged (± confidence intervals) equating to 5% increments in yield loss were estimated at 2, 4 and 6 weeks into the first growth cycle. These are proposed as damage or treatment thresholds for testing.