PEST STATUS OF LYGUS BUGS (HEMIPTERA, MIRIDAE) IN OILSEED BRASSICA CROPS

The feeding damage of lygus bugs and their effects on yield of oilseed rape were assessed in commercial fields and plots of Brassica napus L. and Brassica campestris L. in Alberta, Canada. Surveys of commercial fields showed that 6% of canola seed in some regions was destroyed by lygus bugs, on average, and that in some fields > 20% of the seed was destroyed. When lygus bugs were controlled with insecticide at the early pod stage, yield increased by 11-35% in five tests. The highest yield loss was associated with a lygus bug density of 52 per 10 sweeps at the early pod stage and this loss was statistically significant. The smallest yield increases occurred when insecticide was applied at the bud or flower stages, but these were not significant. The percentage of seeds injured by lygus bug increased and the yield decreased as lygus density increased. In three of four trials, yield declined as the percentage of seed injured by lygus bugs increased. These trends were similar in a number of experiments conducted in different years and locations, but many of the slopes defining the relationships were not statistically significant, and the predictive variables accounted for relatively small proportions of the variation. The percentage of seed injured by lygus and the relationships between lygus density, seed damage, and canola yield support the conclusion that lygus bugs can cause agronomically important losses of canola under field conditions in Alberta.