Economic threshold for plant bugs, Lygus spp. (Heteroptera : Miridae), in canola

Plant bugs in the genus Lygus infest canola (Brassica napus L. and Brassica rapa L.) when the crop is producing buds, flowers, and pods. Field studies in cages and open plots show that plant bugs can reduce yield by 20% or mo re, but have little effect on seed size. A single application of a foliar i nsecticide when the crop has finished flowering and is beginning to produce pods will prevent most or all of the yield loss. The yield loss of canola that can be prevented by control is 0.007 t/ha per plant bug per 10 sweeps sampled at the end of flowering or the beginning of pod formation. The yiel d loss that can be prevented by a later application drops to 0.005 t/ha. Wh en precipitation is greater than 100 mm from the onset of bud formation to the end of flowering, the crop may partially compensate for plant bug damag e. The economic threshold for control of plant bugs in canola at the end of flowering or at the beginning of pod formation is 15 plant bugs per 10 swe eps, based on crop prices and control costs from 1989 to 1992. If plant bug s are present but control is not warranted when most flowering is complete, plant bug densities should be assessed again 5-7 days later as pods develo p, but at this stage the threshold is 20 plant bugs per 10 sweeps. The use of economic thresholds for chemical control of plant bugs will maximize see d yield and minimize unnecessary or ineffectively timed insecticide applica tions.