Sampling plant bugs, Lygus spp. (Heteroptera : Miridae), in canola to makecontrol decisions

Plant bugs in the genus Lygus are pests of canola (Brassica napus L. and Br assica rapa L.) in western Canada and may require insecticidal control. Swe ep-net sampling of field plots and commercial fields in southern Manitoba b etween 1988 and 1995 was used to develop sequential sampling plans for plan t bugs in canola to facilitate control. The variance-mean relationships for plant bug catches were defined by Taylor's power law, and the parameters o f the relationships were the same for field plots and commercial fields. Sa mpling units of 10, 20, 50, and 100 sweeps per sample had variance-mean rel ationships with the same slope but different intercepts and required differ ent minimal sample sizes. Samples taken at two crop stages had similar vari ance-mean relationships, but at a later crop stage the intercept of the rel ationship differed and the parameters were estimated with less precision. S amples taken in two ways along the edges of commercial fields and at variou s distances into the fields all gave similar estimates of plant bug density , justifying the use of edge sampling. Experienced samplers caught more pla nt bugs than inexperienced ones, although the difference was primarily due to the number of nymphs rather than adults that were collected, and this di fference was less pronounced in the edge samples. Sweep-net sampling collec ted less than 10% of the plant bugs present in the sampling area. Sequentia l decision plans are presented for four sampling units and three crop stage s. Sampling commercial canola with a sweep net to make decisions on the nee d to control plant bugs can be completed in as little as 28-35 min. The sam pling is most efficiently conducted with a sampling unit of 10 or 20 sweeps taken along the edge of a field. In an independent test of the sampling me thod, plant bug densities were classified correctly in relation to the need for control in 20 fields using the minimum sample size.