SAMPLING PLANT BUGS, LYGUS SPP. (HETEROPTERA: MIRIDAE), IN CANOLA TO MAKE CONTROL DECISIONS

Abstract

AbstractPlant bugs in the genus Lygus are pests of canola (Brassica napus L. and Brassica rapa L.) in western Canada and may require insecticidal control. Sweep-net sampling of field plots and commercial fields in southern Manitoba between 1988 and 1995 was used to develop sequential sampling plans for plant bugs in canola to facilitate control. The variance–mean relationships for plant bug catches were defined by Taylor’s power law, and the parameters of the relationships were the same for field plots and commercial fields. Sampling units of 10, 20, 50, and 100 sweeps per sample had variance–mean relationships with the same slope but different intercepts and required different minimal sample sizes. Samples taken at two crop stages had similar variance–mean relationships, but at a later crop stage the intercept of the relationship differed and the parameters were estimated with less precision. Samples taken in two ways along the edges of commercial fields and at various distances into the fields all gave similar estimates of plant bug density, justifying the use of edge sampling. Experienced samplers caught more plant bugs than inexperienced ones, although the difference was primarily due to the number of nymphs rather than adults that were collected, and this difference was less pronounced in the edge samples. Sweep-net sampling collected less than 10% of the plant bugs present in the sampling area. Sequential decision plans are presented for four sampling units and three crop stages. Sampling commercial canola with a sweep net to make decisions on the need to control plant bugs can be completed in as little as 28–35 min. The sampling 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 method, plant bug densities were classified correctly in relation to the need for control in 20 fields using the minimum sample size.