Determining the critical density, the spatial distribution, and a sequential sampling plan for Scirtothrips dorsalis (Thysanoptera: Thripidae) in Florida strawberry

Field surveys and field-cage studies were conducted to develop and validate a sequential sampling plan of S. dorsalis in Florida strawberry. Surveys of ten commercial fields in 2016 and 2017 showed S. dorsalis was highly aggregated, using Taylor’s power law (aggregation parameter b = 1.72). A threshold level (or critical density) for S. dorsalis was determined to be two S. dorsalis at any life stage per strawberry trifoliate. It was associated with a 10-30% injury to strawberry trifoliate. Aggregation values and critical density were used to parameterize various sequential sampling plan models (Iwao’s sequential procedure, converging lines, and sequential probability ratio test). Sampling plan simulation was used to compare sampling plans, with a sampling unit of ten strawberry-trifoliate from ten adjacent strawberry plants. The most accurate and least labor-intensive sampling plan, Iwao’s sequential procedure, required a minimum of four sampling units, but up to 20 sampling units when the field population means was near the critical density value of two S. dorsalis per trifoliate. The operating characteristic, or probability of incorrectly categorizing an S. dorsalis population as below threshold, was 35% when the field population means was near the critical density. Using a sequential sampling plan to scout S. dorsalis on young strawberry plants during Oct-Dec in central Florida fields will allow accurate decision-making for timing an initial insecticide application and will eliminate unnecessary repeat applications.