Evaluating broad mite (Acari: Tarsonemidae) sampling techniques in blackberry

Broad mite, Polyphagotarsonemus latus (Banks) (Acari: Tarsonemidae), is an emerging pest impacting blackberry (Rubus subgenus Rubus Watson) production globally. Small plot research recommends applying miticides when mite density averages 5 mites per leaflet or one egg per leaflet. However, an optimal scouting method has not been investigated at the field scale, and many growers rely on assessments of visual injury. This study aimed to identify an effective scouting strategy to estimate broad mite density, investigate the relationship between mite density and visual injury, and develop a visual injury sampling strategy for broad mite. We evaluated 3 leaflet sample sizes (5, 10, and 15 leaflets) in 12 commercial fields, and visual injury was rated on a descriptive scale (1 to 5) in 33 commercial fields. Regression analysis was used to correlate mite density with visual injury assessment, and the optimal sample size was determined. We determined that 15-leaflet samples provided the best estimate of mite populations due to it having the lowest variance-to-mean ratio of 12.2 for motile mites. At 70% accuracy, 31 samples of 15 leaflets were necessary to estimate mite density at threshold in blackberry fields. A positive correlation was observed between broad mite density and visual injury; when an injury rating of two was observed, 72% of samples were above threshold. Additionally, only 11 replicates, 10-cane visual injury samples were necessary per field. Our findings suggest that visual injury can be used to estimate broad mite populations and may encourage better adoption by growers than labor-intensive leaf sampling methods.