Citrus thrips, Scirtothrips citri (Moulton), is a foliage-feeding pest of blueberries in the San Joaquin Valley of California. We conducted a 4-yr field study to determine the type and amount of damage caused by this species. Using pesticides, we established gradients of citrus thrips in commercial blueberry fields near Richgrove, CA, in the fall of 2006, 2007, 2009, and 2014. Thrips densities were evaluated weekly for ∼1 mo to determine cumulative thrips-days and correlate levels with the average length of new growth. During all four years of the study, there were significant negative correlations between thrips-days and shoot length (for every 100 thrips-days over a period of 4-5 wk there were reductions in the length of new shoot growth of 0.41 to 2.45 cm, 6.4-10.3%). During the spring following each trial, we evaluated the impact of thrips-days on blueberry yield and quality. During the 2006 trial, there was a significant negative correlation between thrips-days and yield as well as the number of berries per plant, but no yield effect was observed in the other three years of the study. No impacts on fruit quality were found any year. A discussion of the complexity of economic injury levels in blueberries is provided, especially considering that the cost of spraying for citrus thrips (estimated at US$150/ha) is almost irrelevant given crop values often in excess of US$100,000/ha.
Keywords: economic injury level; high-value crop; monitoring; pesticide resistance management; yield.
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