Evaluation and improvement of a sampling technique to estimate densities of Oligonychus perseae (Acari: Tetranychidae) on Hass avocado leaves

The persea mite, Oligonychus perseae Tuttle, Abatiello and Baker (Acari: Tetranychidae) is a foliar pest of Hass avocados and densities of O. perseae exceeding 100 mites per leaf have been linked to yield reduction. Counting O. perseae on leaves as part of pest management is a tedious process because of the mite’s small size, constant movement and dense webbing. Using 3,791 Hass avocado leaves sampled across 9 orchards in Southern California, we evaluated the performance of an existing simple linear regression model designed to quickly estimate y, the total number of mites on a leaf, based on predictor x, a partial count of mites found along the second major vein from the left half of the avocado leaf undersurface. The results indicate that the ‘half-vein’ sampling technique, in its current state, can underestimate O. perseae mean densities with up to 70% and 60% error when a hand-lens or microscope is used to measure predictor x. Therefore, we developed a series of regression models with updated parameter estimates to improve the accuracy of the ‘half-vein’ sampling technique. The implications of the modeling results and practical applications of this sampling technique for research purposes are discussed.