Effect of abundance of the navel orangeworm on sampling range and interference between pheromone traps

The sampling range of pheromone traps for the navel orangeworm Amyelois transitella (Walker) (Lepidoptera: Pyralidae) and its association with abundance was investigated by examining mutual interference within cross-shaped arrays of nine wing traps baited with virgin females and placed at 400-m intervals in three 256-ha blocks of almonds (Prunus dulcis [Miller] D. A. Webb), and three of pistachios (Pistacia vera L.). The proportions of males captured in the different positions were compared with the mean males for all traps, used as an index for abundance. For means between zero and 50 males per trap per week, the distribution was unequal between trap positions and the greatest proportion of males were captured in the northern-most trap (i.e., the within-row direction). Between 50 and 100 males per trap per week, most males were captured in the western-most traps and fewest in the center, and proportions were equal in other trap positions. Above 100 males per trap per week, the proportion of males captured was more nearly equal for all trap positions. These results demonstrate that the sampling range of pheromone traps for navel orangeworm is density dependent and, at low densities, is >400 m. They also indicate that abundance affects the impact of direction (orientation) of trap interference. At low density, female-strength pheromone traps sample males from beyond the block in which they are placed for orchard blocks of <50 ha.