Effect of population density on the sampling range of pheromone traps for the navel orangeworm (Amyelois transitella) (Lepidoptera:Pyralidae) Charles S. Burks & Bradley S. Higbee USDA-ARS, Parlier, CA Paramount Farming Company, Bakersfield, CA Introduction The navel orangeworm is the primary insect pest of almonds and pistachios; California crops collectively worth ≈$5 billion in Population growth of this pest varies greatly depending on the host, cultivated variety, and stage, and abundance is greater in mature pistachios compared to mature almonds. Seasonal trends in abundance also differ between these crops. Despite advances in characterizing the pheromone blend of this species, there is still no pheromone lure available for this species with sufficient field stability for practical use. Unmated navel orangeworm females are used as a pheromone source in lures for research, and are also in limited commercial use. Mating in this species occurs in the last 2 hours before dawn above 17°C, and begins earlier in the night as temperatures fall closer to the 12°C threshold for mating activity. Abundance and sampling range Sampling range is the maximum distance from which the target species is known to be captured by an attractive trap over a given time. Distance over which mutual interference between traps can be demonstrated has been used to estimate sampling range. Several studies have shown that fewer males are captured in pheromone traps with more calling females in the area. Such demonstrations imply that the number of calling females would affect the sampling range of a pheromone trap, but few studies that have examined this hypothesis. Objectives of current work 1)Determine whether sampling range varied with abundance 2)Determine sampling range for navel orangeworm using female-baited traps Methods Unmated females used as pheromone source were from a USDA laboratory colony. Females were isolated at mature larvae, and placed in plastic mesh cages shortly after eclosion for transport to the field were they were placed in wing traps. Grids of 9 pheromone traps were hung from trees in the center and 402 and 805 m in each cardinal direction in three almond and three pistachio orchards, each of approximately 256 ha. Each ranch had north-south rows, and each was ≥5 km from any of the other five orchards. Traps were placed and baited with three unmated females starting 27 May, and monitored weekly until 2 September. Fig 3 Seasonal abundance in almonds and pistachios. Colored lines represent means of individual 9-trap grids. Fig. 1 Plot locations and arrangement.Fig 2 Caged females as pheromone source. Fig 4 Proportion of males in the center trap v. mean males for the 9 traps. There is sig. correlation for mean < 100 males per trap, but not when the mean is greater. Fig 5 Mean proportion of males by trap position (mean <100 males per trap). The prevailing wind was from the west- south-west, and the rows were north-south. Across rows the upwind trap captured the most males, but along rows the downwind trap captured most males. Results Abundance over the 14 weeks varied substantially between the two crops (Fig. 3). The percentage of males in the center trap was therefore used as an index of unequal distribution of males among the traps. Center traps averaged ca. 11% (i.e., 1/9) when there were high overall numbers of males captured in the 9 traps, but the average percentage was lower at lower abundance (Fig. 4). Linear regression was significant for males per trap per week (n = 49; r 2 = 0.22, P 100 males per trap per week (n = 33; P > 0.95). For weekly observations with a mean of <100 males per trap for the three traps, there was a trend of more males in peripheral traps in each cardinal direction (Fig 5), although this trend was stronger in the north-south direction (with the rows) than in the east-west direction (across the rows). Nighttime winds in the study area were generally light; i.e., <3 m/s, and the wind generally came out of the west or southwest. Conclusions 1)Sampling range varied with abundance. 2)At low to moderate adult density, the sampling range for navel orangeworm adults using female- strength lures is >400 m (i.e., >40 acres).