Western cherry fruit fly, Rhagoletis indifferens (Curran; Diptera: Tephritidae), occurs in unmanaged cherries (Prunus spp.) across dry climates in Washington State and other regions in western United States and Canada. To help explain the fly's distribution in arid climates, we determined the effects of adding water to bare and grass-covered soils on pupal survival rates and adult emergence patterns of R. indifferens in central Washington. Water was added ('irrigated') or not to bare soil and grass-covered soil in cylinders pressed into the ground. Larvae were added to cylinders and pupal survival and adult fly emergence determined over three seasons. Irrigating bare soil had no effect on pupal survival or adult emergence, but in grass-covered soil, irrigation increased pupal survival (15.217.0% vs 6.7-9.5%) and adult emergence (2 of 3 yr, 12.8 and 24.1 vs 1.1 and 11.6%; no difference in 1 yr). Pupal survival was greater in bare (24.5-47.7%) than grass-covered soil (6.7-17.0%). Moisture was higher in irrigated than unirrigated treatments but within irrigated treatments, moisture in bare versus grass-covered soil did not differ. In both irrigated and unirrigated treatments, humidity in soil 5 cm below the surface where many pupae reside was >60%, including during summers. Results suggest R. indifferens pupae tolerate unirrigated soils during summer because soil moisture and high humidity there can prevent desiccation, contributing to the fly's wide distribution and abundance in dry climates, and that dry, grass-covered soils or mulch barriers in mid-and late-summer could reduce fly survival under some conditions.
