How low can you go (and live): Determining the sub-lethal exposure time to desiccation in snowberry maggot flies (Rhagoletis…

The fruit infesting snowberry maggot (Rhagoletis zephyria) inhabits a broad range of habitats across the northern United States, including the humid and arid parts of Washington State. Pupating snowberry maggots (the most vulnerable life stage) exhibit local adaptation, with flies being more desiccation resistant east than west of the Cascades. Previous experiments have measured this difference at eight days after the larvae leave the fruit. However, desiccation impacts on survival may occur much earlier. To better understand the mechanism(s) by which flies protect themselves from desiccation we need to study flies at a sub-lethal level of stress, as dying flies may display general stress, rather than desiccation-specific responses. We sampled larvae from both western and central WA, and exposed them to varying lengths of low humidity before 'rescuing' them in 100% relative humidity. Survival was measured indirectly as water loss. Desiccation sensitive flies from western WA experience lethal effects as early as day one of treatment whereas central WA flies did not differ from a no-treatment control. These results are consistent with previously reported gene expression differences between the two populations. Our results suggest that functional studies of desiccation resistance should focus on the time period immediately following the larva's exit from the fruit. These results have agricultural implications, as R. zephyria populations hybridize with the related R. pomenella (the apple maggot) and may transfer their desiccation resistance to this invasive pest.