Enhanced stress responses and metabolic adjustments linked to diapause and onset of migration in the large milkweed bug Oncopeltus fasciatus

In response to short-day photoperiods in the autumn, the large milkweed bug Oncopeltus fasciatus Dallas enters a reproductive diapause and migrates south to avoid the adverse environmental conditions and food shortages that prevail in the winter. Milkweed bugs are one of only a few temperate insects that undergo long distance migration during diapause, making them a good model for investigating trade-offs associated with migratory diapause. Although enhanced stress tolerance is typical of diapause, it is unclear whether this aspect of diapause would be retained in a species that migrates to more favourable conditions. The present study tests (i) whether diapause enhances thermal tolerance; (ii) whether food shortage, which is required for maximal expression of diapause, influences thermal tolerance during diapause; and (iii) whether the potential changes in stress tolerance are associated with upregulated heat shock protein expression or metabolic adjustments (including cryoprotectant synthesis), or both. Both cold tolerance at -10 °C and heat tolerance at 43 °C are significantly higher in diapausing O. fasciatus, whereas food restriction has no further effect on thermal tolerance. None of the heat shock protein transcripts measured are significantly upregulated in response to diapause, and the experiments also fail to detect any cryoprotectant accumulation during diapause. Thus, although heat shock proteins and cryoprotectants are common mechanisms for enhancing thermal tolerance in many diapausing insects, the results of the present study suggest that alternative mechanisms are responsible in milkweed bugs.