The effect of diapause on stress tolerance in migratory milkweed bugs, Oncopeltus fasciatus

In response to short photoperiods in the fall, the large milkweed bug, Oncopeltus fasciatus, enters a reproductive diapause and migrates south, presumably to avoid adverse conditions and food shortages. Milkweed bugs are unique in that they are one of few temperate insects to migrate during diapause. Because milkweed bugs avoid adverse conditions by migrating, it is uncertain whether diapause increases environmental stress tolerance, as is typical of other types of insect diapause. In this study, we tested 1) whether diapause increases environmental stress tolerance, 2) whether food shortage during diapause further influences stress tolerance during diapause, and 3) whether heat shock proteins are upregulated during diapause to enhance stress tolerance. Milkweed bugs were reared under nondiapausing (25°C, 16:8 L:D) and diapausing (25°C, 8:16 L:D) conditions, and diapausing conditions with no food, to simulate seasonal changes in food availability. We found that both cold tolerance at -10°C and heat tolerance at 43°C were significantly higher in diapausing bugs relative to nondiapausing controls. However, restricting access to food had no further effect on thermal tolerance. To see whether heat shock proteins (hsps) were in part responsible for the increase in thermal tolerance, we measured expression of ten different hsp transcripts from five families. None of the transcripts were significantly upregulated in response to diapause; thus, we conclude that some other mechanism is responsible for increasing environmental stress tolerance during diapause.