Oosorption and migratory strategy of the milkweed bug, Oncopeltus fasciatus

Migration evolves as a response to seasonally unfavourable environments but plasticity in reproductive physiology is another avenue by which insects can respond to resource-poor conditions. We investigated the relationship between individual variation in migratory propensity and the level of response to poor conditions modulated by the female reproductive physiology. We tested the hypothesis that, compared to migrants, residential behaviour is associated with a higher degree of phenotypic plasticity in oosorption, an adaptive physiological mechanism that allows females to recoup resources from undeveloped oocytes. Reallocation from reproduction to survival would allow females to skip migration and to cope with unfavourable environments. If this plasticity is evolved, we further predicted it would vary between as well as within populations. We examined variation associated with migratory behaviour in females from four populations of the milkweed bug, Oncopeltus fasciatus, using a behavioural assay to categorize females as either migrant or resident and observing the differences in oosorption between these groups. As expected, food availability, source population and wing length influenced the propensity for migratory flight, and food availability influenced levels of oosorption. We also found support for our key prediction that resident females are characterized by higher levels of ovarian oosorption than migrant females. Our study provides support for a physiological difference between migrant and resident females and suggests the presence of both physiological and behavioural tactics that interact with the potential for migration to provide adaptation to seasonally challenging environments.