In the fall, decreasing daylength, cooler temperatures, and aging milkweed (genus Asclepias; the larval food plant) contribute to the environmental events that trigger the migratory generation (Goehring and Oberhauser, 2002). Migrant flight is directional; they orient to the south/southwest using a time-compensated sun compass.
Thanks to a deficiency in juvenile hormone (JH), migrants are reproductively inactive (a state called diapause), have increased abdominal fat stores, and live longer. They remain in this limbo throughout the winter until warmer temperatures and longer day lengths coax them out of diapause. Then they mate, fly northward, and lay eggs on newly emerging milkweed plants.
The summer butterflies hatched from these eggs are reproductively active and short-lived. In two successive generations, these monarchs leapfrog north from the overwintering sites to repopulate their entire range. We have found in flight simulator studies that at the end of summer—quite unlike the fall migrants—monarchs don’t exhibit oriented flight at all (Zhu et al., 2009).
What are the physiological and molecular differences between migratory and non-migratory monarchs? How is the switch to reproductive diapause triggered, and what are the genetics underlying that change?