|Female monarch feeding on tithonia|
|Laying eggs on Arizona Milkweed, Asclepias angustifolia|
Dr. Chip Taylor, Director of Monarch Watch, explains: "Migratory monarchs are in what is known as reproductive diapause. This simply means that the butterflies have suspended reproduction. This condition is very labile, that is to say, easily broken. The non-reproductive condition is probably maintained by a balance of hormones (specifically one called "juvenile hormone" that triggers reproduction) and enzymes that break down this hormone. The diapause system breaks down, i.e.reproduction starts, when the butterflies are exposed to winter temperatures in the high 70s and low 80s over a few days. Warm nights and days accelerate the process. Monarchs can go from being non-reproductive to being ready to lay eggs in 4-5 days. Some of my colleagues believe that the presence of milkweeds in the winter is sufficient to trigger reproduction. Perhaps, but it is difficult to design experiments that separate a response to plants from that of temperature alone. Our experiments suggest that low temperature is both the main initiator and maintainer of diapause in monarchs. Once in diapause, monarchs exhibit a number of behaviors that seem to be designed to keep their body temperature and metabolism from becoming elevated. Once butterflies in transit become reproductive, the behavior shifts from migratory to local, mating occurs and females begin looking for milkweed plants."
|Monarch egg on Desert Milkweed, Asclepias subulata.|