In early spring, as Monarchs move north through the Texas funnel, they need milkweed on which to lay the first generation of eggs. They continue to need milkweed through the summer as subsequent generations reproduce as they move north.
But in the fall when the Monarchs are heading south, they don’t need milkweed since they enter their reproductive diapause–that is, a temporary stage in which they postpone sexual activities and save their energy to migrate to Mexico. At this point, they need water, ample nectar sources, trees or other protection at night for roosting, and connected habitats.
Agrawal’s point is well taken. Immense focus has been devoted on increasing the Monarch butterfly host plant, milkweed. Milkweed, anything in the Asclepias family, is necessary in the early stages of the migration when the Monarchs are reproductive. But what about later in the game, when Monarchs are NOT reproductive? They require nectar to fuel their flight. Agrawal and his colleagues suggest that late season nectar plants and perhaps interconnected habitats–ie, pollinator corridors–would make their continuous route south more doable.
University of Georgia ecologist Andy Davis made a similar point last year when he spoke to the Washington Post. Davis challenged the idea of basing the Monarch butterfly population census on counting the number of hectares in Mexico occupied by the migrating creatures each winter.