It turned out that the majority of monarchs (88%) originated in the midwest and Great Lakes regions, providing the first direct evidence that second generation monarchs born in June complete a (trans-) longitudinal migration across the Appalachian mountains. The remaining individuals (12%) originated from parents that migrated directly from the Gulf coast during early spring. The results provide evidence of a west to east longitudinal migration and provide additional rationale for conserving east coast populations by identifying breeding sources.
The Monarch butterfly (Danaus plexippus) travels thousands of kilometers from breeding grounds in eastern North America to wintering sites in central Mexico. During early spring, over-wintering Monarchs and their first generation offspring recolonize eastern North America, becoming abundant in the mid-west and Great Lakes regions in June. Curiously, Monarchs do not become abundant in areas east of the Appalachian mountains until July, leading Brower to hypothesize that the recolonization of the east coast is accomplished by second generation individuals born in the central and northern mid-western states in June that migrate eastwards across the Appalachians; the ‘range expansion’ strategy or "RE").
Alternatively, recolonization of the east coast could be accomplished by the first spring generation produced in the Gulf coast region. Under this strategy, individuals migrate northwards from the Gulf coast to areas east of the Appalachians in late May and early June, lay eggs and produce a cohort of second generation monarchs along the east coast that emerge in early July (‘coastal migration’ strategy or "CM").
They collected Monarchs at 17 sites in the eastern United States and used stable-hydrogen and -carbon isotopes to estimate natal origins. Following the RE strategy, they predicted that monarchs would have isotopic values consistent with the centralwest and northwest regions of the breeding range, while those following the CM strategy would have isotopic values indicative of either the Gulf coast region (migratory adults) or the central-east and northeast regions (offspring of Gulf coast adults).
The results? Wow!...88% originated in the midwest and Great Lakes region! As a lover of Monarchs (That's why I capitalize the name) and ardent observor of them, I have noticed each year a sudden dearth in eggs and catepillars collected in the latter part of June. This is after large numbers were collected earlier along the roadways and in my backyard Monarch Waystation No. 613 in NW Ohio. Could this late June "lull" actually be that many Monarchs made the eastern journey? I never grew concerned, because the numbers of eggs and caterpillars collected skyrocketed up again in July.
Is it possible that the Monarchs that did not make the eastern journey finally boosted the numbers of eggs and caterpillars collected in July? Case in point. One of my collection sites is a small "waste" area behind a store in downtown Fremont, Ohio, consisting of 39 Common Milkweed plants. I inspect each leaf on each plant for eggs and caterpillars; no egg is left behind! Last July, I collected 33 eggs in one day in the waste lot behind the store and the next day collected 18, and the very next day collected 23...74 eggs in three days from the small waste lot! This is after a short "lull" in collections (-0- eggs) from the waste lot during late June. Ironically, I collected up to 3 eggs on one leaf and 6 eggs on one plant.
The Monarch migration study raises many questions in my mind. Could this west-to-east migratory phonomena in June be a way to prevent over-crowding and thus reduce the spread of disease among the Monarch population or to minimize parisitization- spread out men, one grenade can kill us all?
Read the full research report: MONARCH BUTTERFLIES CROSS APPALACHIANS FROM WEST TO RECOLONIZE N. AMERICA'S EAST COAST
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