Recent Boston Globe "monarchs are threatened" article

[Paul Cherubini]

Pat Foley wrote:

> You make an interesting point that migrations reappear in farflung
> populations of the Monarch. That is well worth studying. Do you think it
> is due to built-in genetically controlled tendencies? Do you think it is
> a matter of immediate good decisionmaking by the butterflies? Is it a
> vestige or an easily selectively reinforced set of traits? All good
> questions. 

At temperate latitudes (approx. 33-57
degrees N or S of the equator) the relatively rapid day to day decline
in daylength after about July 1  somehow triggers adult reproductive 
diapause in monarchs and the associated "instinct" to "seek" a 
fall / winter diapause site where the climate is consistently cool and 
humid enough to maintain the diapause. So when monarchs became 
inadvertently transported by man to farflung places like southern 
Australia, the seasonal migration / overwintering phenomenon 
becomes quickly established.

> But if the eastern NA Monarchs do not achieve sufficient
> Darwinian fitness by their migration, the migration will sputter out. As
> it may have in untold other species. And it will sputter out either
> because the wintering sites or the feeding sites become compromised.

Well the monarch migration phenomenon has a long established
record of persisting even in cases of extremely small migratory 
populations. For example, the monarch 
overwintering colonies in Sydney, Australia contain only hundreds of 
monarchs rather than tens of thousands as on the California coast or
tens of millions as in central Mexico. And the monarch overwintering 
colonies in Adelaide, Australia contain merely dozens of monarchs. 

Also, tagging has established that western and Midwestern
USA fall migrant monarchs suffer relatively little mortality during 
their long distance migrations.  Example: Approximately 1 in every 
50 fall migrants tagged in the Midwest USA are recaptured at the
overwintering sites in Mexico regardless of whether they were tagged 
way up north in Minnesota or way south in central Texas.  And in
a 1990 study, 1 in  every 10 monarchs tagged in the state of 
Washington was recaptured along the California coast 
vs. 1 in every 11 tagged in Nevada or northeastern California.

> If the wintering site for the Eastern NA monarchs has no special
> features, could you explain why the Monarchs don't overwinter in all the
> closer (to the NA milkweeds) habitats on their routes? 

The fall migrant monarchs thoughout the northern & central USA 
migrate mostly south and southwest in September / October until 
they reach geographical locations where the climate is consistently cool 
and humid.   So monarchs arriving at the California coast in September / 
October stop migrating while those arriving along the south 
Atlantic coast, Gulf Coast and in Texas keep on migrating because 
the climate is still so warm.  Considerable numbers of south Atlantic 
coast monarchs enter Florida and don't live 
too long because it's warm everywhere in Florida which breaks the 
reproductive diapause and causes rapid aging.  A portion of the fall 
migrants arriving along the Gulf Coast flies westward to Texas and then 
southward into Mexico. Another portion simply heads out over the waters
of the Gulf of Mexico and drowns in the ocean at nightfall (except for some
that find temporary refuge on boats and oil drilling platforms).

In other words, although a high percentage of western and Midwestern 
monarchs survives the journey to the overwintering sites in California 
and Mexico, only a small percentage of Atlantic coast monarchs does 
so because fall migrant monarchs have limited navigational capabilities.

> Paul, how are milkweed populations doing in the Midwestern US? 

Milkweeds are virtually everywhere in the Midwest and almost every
city and county of every midwestern State becomes practically inundated 
with monarchs for a week or two every year during either September 
or October depending on latitude.

Paul Cherubini

 
 ------------------------------------------------------------ 

   For subscription and related information about LEPS-L visit:

   http://www.peabody.yale.edu/other/lepsl