the mystery of Vanessa unidirectional migration

[Royce J. Bitzer]

At 08:57 PM 04/30/2001 -0500, Liz Day wrote:
>I don't understand what advantage this bug gains by migating north every 
>spring, if the adults don't then migrate back south in the fall, and (?) 
>thus their offspring are instead killed off by winter.  Isn't this genetic 
>suicide?   I understand dispersion is good, but it seems like consistent 
>directional dispersion in a direction that is ultimately death would be 
>selected against, eventually.   What is going on?  I must not be seeing the 
>whole picture.
>
>Thanks,
>
>-------------------------------------------------------------
>Liz Day
>Indianapolis, Indiana, central USA  (40 N, ~86 W)
>USDA zone 5b.  Winters ~20F, summers ~85F.  Formerly temperate deciduous 
>forest.
>daylight at kiva.net
>www.kiva.net/~daylight
>-------------------------------------------------------------
>
To Liz Day and others,

I think that Red Admiral migration in North America is actually
bidirectional. 
My own observations do suggest that the offspring of the spring arrivals don't
themselves migrate south. But is that a genetic "dead end?" Most likely not.
The reason why it's probably not is because their offspring, which are our
"summer brood" and appear to be stationary here, have enough time to breed yet
another generation during the summer. Butterflies of this third generation
start emerging in mid- to late August, and these in turn probably are the ones
that fly south from here during September and early October.

This suggested phenology somewhat parallels that of Monarchs in the
midwest, in
which one generation arrives here from more southerly regions in the spring
and
gives rise to a summer, stationary brood, which in turn breeds yet another
generation that is the one that actually migrates south.
 
To some extent, these generations are distinguishable by body size. In Iowa,
the springtime Red Admirals are smaller-sized, with forewing lengths mostly
between ca. 25 and 30 mm. Summer brood individuals are larger, with forewing
lengths mostly between 28 and 34 mm. The fall generation of individuals that
are their offspring are also large. Because of the body size difference
between
this fall generation and the succeeding spring generation, it seems unlikely
that the fall butterflies overwinter here as adults and then become active the
following spring. Instead they are most likely different butterflies, as R.R.
Baker claimed from a similar study he made of the European population in the
early 1970's.

Royce J. Bitzer
mariposa at iastate.edu

Dept. of Entomology 
113A Insectary 
Iowa State University 
Ames, Iowa 50011 
Phone: (515) 294-8663
 
http://www.public.iastate.edu/~mariposa/homepage.html
The Red Admiral and Painted Lady Website 
A web site to encourage and coordinate 
field studies of territorial behavior 
and migration of Vanessa butterflies 

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