Possible Painted Lady Migration this spring due to El Nino?

[Royce J. Bitzer]

Is anyone here planning to carry out systematic observations of Painted
Lady (Vanessa cardui) migration this spring?

In a letter dated December 24, 1997, Derham Giuliani [of Big Pine,
California] wrote to me:

"I have been taking observations on V. cardui migrations for several
years and have always wished that there could be organized a group of
individuals who could take simultaneous measures of the flights from many
geographic points....Since the proper rains in northern Mexico in winter
are what seem to trigger mass migrations, it is possible that this El
Nino could make 1998 a migration year.  The migrations usually start in
March and peak in April [in Big Pine, California].  Persons in N Mexico
or S Arizona could alert those of us further north that a flight has
begun...."

He has been taking the following two basic observations:

Migration size:  count number crossing a line perpendicular to the line
of flight and convert to a standard (number/5 minutes/15 meters).  15m is
approximately equal to 50 feet.  So, 20 butterflies crossing a 30m line
in 15 minutes = 3.33/5 minutes/15m.

Flight direction: step into the line of flight of a passing butterfly and
aim a compass at the retreating insect.  There is always variation but
central tendencies can be seen from a proper sample (try to record at
least 20 butterflies per observing session.)  Be sure to correct your
compass readings to true north after taking your data.

Some other interesting observations are the following:

Body orientation during flight: This is often difficult to
measure accurately.  Perhaps the best way with such small butterflies is
to note whether the butterfly is facing either to the left or to the
right of its flight direction.  Also note wind speed and direction during
your observations.

Possible changes in mean body size during the migration:  This can be
done by measuring the forewing length, the mass, or both, of a sample of
migrating Painted Ladies each day as the migration proceeds.  A
good daily sample size might be on the order of 40-60 individuals
(more if you're feeling ambitious). Forewing length (in mm) can be
measured with an accurate ruler or with calipers.  Mass is taken by
placing a butterfly with its wings closed in a small paper or plastic
envelope, weighing the butterfly and envelope together to the nearest
0.01 gram, then subtracting the weight of the envelope.
These data can tell you if there is any tendency, for example, for the
butterflies in the forefront of the migration to be larger.  In addition,
any change in the ratio of mass/winglength as the migration proceeds
might give one an idea of the amount of stored energy reserves that the
butterflies have available, or the amount of nectar they are able to
locate along the way, etc.  Presumably, late-arriving butterflies might
not reach an area until most nectar sources have been depleted.  As a
result, their mass could be lower relative to their size.

Possible change in sex ratio during the migration:  Is there a tendency
for one sex to start migrating before the other?  If so, that sex will be
predominant in the early part of the migration.  As the migration
continues, more butterflies of the other sex will arrive.  The sex ratio
should then reach 50-50 at some point, and then reverse as more
individuals of the later-arriving sex pass through.  Again, a sample of
about 40-60 individuals per day is recommended.  Sexing Painted Ladies
requires viewing the genitalia.  For this, a 10x or 16x doublet or
triplet folding magnifier should be sufficient.  The genitalia are
somewhat hidden beneath a ring of scales at the end of the abdomen.  Look
for large, spiny, flaplike claspers of the male.  Females have an opening
which resembles a small, vertical pair of lips.  An alternative method of
sexing is dissecting to find the internal genitalia.  One might then
examine a sample of females to find out how many are carrying male
spermatophores.

More information on observing migrating butterflies and the
accompanying weather conditions can be found at these web sites:  Monarch
Watch, Tactics and Vectors, and the Red Admiral and Painted Lady Web
Site.

Derham Giuliani can be reached by mail at:

P.O. Box 265
Big Pine, CA  93513
U.S.A.

My mailing address is:

Royce J. Bitzer
Department of Zoology and Genetics
Iowa State University
Ames, Iowa  50011
U.S.A.
e-mail: mariposa at iastate.edu