We have been observing with great interest what we believe is
the third generation of Monarch caterpillars, soon to be followed
by adult butterflies, that have taken advantage of the half dozen
Mexican milkweed plants (Asclepias currisavica) growing in the
bed of annual flowers along our front sidewalk. A female Monarch
butterfly laid an egg in June on one of the plants when the
little milkweed was only around ten inches tall.
The caterpillar was so easy to watch from day to day and by
around its third instar it had stripped, literally gobbled up,
every leaf on that plant and had to move to another one where it
completed its growth before entering into its chrysalis stage.
Slowly all of those milkweed plants continued to grow and form
new leaves and, just as fast, other monarch eggs were laid on
them. Even the first plant that had been completely stripped of
its leaves began forming new foliage.
Now there are two nearly full-grown Monarch caterpillars on
one of the plants and, hopefully, they will soon form chrysalids
and develop into that magical generation of adult Monarch
butterflies that will complete the most incredible migration of
any insect on Earth. In fact one of the Monarch butterflies
tagged in Canada, and later recovered on their Mexican wintering
site, was proven to have flown at least 1,870 miles. It had been
originally tagged on September 18 in Highland Creek, Ontario and
was spotted again in San Luis Potosi, Mexico, four months later.
Another had been marked in Campbell, Minnesota and was
recovered on its winter roosting site 1,844 miles to the
southwest in El Rosario, Mexico. Bear in mind that this distance
was obviously not achieved in a straight line, so the total
flight may have been a few hundred miles further. Estimates are
that the migratory distances of some may be 3000 miles.
Naturally our interest in Monarch butterflies is shared with
countless other nature watchers and was delivered some especially
shocking news this past January. A devastating storm on Jan.
12th and 13th dropped approximately four inches of rain in the
Monarchs’ wintering area at an elevation between 10,000-11,000
feet in the Oyamel Fir forests in the mountains west of Mexico
City, their primary wintering site.
The crippling blow to the butterflies occurred when the
torrential rainfall was followed by freezing temperatures.
Unfortunately monarch butterflies, even though they can withstand
temperatures to around eight degrees F. below freezing, are
particularly susceptible to freezing if they become wet. It was
reported by Dr. Lincoln Brower, eminent biologist of the Monarch
butterfly, that the temperatures following that crippling storm
were the lowest recorded in the Monarchs’ wintering colonies
within the previous 25 years.
Fortunately at least approximately 25% of the Monarchs
survived and by around the third week of March this year began
their journey northward to find the first milkweed plants.
Mating had previously occurred, fertilized eggs were laid on the
milkweed plants and the adults then died, having lived for about
eight months.
It would be that first generation of new monarchs, having
developed into flying adults in around 25—30 days, to head
northward eventually arriving in northeastern Wisconsin in late
May to early June. This first generation, along with the second
and third, will have life spans of between two and five weeks.
It is only the fourth and final generation that, unlike the
preceding three, which were hatched and were sexually mature and
ready to mate, hatches having reproductive organs that are
immature. In other words they are in a state of reproductive
diapause. They will have to wait until the following spring to
mate.
What’s so incredible about this fourth generation is that they
will have a life span of around eight months and, quite
miraculously, find their way to precisely the same wintering
site, to which they have never been, where their
great-great-great-great grandparents spent the winter of 2001!
Dr. Lincoln Brower and some of his colleagues believe that a
magnetic mineral in the Monarch’s body may operate as a direction
finder thereby enabling these fragile butterflies to locate one
of about a dozen wintering sites all contained within an area of
roughly 300 square miles, amounting to slightly more than 17 by
17 miles in size.
Enough recoveries of marked monarch butterflies along their
southerly migration route indicate that they fly on the average
about 50 miles a day. You may be one of the lucky people to
witness a large "flock," or perhaps a
"flutter," of hundreds of Monarchs spending the late
afternoon, throughout the night and into early morning, roosting
in a tree.
Such was the discovery at around 3 p.m. on August 17, 1997 by
two of our very good nature observing friends, Dean and Bernice
Shumway, in the rural area southeast of Sister Bay. We arrived
at their place early the following morning only to find that most
of the butterflies had already left for the South. The Shumway’s
estimate of several hundred Monarchs roosting in one Sugar Maple
tree was surely a thrill of a lifetime.
All we can hope for is that these unusually wonderful
butterflies have had a good summer in this region and that next
spring they will return in greater numbers to provide nature
lovers with more exciting observations. Summer would be
incomplete without Monarch butterflies.
This gathering of monarch butterflies congregated late one afternoon on a sugar maple
tree to rest and spend the night before continuing their long flight to Mexico.
This column appeared in the Door County Advocate
on 08/30/2002.
© Copyright 2002 Roy Lukes. All rights reserved.
