Thoughts
About Early Reports of Arrivals in Mexico
Contributed by Dr. Bill Calvert
Students
in the mountain schools surrounding Angangueo, Michoacan reported the
presence of monarchs in mid September, over a month earlier than previously
reported first arrivals. It would seem that three factors could account
for these very early sighting of monarchs near the overwintering areas
in Mexico.
1) The first question, of course, is whether these were truly monarchs
or a mistaken identification.
2) If monarchs were truly seen, they may simply be a few monarchs that
breed locally in the area and have wandered into the region.
Milkweed (Asclepias curassavica) in Angangueo is not abundant being planted
in only a few gardens. There are patches of A. curassavica near most reservoirs
and rivers in the nearby mid-elevation (ca. 1500 m) areas of Zitacuaro
and Tuxpan, Michoacan. The closest of these watercourses is about 20 kilometers
from Angangueo, well within the range of an exploring monarch. Also in
these areas is the local native milkweed, Asclepias glaucesens. The phenology
of this species is not well known. It develops in the late winter and
spring months at the end of the dry season and at that time is an important
food plant for monarchs. Whether or not it support monarchs during the
wet season needs to be investigated.
Another milkweed, Asclepias linaria, is present a scant 15 kilometer north
of Angangueo near and around the town of Sengio. This is a strange looking
milkweed that has very thin needle-like leaves and in general resembles
a pine seedling. Although eggs have been found on it, it is not known
if it is an important host for monarchs.
3) A third possibility, and by far the most interesting one, is that
the monarchs have truly arrived early this year. This could have at least
two interpretations.
a) A few of the monarchs always arrive early and go unnoticed because
they are insignificant compared to the numbers that come later. In support
of this idea are reports of early arrivals in Texas and in northern Mexico.
Although it has not been verified for these particular sightings, an early
contingent of reproductive monarchs usually arrives in Texas to take advantage
of late summer milkweed growth. Could the monarchs reported so early in
Angangueo simple be early reproductives that have expanded their range
southward in the same manner that they do in Northern Mexico, Texas and
other southern states?
b) The last interpretation is the most intriguing: This really is a very
special year. Monarchs have migrated early. The ones seen near Angangueo
are true non-reproductive migratory monarchs ? an advance party of the
hordes that will follow.
One piece of evidence that is missing is the reproductive status of the
early arrivals. If they were reproductive, we could rule out their being
part of the main migration, for these are believed to be in reproductive
diapause. If they were reproductive, they could be local or they may have
come down form the north.
Another piece that would be useful is are the early arrivals present in
the sanctuaries in the mountains above the town. If there were there and
were roosting gregariously, it would be further evidence that they are
true migrants.
So what is the fate of early migrants? Is there any cost to arriving early?
One question that quickly comes to mind is is there enough nectar available
to feed them and for them to accumulate the fat reserves that they need
to survive the overwintering period? In the fields below the sanctuaries,
several species of fall plants normally provide nectar for monarchs. Among
these are several late season composites such as Tithonia and Viguiera
and several species of Eupatorium. If the monarchs arrive during the rainy
season, will these nectar sources be available to them? If not will there
be other plants that can provide them the needed nectar? All these questions
point to the importance of ground truthing observations to augment and
verify our interpretations and speculations!
It seems like each year observations generate more questions. But each
time they also clarify our understanding of the biology of this most interesting
insect.
