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.