Greetings monarch and milkweed enthusiasts from Oaxaca! This is Part III in my series from Mexico where I am based on sabbatical leave from Cornell (see the second post
here). This post follows up on observations here that are laying the foundation for my next research and writing projects, continuing on from my recent book . Monarchs and Milkweeds
I recently visited Monte Alban, a remarkably intact archaeological site just outside of Oaxaca City. This was the site of one or the largest pre-Columbian cities in the region, established some 2500 years ago.
Monte Alban was a cultural and economic center of the Zapotecs for nearly 1000 years.
The nodding milkweed ( Asclepias glaucescens) was the most abundant Asclepias on site. This species produces copious latex and ranks relatively high on the scale of cardenolides (the poisonous cardiac glycosides that make milkweeds to most insects).
Monarch butterflies were abundant, flying, mating, and laying eggs. These are non-migratory monarchs, likely resident in Oaxaca all year. The importance of these non-migratory populations for monarchs overall, and exchange between the populations is largely unknown.
The nodding milkweed, Asclepias glaucescens
Two other milkweeds are common year-round in Oaxaca. Here, high and dry at Monte Alban, Zizotes milkweed ( Asclepias oenotheroides) is the other abundant species. Tropical milkweed ( Asclepias curassavica) seems to be always flowering in this region, but favors wet soils, often stream-side, and was not seen at the site.
What I have observed at several sites across the region is that monarchs are frequently laying several eggs per plant. As my son points out here, at least 4 eggs were on this plant. This behavior, sometimes termed “egg dumping” is not common in the Midwestern and Northeastern USA, where most plants lack eggs, and there are typically not more than 2 eggs laid pr plant. Indeed, monarchs usually avoid laying multiple eggs on plants as a means to avoid egg cannibalism. Monarch caterpillars are mostly vegetarian, but the one piece of meat they will eat is the eggs of their own species. Such egg dumping has also been observed occasionally in the north and in other localities such as Australia where monarchs and milkweeds are introduced species.
The moment my wife, Prof. Jennifer Thaler, and I realized that we were living aspects of the graphic novel by Peter Kuper, Ruins.
Ruins is the story of academics on sabbatical in Oaxaca, and their story is intertwined with the monarch butterfly’s annual migration.
Other spectacular tropical natural history abounds, some associated with milkweeds like this seed bug, Lygaeus reclivatus, a relative of the small milkweed bug ( Lygaeus kalmii) common in the Northeast. Here drinking nectar of the nodding milkweed’s flowers.
A leaf-mining fly (that’s right, a fly that lays its eggs on milkweed with the young feeding between the leaf layers) on nodding milkweed. I hope study this fly, which very well may be a new species to science.
I had previously reported this bug, a species from the genus Largus, associated with milkweed in Mexico, but I didn’t realize that it could be an important herbivore. Here the brownish adult is about to take off from a nodding milkweed, Asclepias glaucescens. But it appears much more common on Zizotes ( Asclepias oenotheroides).
In a mixed population with abundant “nodding” and many fewer “zizotes”, Largus seems to aggregate on zizotes. Here the nymphs (juveniles) of Largus are intermixed with the adults, and their feeding appears to be causing leaf necrosis. The steely blue nymphs with a red dot on their back (not visible here) are reminiscent of other milkweed insects and suggests that they may be chemically defended with milkweed poisons. Another hypothesis that should be tested!
Back at the ranch, I am continuing to rear monarchs, primarily on nodding milkweed, Asclepias glaucescens. The larvae grow well on these cut leaves, with little mortality or evidence of disease.
Rearing monarchs from eggs takes time, but usually ends well. Interestingly, in the field, I seem to only see 1 caterpillar for every 100 or so eggs. In other words, predation on eggs or young caterpillars seems to be very high. When I find larger monarchs in the field and bring them in to rear, they seem inevitably infested with parasitoids. Here one monarch (top) and one queen butterfly (bottom) collected in the field attempted to pupate at a rather small size. Four fly larvae emerged from each of them and pupated themselves instead. These parasitoids were laid as eggs on the caterpillars, they hatched and burrowed into their bodies, and fed until maturity. They emerged like aliens.
The milkweed vines continue to befuddle me. Although not in the genus Asclepias, they are very much milkweed vines by virtue of their latex, fruits and seeds, and perhaps by their insect herbivores. It has been difficult to study their insects because most appear to be at the end of their life-cycle, such as this one in February 2018. Note the flowers of the tree that the milkweed vines is climbing on.
Opening pods of the milkweed vine.
Spiny pod of a different milkweed vine. Seeds are already gone.
Clearly another species, with highly distinct fruits. Several genera occur here, including Cynanchum, Marsdenia, Funastrum, Matelea, and Gonolobus (with many species in each)! A tremendous diversity of milkweed vines.
Oaxaca sits at mid-elevation (4500 ft)… In my hikes to look for milkweeds and their insects, I have been hiking up to over 9000 feet of elevation in the hills. In the grasslands just above Oaxaca, this very unique Euphorbiaceae appears to be a common early spring plant. It is tiny, just an inch wide.
Higher up, one passes through oak forests, which transition to pines. In and amongst these higher elevation trees, was a massive tree from the Malvaceae, the botanical family with cotton and hibiscus. The flowers are huge with cups of nectar which are apparently visited by hummingbirds (but also buzzing with bees). This species, Chiranthodendron pentadactylon, or Devil’s hand tree, was used medicinally by the Aztecs.
And finally, some humor.
Anurag Agrawal , photographed by his son, behind a katydid near San Jose del Pacifico, Mexico.