I am getting settled in Oaxaca, on sabbatical leave from Cornell studying milkweeds and monarchs in Mexico. Here is my initial report on plants and insects from the first couple of weeks. The people, culture, food, and biodiversity have all met my expectations so far. What a great country! This is providing inspiration for the next chapter of what I would like to research and write about, following up on my recent book
. Happy new year! -Anurag Monarchs and Milkweeds
January 2018. In downtown Oaxaca city, a mature stem-succulent plant from the milkweed family, Pachypodium… I assume it is P. lamerei from Madagascar. My kids call it Elephante.
In Mazunte, on the coast, about 250 km south of Oaxaca city, seed-eating Oncopeltus, it could be O. sexmaculatus on a milkweed vine. Note the bright red abdomens of these toxic milkweed specialist bugs.
Also in Mazunte (the southern most part of Mexico), another milkweed vine, likely Marsdenia sp. (thanks to Mark Fishbein for the ID).
Marsdenia latex, as distasteful as ever.
The Marsdenia were all senescing, as January is the beginning of the dry season, and the vines die back each year much like most milkweeds. Despite the lack of many herbivores, there was plenty of evidence for leaf chewing and the typical insect behavior of deactivating latex canals by cutting the veins before feeding on leaf tissue. In the US, this type of damage would be done by a leaf beetle. Here, who knows?!
In downtown Oaxaca city, tropical milkweed flourishes, although only alongside streams where there is plenty of moisture and shade. The afternoon sun, even in here, causes them to wilt. The oleander aphid, Aphis nerii, a specialist on milkweeds is nonetheless a “generalist” — it eats nearly all milkweeds and is on 6 continents (save Antarctica). Here it was doing its typical thing. A. nerii has almost never been reported to make sexuals — it is a highly clonal reproductive machine.
Surprisingly to me (it shouldn’t have been) monarchs ( Danaus plexippus) we abundant in this population of several hundred milkweeds.
At least two adults were flying around, and there were plenty of eggs… relatively few larvae, as is typically the case. Predation pressure is likely very high: most all eggs and caterpillars are food for others.
The big surprise was this spectacular Diabrotica beetle, likely D. nummularis. It has previously been collected from milkweed, and was feeding on flowers here. Their larvae are typically root feeders (!), but probably on other plant species. Thanks to several coleopterists for help with the ID.
Asclepias curassavica also had a few leaf miners, likely in the genus Liriomyza (its a fly that feeds between leaf layers). Although I believe that L. asclepiadis is the only known milkweed feeder, there are likely many undescribed species. Their host range and evolutionary ecology mostly unknown. A great PhD thesis waiting to be done.
But tropical milkweed was well-known to me. From hikes around the city and taxi rides, I have been looking for another somewhat common milkweed, but one that I have only grown (never seen in the wild) ( Asclepias glaucescens). And there it was — from the back seat of the taxi “ por favor, para, necesito ver una planta” Complete with plenty of insect damage and A. nerii. Interesting to me that milkweeds, several thousand miles removed from Ithaca, appear to be subject to the same consistent selection by similar, if not the same, insect pests.
On a jog through the city, towards the edge of town, Jennifer and I found a farmer’s field with tens of A. glaucescens (aka nodding milkweed) . This primarily Mexican species is widely distributed, has a tall upright structure, and is VERY waxy. It looks purple-ish, no leaf hairs to speak up, and has big creamy flowers. Very latexy.
And quite predictably, a familiar caterpillar was munching away. But upon closer inspection, it was not familiar to me… not a monarch, but its sister species, The Soldier, Danaus eresimus. Although long-thought to be more closely related to the Queen butterfly ( D. gilippus), recent work shows that it is closer to monarchs. Related milkweeds, related butterflies, the whole community repeats itself.
The classic vein drain followed by feeding in V form… in the US this type of damage would most often be done by the four-eyed Tetraopes beetles. This is is not how a monarch typically disarms milkweed. Its sister, the soldier, has found a very similar, but distinct counter-ploy to milkweed’s latex.
Until next time, all my best, -Anurag