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Excerpts from Jim Conrad's
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Entry from field notes dated June 29, 2023, taken along one-lane gravel road in valley between Puerto del Zenthé and Chavarrías in mountainous area with a maze of roads too complex to say how to get there; in general it's in the mountains of east-central Querétaro state, municipality of Cadereyta de Montes, 12 straight-line kms due east of Vizarrón de Montes but much farther by twisting roads; juniper-scrub forest; limestone bedrock; elevation ~2860m (~9400 ft), Querétaro, MÉXICO, (N20.84122°, W99.60509°)
ASCLEPIAS OVATA
On the valley floor in thin, rocky soil at the base of a limestone outcrop, in a scrubby area overgrazed by cattle, burros and sheep, the above milkweed was about 1m tall (3.3ft). Its stem was unbranched but other new and old stems rose from the base. The plant displayed all the typical milkweed field marks: thick, simple leaves occurring in pairs at stem nodes and fairly far apart, leaf margins without lobes or teeth, and conspicuous clusters of distinctive flowers with their unique features:
Above, each flower's center is occupied by a tin-can-shaped, white-topped item formed of the flower's female style and stigma united with the male stamens; that's the gynostegium. Surrounding each gynostegium stand five scoop-shaped structures called hoods, and note how the hoods' colors vary within a single inflorescence. The sharp-pointed, fingerlike items arising within each hood and extending over the gynostegium is the horn. Below all the above, there are five down-turning, greenish petals.
For identification to species level, in the above picture critical features to note are that the hoods do not rise above the gynostegium, a matter that separates this species, for example, from Asclepias notha, which also appears in this same valley. In fact, our plant's hoods are exceptionally short in a general sense, only about 3mm tall.
A couple of the above insects were attracted to the flowers and hesitant to leave. They were tarantula hawk wasps, Pepsis grossa, predators of tarantulas of the family Theraphosideae. The wasp's larvae feed on tarantulas supplied by the mother, but adults feed on flower nectar. The 2006 study by Fred Punzo entitled "Plants Whose Flowers Are Utilized by Adults of Pepsis Grossa Fabricius (Hymenoptera: Pompilidae) as a Source of Nectar" tells us that milkweed species are among the most important sources of nectar for adult Pepsis grossa, and that the wasps pollinate milkweeds. I can visualize the wasps' legs as they wander over the flowers slipping between the hoods, where they snag the gynostegium's packets of pollen called pollinia, and transferring the pollinia to other flowers.
At the right, immature fruit pods are attached to pedicels which curve downward, then abruptly turn upward, directing the fruit's slender end skyward. The picture also shows that pairs of leaves are widely spaced from one another. Altogether, this is a peculiarly lanky species. Note that at least some leaf blades are wider above their middles than below, and are neither very narrow nor very wide. Their bases are somewhat rounded and are attached to definite petioles much shorter than the pods' peduncles.
The above leaves tend to be wider below their middles, so leaves on this species are variable in shape. Leaves and stems are hairless, and the dark purple stem bears a frosty coating, or glaucescence, which easily rubs off. Finally, here's what our plant looked like in its habitat:
All the above details point to ASCLEPIAS OVATA, endemic just to Mexico, and occurring in uplands north of the Isthmus of Tehuantepec, except the more arid northern parts. It's found in various forest types, as well as grasslands and dry scrub, and is considered rare, despite its wide distribution.
This identification was made primarily with the help of the 2018 biology-degree thesis of Christopher Ostwaldo Cervantes Meza at the Autonomous University of Hidalgo State; the online thesis is entitled "Sinopsis del Género Asclepias (Asclepiadoideae, Apocynaceae) de Hidalgo, México." Our plant's location was only about 12kms west of the border of Hidalgo state, in a mountain range extending into that state.
The above study remarks that this species is highly variable in size and morphology of leaves and hoods, and that it commonly hybridizes with A. pringlei, A. notha, A. otarioides and A. similis.
There's little information about human use of Asclepias ovata, other than in the above thesis it's reported that sometimes it's eaten. In general, many milkweed species have been used traditionally. Fibers from the stems can be used to make bindings and woven textiles. Young milkweed shoots and stems, flower buds and immature pods can be boiled and eaten. Cooking is important since the plants' white latex contains various powerful alkaloids which often are medicinal in certain dosages but toxic in others.