Milkweed

The milkweeds, Asclepias (L. 1753), are a genus of herbaceous perennial, dicotyledonous plants that contains over 140 known species.

Milkweeds
Asclepias + ant feeding on nectar
Scientific classification
Kingdom:	Plantae
(unranked):	Angiosperms
(unranked):	Eudicots
(unranked):	Asterids
Order:	Gentianales
Family:	Apocynaceae
Subfamily:	Asclepiadoideae
Genus:	Asclepias L.
Species
See text.

Milkweeds are an important nectar source for bees and other nectar eaters, and a food source for caterpillars. The weeds are eaten by caterpillars of the monarch butterfly and its relatives, and by other herbivorous insects (such as beetles, and true bugs). These insects are able to feed on the plants despite their chemical defences. Milkweed is named after its milky sap, which contains alkaloids, latex, and several other complex compounds. Some species are toxic.

Pollination in this genus is accomplished in an unusual manner, as the pollen is grouped into pollen sacs. The feet or mouthparts of flower visiting insects such as bees, wasps and butterflies, slip into one of the five slits in each flower formed by adjacent anthers. The bases of the pollinia then mechanically attach to the insect, pulling a pair of pollen sacs free when the pollinator flies off. Pollination is effected by the reverse procedure in which one of the pollinia becomes trapped within the anther slit.

Milkweeds have three defenses to limit damage by caterpillars and other insects: hairs on the leaves, toxins, and latex fluids. Some more recent milkweed species grow faster than older species. They have the potential to grow faster than caterpillars can eat them. Caterpillars which eat milkweeds, and their adult butterflies, may be protected by the foul taste of the milkweed chemicals. Such butterflies and caterpillars usually show warning colors (see mimicry#warning coloration).

Species

Asclepias albicans Asclepias amplexicaulis Asclepias asperula Asclepias californica Asclepias cordifolia Asclepias cryptoceras Asclepias curassavica Asclepias eriocarpa Asclepias erosa Asclepias exaltata Asclepias fascicularis Asclepias fruticosa Asclepias humistrata Asclepias incarnata Asclepias lanceolata Asclepias linaria Asclepias linearis

Asclepias meadii Asclepias nyctaginifolia Asclepias obovata Asclepias physocarpa Asclepias purpurascens Asclepias quadrifolia Asclepias rubra Asclepias solanoana Asclepias speciosa Asclepias subulata Asclepias sullivantii Asclepias syriaca Asclepias tuberosa Asclepias variegata Asclepias verticillata Asclepias vestita Asclepias vincetoxicum Asclepias viridiflora

Milkweed Media

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  A monarch butterfly on swamp milkweed

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  Asclepias syriaca seed pods, upper image from August and lower from December

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  Milkweed sprout, a few days after sowing

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  Chemical structure of oleandrin, one of the cardiac glycosides

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  Male Pepsis grossa, a typical milkweed-pollinating wasp

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  Honeybee on antelope horn (Asclepias asperula) showing pollinia attached to legs

• milkweed seeds dispersed by the wind.

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  Asclepias albicans National Park Service photo

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  Asclepias amplexicaulis Arkansas

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  Asclepias asperula - Antelope Horns

References

• Everitt, J.H.; Lonard, R.L. and Little, C.R. (2007). Weeds in South Texas and Northern Mexico. Lubbock: Texas Tech University Press. ISBN 9780896726147.{{cite book}}: CS1 maint: multiple names: authors list (link) ISBN 0-89672-614-2