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Milkweed Bugs - December 10, 2007


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Milkweeds belong to the genus Asclepias named from Greek mythological god of healing. The plant has been used for centuries in folk cures.  The common name comes from the milky sap produced by the plant.  The sap contains a class of toxins called cardenolides - forms of sterols.  Cardenolides are known as heart arresting agents in mammals and apparently have both a foul taste to animals and perhaps neurotoxic capabilities.  

It’s this toxin that makes milkweed a host to a number of insects.  They absorb the toxin in their system and are thus rendered “inedible.”  Insects that make use of the toxin include the monarch butterfly, milkweed bugs, and aphids.

Monarch butterflies lay their eggs on the milkweed or nearby plants.  Once the eggs hatch, the larva (caterpillar) feeds on the tender vegetation of the plant, ingesting the milky sap, and thus the toxins.  This renders the caterpillar and the adult inedible by predators.  Interestingly, the viceroy butterfly is close in appearance to the monarch.  At one time, it was thought the viceroy was an example of Batesian mimicry where it mimicked the look of the monarch so predators would not eat it.  However, recent evidence (as stated in Wikipedia and attributed to Pinherio (1996) the viceroy actually tastes worse than the monarch and therefore is an example of Mullerian mimicry.

Milkweed bugs are members of the Class Insecta, Order Hemiptera and are considered “true” bugs by entomologists.  Hemipterans have forewings which are half leathery and half membranous. 

These bugs feed off the sap of the common milkweed of the genus Asclepias.  They are highly colored which, at first glance, seems a bad idea particularly around birds who eat insects.  However, the coloration is a warning mechanism called aposematism. 

Adults lay eggs on the milkweed plants.  The life cycle includes five nymphal instars (stages between molts).  Hemipterans or true bugs exhibit hemimetabolous development or incomplete metamorphosis.  This means the instars resemble the adults during their development. Only adults have wings. 

There are two species common in South Florida: Oncopeltus fasciatus, the large milkweed bug and Lygaeus kalmii,Oncopeltus the small milkweed bug.  From these photos, it appears the bug has a red head and thus should be .

These adults were found on the butterfly milkweed, Asclepias tuberosa, from a cutting of a plant given to me by Terri Hayes of South Campus. Instars are also shown.  Note the wing pads showing up as dark areas on the thorax of the insect.

Milkweeds produce a specialized flower with a complicated modification of flower parts to include a corona and pollinium - a mechanism to transfer large quantities of pollen.

Another insect to feed on milkweed are aphids.

It’s not unusual for the nymphs to cluster together on the stem of the milkweed.  Some suggest this is a mechanism to intensify the color on the stem and thus warn predators.  However, I think it possible itís a means of control by higher instars ñ much like cattle are herded.  This idea of herding is common in plant aphids. Ants often herd aphids and use them as sources of sap and I have found ants on the flowers of these milkweeds.

© Fred Searcy 2017