Monarch Caterpillar Host Plants

Butterflies and moths will drink nectar from many different kinds of flowers, but caterpillars are much more fussy! They will only eat particular species of plants, known as their host plants. This diagram illustrates the relationships of the Monarch caterpillar’s better-known host plants in New Zealand.

Monarch Host Plants
Monarch Host Plants

Gomphocarpus or Asclepias?

In 1753 the famous botanist Carl Linnaeus  decided the swan plant belonged in the same genus as the milkweeds, and named it Asclepias fruticosa. This is often written A. fruticosa (L).

Like the milkweeds, swan plant uses silk strands to spread its seeds on the wind. However swan plant has a hairy inflated double layer around its seed pods, and the other Asclepias milkweeds don’t.  In 1810 the botanist Robert Brown decided that there were enough differences between swan plant and the other milkweeds to form a new genus, which he named Gomphocarpus. He renamed the swan plant Gomphocarpus fruticosus. This is often written G. fruticosus (R.Br).

The debate as to which name was better continued, and both names were used. Recently further developments, including the development of DNA analysis, have resulted in the name G. fruticosus (R.Br) being preferred. In their 2001 paper, “A revision of Gomphocarpus R.Br. (Apocynaceae: Asclepiadoideae)”, Goyder & Nicholas came up with the following definitions:

  • Gomphocarpus: Rootstock fibrous, sometimes woody, but never formed into a napiform tuberous caudex; short-lived perennial subshrubs with branched stems, or if pyrophytic herbs with unbranched annual stems, then leaf lamina broad; flowers in extra-axillary inflorescences.
  • Asclepias: Rootstock forming a slender to stout napiform tuberous caudex; pyrophytic herbs with annual mostly unbranched stems; flowers generally in terminal inflorescences.
    Gomphocarpus fruticosus showing extra-axillary inflorescence
    Gomphocarpus fruticosus showing extra-axillary inflorescence

    Gomphocarpus physocarpus showing its fibrous Rootstock. Note the absence of a napiform tuberous caudex.

There’s a lot of botanical jargon in there, so lets break it down a bit:

  • Napiform means round at the top and tapering down sharply at the bottom, rather like a Turnip. 
  • Tuberous means full of  swellings.
  • Caudex means the base of the stem, at or just below ground level.
  • Pyrophytic means they ignite readily and burn intensely.
  • Inflorescences are clusters of flowers. 
  • Terminal means from the end of a stem.
  • Axial is the point where the upper side of a branch, leaf, or petiole comes away from its stem or branch, so axilary means from the upper side of the base of a leaf, while the more uncommon extra-axillary means growing outside of that point.
  • Herbs to a botanist means non-woody flowering plants,while Subshrub means a small woody plant.  Woody plants generally increase the diameter of their trunk every year.

So, in simple English, An Ascepias has a turnip shaped swelling at the base of its stem.  They are non-woody and burn easily, and grow new unbranched stems every year.  Their flowers grow in bunches at the end of the stems.  But Gomphocarpus never have a turnip shaped swelling at the base of their stems.  Their flowers grow in bunches near the base of a leaf, but not exactly between the leaf and the stem.  They can be non-woody and burn easily, but they can also be a small woody plant with broad leaves.

Using this definition means that while Asclepias is found in the Americas and Africa, Gomphocarpus is only found in Africa and Arabia, and includes swan plant and balloon plant. Both genra are part of the Asclepiadoideae sub-family, which confusingly is also known as the Milkweeds.