Illinois Natural History Survey - University of Illinois

Hi Cheryl,

Good question.

What exactly do Olympia Marble caterpillars feed on?

I have not seen any references for Arabis shortii in particular being a host for Olympia Marbles. The Butterflies of Illinois lists the Olympia Marble as feeding on Rock Cresses. Unfortunately, that could mean several species. The references I have seen most often mention Sand Cress, Arabis lyrata as a common host plant. However, this plant does not occur in the southern half of Illinois (including Mason County), and the range of the Olympia Marble spans the north-south range of the state, especially along the Illinois and Mississippi Rivers in the south.

See range map here
https://drive.google.com/open?id=1Xs_6G-skRHExwFyFMlGw5CkeS22T6dbi

So it must be feeding on other Arabis species. The taller, so called, Woodland Rock Cresses such as Arabis shortii, seem less likely to be common larval foods because they typically grow in more shaded woodlands. Most butterflies do not tend to spend much time in the woods. But perhaps when Woodland Rock Cresses grow in, or nearby more open areas, they could work as host plants.

Perhaps this butterflys preferred habitat across Illinois is a clue to which rock cresses that it uses. It is most often encountered in sandy areas, dry hill prairies, and oak savannas. Determining which cress’s are the most common in these types of habitats, in the Marbles Illinois range, would probably winnow down the most likely candidates that serve as the caterpillar food source.

In other parts of the country, the following plants have been listed as larval hosts: Arabis glabra, A. canadensis A. drummondi, A. serotina, A. petiolaris, A. laevigata, A. divaricarpa, and A. missouriensis. Some sources even list Marbles feeding on weedier mustard species such as Draba spp., Descurainia pinnata, and Sisymbrium officinale, though such reports are rare. Since this butterfly is rather uncommon throughout its range, having a better knowledge of its preferred caterpillar food sources would certainly be helpful for its conservation. Answering this mystery would take a good bit of walking around and finding Cress plants of different species across the region and checking the plants for Marble caterpillars. No easy task.