Margined white butterfly, also called the mustard white butterfly.

Margined white butterfly on wild dandelion.

If you have seen a white butterfly flitting around in your garden, it likely is the cabbage white butterfly.  This European invader has supplanted the native margined white butterfly shown above.  This is actually not the cabbage white butterfly’s fault; it is capable of feeding on a wider variety of plants than the margined.  To complicate matters, the primary food of both is the mustard plant.  Much of the native mustard varieties, which the margined primarily feeds on, has been replaced by invasive European ones.  The cabbage white can eat both plants while the margined white can only eat native species.  If eggs are mistakenly laid on the foreign variety, the growing caterpillars die.

According to William Neill, author of Butterflies of the Pacific Northwest, the margined white butterfly used to be common throughout most of mid latitude North America.  I found this one in a very rural area of Washington state, far away from the influence of man. 

The cabbage butterfly is aptly named, because (believe it or not) cabbage is one of the plants in the mustard family.  So are broccoli, cauliflower, rutabaga, Brussels sprouts, and rapeseed.  Guess how many of those are native to North America?  None.  Even many of the wild members of the mustard family are introduced.  Shepard’s purse is an example. 

This is why the cabbage white butterfly’s population has soared while the margined white’s has plummeted.  So many of the native plants have been replaced by introduced species.  A margined white butterfly female cannot tell if the plant is native or not, whether her young will survive to adulthood on the food source she has placed them upon.  Chances are they will not.  It is only in the areas where man’s introductions have not replaced native species where her eggs will really stand a chance to grow. 

It may not be long before they no longer exist.