California’s Butterflies

Food Plants

Butterfly larvae are largely herbivores.  There is only one North American exception and that is the harvester, Feniseca tarquinius.  The harvester is a small butterfly endemic to Northeastern North America.  The larvae of this species feed specifically on wooly aphids largely found on alders.

 

Many butterfly larvae are selective on which plant species they will feed upon.  To understand why most butterfly larvae would specialize on a small group of plants, it’s helpful to think about the co-evolutionary relationship between food plant and butterfly larvae.  In a sense, there has been a chemical war going on for millions of years between insects and plants.  Plants have evolved chemicals that fend off insect feeding and initially insects will not be adapted to these new chemical defenses.  This gives these plants an advantage and they are able to expand in numbers and geographic range.  Eventually insects become adapted to these chemical defenses.  Those insects that adapt to these chemicals will in turn have an advantage and be able to rapidly expand in numbers and geographic range to encompass the area of the plant.

 

Once these insects have become adapted to specific chemicals, these chemicals become oviposition and feeding stimulants.  This means that female butterflies will only oviposit upon those plants that contain these chemicals.  Larvae in turn will only feed upon those plants with these same chemicals.  Food plant shifts can be made through ovipositional mistakes by female butterflies onto unrelated plant species if these plants have evolved similar chemicals.

 

As an example, the monarch butterfly feeds on milkweeds.  These butterflies select plants that contain cardiac glycosides which are highly toxic chemicals, particularly to birds and mammals.  The cardiac glycosides are used as ovipositional and feeding stimulants by these butterflies.  The larvae and adult butterflies in turn protect themselves from being fed upon by concentrating these chemicals in their bodies.  Birds that feed upon these butterflies and larvae will quickly learn of their toxicity.  Because of their characteristic colors and patterns, potential predators will be able to identify them from other butterflies and choose not feed upon them again.  Butterflies that are not toxic and evolve coloration to mimic these toxic butterflies (also called “mimicry”) can in turn receive protection from bird predators.