Milbert's tortiseshell - a nettle-eating caterpillar

I photographed this Milbert's tortiseshell near the H2O center in Kelowna.

At first, I thought it was a painted lady.  However, the colouring was not right, although similar.  This is the first Milbert's tortiseshell butterfly I have seen.  I did not get a very good photo of it;  It landed only for a moment and was gone again before I could get into a better position.  If you look closely, you can see that the image has been doctored.  I have the original image below; I erased the right wing and replaced it with a copy of the left one.

You can see how my original image was edited to create the top image.

The host plant for the caterpillars is stinging nettles.  I have always been impressed by the pain that one of these plants can impose on one who passes by a little too closely.  The pain is caused by formic acid, the same nasty chemical found in some stinging/biting ants.  The milbert's tortiseshell larvae have several defences against the trichomes (stinging hairs) of the plant.  First off, their physiology provides protection against the acid attack.  Then, they work in groups, build shelters, and eat the younger parts of the plant that are less well protected.

The caterpillars (click here to see them) have a sweet deal in that they are somewhat protected by the very thing they are eating.  Nettles are avoided by a host of would-be predators.  Thus, the caterpillar has both a food source and a defence strategy.  The adult butterflies lay their eggs in groups on the underside of stinging nettle leaves.  The young caterpillars are communal and even build silken nests on the plant as an additional means of protection.  

The adults will either lay eggs to start a new generation, or they will seek shelter to overwinter and begin again in the spring.  It is possible that two or even three generations of adults is possible within a year.

Thanks for reading.

Eric Svendsen     www.ericspix.com