Geranium Bronze Butterfly

I noticed this butterfly for the first time in August of this year sheltering from a shower of rain in the tomato plants, and being unable to find it in either my guide to European insects (a French, or better still Pyrenean butterfly guide is on the to buy list) or on a couple of good identification web sites I thought I might be onto something rare or unusual. 

But no, this is another invasive species, a geranium bronze butterfly from southern Africa. Presumably it arrived on imported plant material. 
I have a couple of hanging baskets with the classic red geraniums (actually pelargoniums but they will be forever known as geraniums here!) paired with spider plants outside their summer holidays, so went and had a quick check on them. 

Sure enough there was one empty chrysalis, but with no sign of damage to the leaves I reckoned it was just a one off. The plants have grown well this year, but flowering has been poor, especially late on in the summer, something I had put down to the very hot weather and possibly inadequate watering on my part.

However, further reading and research revealed that it is actually the flower buds and not the leaves that are favoured by the caterpillars. Closer inspection showed that almost all of the flower buds were just empty husks, entirely consumed, with a couple small fat caterpillars and quite a bit of frass on each plant.

I now face a quandary. My normal reaction to an invasive species is to try and eliminate it, but I'm not so sure in this case. After all, our bird life is in crisis in part due to a lack of insects, and I actively plant food for insects in the hope that supporting the bottom of the food chain will help those further up. As far as I am aware, the butterfly is not out-competing any native species or devastating an important food crop, but it is destroying an iconic plant of the French domestic landscape!

It will be interesting to see both the progress of this butterfly and the reactions to it over the next few years. My geraniums will be coming inside for the winter soon giving me a few months to decide quite what to do about this new arrival.