We have the occasional caterpillar on ours too
to answer Tracie's question... they are little green ones
then HUGE green ones by the time they have done!
we pick them off when we find them.
Oh and a little extra, if you bring your dahlias inside as cut flowers and they have caterpillars on them, the caterpillars have a wonderful habit of excreting highly colourful poo that stains whatever it lands on!!!