The only caterpillars that I've found to eat pelargonium leaves are the larvae of a small moth that curls up the leaves with silk webs. They aren't particular about what they eat.

Roseana,
I had those too. I saw in some book somewhere that they are a geranium-specific worm. They never ate anything else in my yard. The thing is, I've been picking the worms off for over a year now (I even was a bit obsessed, every day checking each flower cluster intensely). A few months ago I finally stopped seeing the damage and I thought I'd finally gotten all the worms. But then recently I started seeing a little damage again. ARG! So I hate to say it, but I'd throw away any new geranium I got with them, before I got too attached to it. But then again, maybe someone out there knows of a good solution that works. Good luck!

~Onion

Yup, people DO have a good solution - check out the thread linked below. I guess they're called Budworms.

I tried cutting off all the flower stalks as well. But I still kept getting them every now and then. I've seen the worms on the stems of leaves, too. I guess they have to find more flowers somehow.

Good luck!
~Onion

Here is a link that might be useful: Budworm thread in Gardenweb

Those are budworms and I've been fighting them for more years then I care to remember. I've given up growing Geraniums for the last few years. I understand they are the small white moths you see flying around in the summer. Have tons of those. BT will work but it also kills the butterfly worms so I won't use it as I plant alot of flowers to attract them. Tried organic measures to no avail. Stopped growing geraniums for 3 years then tried again and they came back. I grow herbs in amoung my flowers in containers so I won't use poison. Good luck, I've had none getting rid of these pests. Jane