Obviously, they must be eating or otherwise damaging the roots right??
I believe they eat decaying vegetation and if there is plenty of organic matter in the soil, maybe they won't need to eat living vegetation.
Can't agree with you there Longy. I've had a lot of problems with curl grubs (a few different species of them) in my Bonsai pots. The very nearly killed a beautiful Champion Green Island Fig of mine. When I found them (because the tree was looking a bit sick) there were eight of them in the pot as big as your thumb and they'd chewed through at least 3/4 of the whole root system. Another week or so and I would have probably lost the tree to the mongrels - or have had major die-back on it.
I've found them in a number of my Bonsai pots - or at least I used to. Now, all my trees get dunked fairly regularly in Chlorpyrifos 500 (Lorsban) mixed with water. It's dangerous stuff to use and is indiscriminate. It will kill any insects that come in to contact with it. I only use it for my Bonsai, I would never use the stuff around the garden. I also use long industrial Chemical gloves when handling it because it is a known carcinogen. That's why I didn't offer it as a suggestion to Michelle's question. On top of that, it isn't available to the general public - you have to be a primary producer to buy it.
My suggestion would be the same as Brill's. Use the granules if necessary but make sure they are covered, so no other creatures - like Maggies etc can come in contact with it. If you bury the granules and give the plant a good watering, they will dissolve fairly quickly. The granular stuff (if it contains Chlorpyrifos) is a lot less potent than what I use - like about 1/10th as strong, so will dissipate quite quickly - but it will do the job in getting rid of the curl grubs.