Typically it is slugs, at least in this part of the country. If so, various methods can work to protect your plantings. Currently it is often recommended to use the iron phosphate baits around the base. These do not pose a threat to dogs or other pets and work fairly well.

Oh boy -- GardenWeb is FULL of advice on slug control!! Just enter "slug" on the search box, and you could be reading for a week!
Slugs don't kill the plant. It grows back fine the next year. Then the slugs come back.
I had good luck this year with a product called Sluggo. It's in pellet form, so you don't have to worry about breathing in spray or dust. It decomposes into the soil.
I started applying it it early May and kept on about every 2 weeks. I have tapered off by now on all the hostas except those right next to my back door.
The ones I stopped treating are now just beginning to get a few holes.
Other people use home-brewed potions. See the many slug entries for more info.
But nothing really eliminates those slugs -- anything you use just keeps them away till the product decomposes.
In general, thicker-leaved hostas have less of a slug problem. Also keeping them in a spot with a little more sun and plenty of air circulation helps (not too closely planted).
But in Michigan, at least, if you have hostas you have slugs.
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Get your slug control going really early in the season, most damage is done when the leaves are just unfurling as they are really succulent then, also being very compact a tiny nibble leaves a big hole.

Once the leaves have opened out they don't seem to get so much attention. So get that bait down in feb and march, or whenever your first growth gets going.

Also worth removing debris and stones from round the plants, leave the slugs with nowhere to hide.

Slugs cannot cross copper, it's like an electric shock to them. If you have a couple of special hostas that always seem to get munged, try putting a little copper collar round the plant in early spring. Hammer a bit of old pipe flat and bend it into a collar. Again you can remove it when the leaves are unfurled.

Cheers Greenmanplants

Look for a product called DEAD-LINE BULLETS ,check out Agway or similar type of supplier.They don,t call it that name just for the sake of it.Slugs will be out of town before sun-up!!!!!!! pardner N.N. P.S, it may be similar to Sluggo, that magazinewriter mentioned.

I had dozens of varities of hosta in the midwest. The thicker the leaf, the less damage. Slugs are at their worst/best in a wet season, but they don't kill the plant...just make it look awful. Have used the various baits/etc and they work but only until the rains come and they need to be re applied. If you have a large collection not much to do but trim and pick by hand...which is a sticky proposition!

I stopped using pesticides and herbicides and put out loads of bird seed. My yard has birds EVERYWHERE and my previously skeletonized hostas had nary a hole this year!

This year I used pine needles as mulch around the hostas, put down before they leafed out, and the plants were alot less eaten up than they were last year when I used beer traps. Plus -- no marinated slugs to dispose of.

Springcherry

If you grow a large number of hostas slugs are going to be attracted to your garden. I grow many hostas and have much success using slug baits with the key ingredient iron phosphate as mentioned above. It is non toxic and can be used safely around children and pets. I would caution anyone with pets or fond of wildlife in their garden to stay away from Deadline which contains a chemical called methaldehyde, it can be fatal to pets and wildlife. Make sure you read the label if you want the non toxic slug bait, it will be 1% iron phosphate and 99% inert ingredients.
If you use Sluggo (which is one of the iron phosphate typed baits) diligently every two to three weeks and remove garden debris you will notice much less slug damage on your hostas. Another good thing with the iron phosphate is the slugs crawl off to die, I haven't had to dispose of any dead slugs since I've used this product, about three years now. We'll never completely eliminate slugs but it is possible to get the upper hand over them.

Just a little anecdotal humor. We just moved to our new home and planted a few hostas here and there at the front of the house and like they knew they were coming, the slugs were right there the next morning. One slug must have crawled across our cement patio and got stepped on but I wasn't in the mood to clean up slug guck so I let it dry out first. Well those slugs must be made of Crazy Glue(c) or something because it's been 7 MONTHS and I still can't get that critter off my patio! What (is) inside those critters anyway?!

Barb
Southern Ontario, CANADA
zone 5b/6a

Frogs eat slugs. Both green and bullfrogs love them. I used to put them on the side of my pond when I would find them. They are like frog candy. There were a few hostas in the yard but I never noticed a slug problem.

I put used coffee grounds around each hosta as it was coming up last spring and had no slug damage at all. Seems they don't like to crawl over anything gritty. Plus the hostas liked the extra nitrogen from the coffee. I think I am on to something and I like the price - free!

I had holes in my hostas that grew under a magnolia. About mid-summer, the magnolia produced large, spear-shaped seeds which dropped from above and punctured the hosta leaves. Took me a while to figure out what was going on. These holes looked different from slug holes; they were very ragged and only in the middle of the leaves. The slugs will eat mainly on the edges.

Firefly larvae eat slugs. Stop using all pesticides. If you see lots of fireflies, I bet you will see very few slugs. The way I think of it is, no slugs, no fireflies.

FINALLY, found a post about those magnolia spears. They do MORE damage than the slugs. However, I've noticed over the past few years that if I do not mulch the hostas under the magnolia I have very, very little trouble with the slugs. I have my army of toads too. Keep areas clean of debris of all kinds. But getting back to the magnolia spears, I have thoughts of rigging up some kind of light weight netting under the magnolia to catch these bombs when they're in their "season" of dropping. Magnolias are glorious in the spring when blooming, but they sure can be one dirty tree. No, their dropped bloom leaves do NOT make a good mulch. I know. They're made of leather. Attracts bugs from the other side of the world - and stink to high heaven when swept up into a pile and left. So much for trying to be resourceful. Now I let those leaves shrink, dry, curl up and go away on their own and/or run mulching blades over them to keep them from suffocating the grass.

Spray twice a week with 10% to 25% diluted household ammonia, more often when it rains. Flood the petioles at the crowns (center) where the little buggers hide and try to get under leaves as well. This kills slugs and the breakdown of nitrites into nitrate and mildly fertilizes the hosta as a bonus. This is an inexpensive and effective way to control slugs and probably other pests as well. Also, slugs will not cross anything that has sharp / rough edges so 'mulching' with gravel, stone, sand will prevent them from going after the hosta. Cans submerged to ground level and half filled with beer are effective traps for drowning slugs as well. I like the ammonia treatment myself.

Jon