How can you tell which insect by shape of holes?

Something is munching on my cannas as they are just coming off the ground! I was told by some they are leafrollers, and by others it's slugs...
Holes are perfectly round and mostly in the middle of the leaves....what do you guys think? Sluggo or Bt? Help!

If you can post a picture that would help. The only thing I've had in my garden that makes perfectly round holes is leaf cutter bees, but I see lots of their holes around the edges of leaves too, so I'm not sure if that's what you have or not. If that's what you have then I'd leave them alone, they're beneficial and really only do cosmetic damage to the leaves. Slugs and snails typically make more irregular shaped holes, not perfectly round (at least I don't remember them being perfectly round!). And leafrollers as their name would imply will roll up your leaves, maybe they make some holes too but the rolling would be more noticeable than the holes.

OK ecrane, I took some pics yesterday. I'll post 3. Help!

Pic #2

and pic #3! What do you think?

I don't think it's leaf-cutter bees, when I've had those it always looks more like someone attacked the leaf with a hole punch...here's a pic of that looks typical of the damage I've seen them do. http://www.icrisat.org/vasat/learning_resources/pigeonpea/pest/pest_fig/leafcutter_damage.jpg

It could be slugs/snails although I'm not sure if it looks quite right. I don't have too much personal experience with what their damage looks like though. Here's a pic for comparison http://oregonstate.edu/dept/nurspest/Images/mollusks/snails/brown%20garden%20snail%20damage%20Hosta%20thumb.JPG

Given that the leaves don't look rolled up I also don't think it's leaf rollers. Hopefully someone else will have some better ideas for you! I wouldn't rule out the slugs though based on my own comparison of your pics with the ones in the link, so I'd check and see if you can see slime trails anywhere on the ground near the plant or on the leaves & stems. Very possible it's something else, but that's an easy check to do while you're waiting to see if someone recognizes this.

The pattern of those holes very definitely looks like leafrollers. Did you open the leaves for the photos?

Bt to the rescue. This is the reason I no longer grow cannas!

You think then? OH mY! argh! I spread sluggo around all the cannas...but I think I'll try Bt too! I thought the patterns were way too symmetric for it to be slugs too...but again I have (or had?) never seen leafroller damage before to be able to tell...

The leafroller lives inside the curled up young leaf, and eats its way though each layer - that is why the holes are all lined up. If you would roll that leaf up just right you could align those holes. On more mature leaves that have unfolded, the leafroller has to roll the edges up themselves, and they they then secure it with silken threads - basically sewing the edge to the leaf. They live inside and munch away. If you were to cut through the silk and open the leaf up, you would find the holes and lots of poop too. And a leafroller.

I just found these bugs crawling nearby! what are those?

canna leafroller

http://aggie-horticulture.tamu.edu/galveston/f&b_11.htm

AAAAAAAAAAAAAAAAARGHHHHHHHHHHHHHHHHHH!!!!!!!!!!!!!!!!!!

Ok folks, I BTd the heck out of those cannas...how long does it take for the caterpillars do die? It really is looking ugly right now!

If the caterpillars are in a late instar or stage of life, the Bt is not very effective. In order for it to work, you must apply it when they are small and eating voraciously.

They ARE eating voraciously! lol! HOw often should I reapply the Bt?