I'm at Seattle. Recently, I found holes on our bell pepper even when it's small.. Any ideas on what caused it? There're also some holes on leaves too, but not a lot. Shall I buy some oil or insect killer? Thanks
That looks like earwig damage (they like to hide in dark, damp places). They aren't really eating the peppers, but using them as temporary homes. There are a number of different ways to deal with earwigs. Although I usually just ignore them, the newspaper trap works well if they become a problem in my yard.
The newspaper trap is simple: roll up a piece of newspaper into a tube or bunch it together like a sausage, wet it enough to make it damp, then put it into the garden at the end of the day. The earwigs (active at night) will hide in the dark and damp newspaper as dawn approaches. In the morning, either place the paper into a bucket of water or dispose of it in your garbage.
For your peppers, you could put the trap at the base of the plant and then remove in the morning. Note that you'll probably have to repeat this for a number of nights in order to make a dent in the earwig population, Here's a ton of information about earwigs, including other methods of removing them from your garden.