I'm watering overhead with a spray nozzle ...

&&&&&&&&&&&&&&&

your watering is the cause, I think.
Pill bugs don't eat plants.

Uh,,,I disagree.
Pill bugs absolutely eat plants. I find them on my young cucumbers in the spring chewing them to pieces. But that may not be the culprit on your Pak Choi.

I see no way that watering can cause that damage. Something is eating it and probably at night. Maybe go out with a flashlight some time?

If you can't find it, don't worry too much. That Choi will kick into fast growth soon and outgrow minor damage like that.

-Mark

{{gwi:109271}}

I found the culprit!
Nasty green caterpillar. Anyone know what this is and how to get rid of them or prevent them?

It's likely a cabbage moth larva, usually from those little white butterflies. But there are a few other species it can be from also, all very similar.

They are a major pest for me and damage all sorts of brassicas. On a small scale you can hunt on the plants they favor and pick them off easily. For larger scale planting, I use an organic spray called BT. You can find it in many gardening supply stores.

-Mark

I thought those little white butterflies were nice and innocent. I used to let them fly freely and land on the plants.
NOT ANYMORE!

Aside from the new discovery of some worm, I still believe that the damage on the leave of the pak choy, SHOWN IN THE PHOTO, is environmental. Just look very closely to the big leaf on the left side. There are 4 tiny holes on it. it cannot possibly be the work of any worm, caterpillar, slug, snail,(maybe only flea beetle)... But it can be due to some object falling from a nearby tree (pine needle, just example). If this incident happens when the leaf is smaller, the holes will grow bigger as the plant tissue are pulling. I have personally, observed and monitored such things on eggplant leaves. The flea beetles make small holes and as leaves grow, the tiny holes get bigger and if it happens to be near the edge, the leaf would look like as if it has been torn.

Harsh water jet can also do similar damage on tender leaves, especially if there was some debries on the leaf.