I've never heard of insect pests with A. donax. I would just pick the worms off with tweezers, and destroy them.

Get some close-up photos of them, and they can be ID'd.

I don't understand why there are open hollow stalks collecting water. If they are remnants of previous season's stalks, they should be cut away, so they don't collect water.

Thanks, donn; that's exactly what I've done so far. If/when another one turns up, I'll snap a photo before I send it to the worm afterlife. They appear to be leaving some dry, crumbly looking...substance behind. Not eggs; it looks almost like dirt, a light tan color.

The places where the water is collecting is basically just the shape that the grass makes as it is growing and forming new segments. Like, the stalk is sort of in a rounded shape, and then out of the center of that springs another new little shoot. It's just a small space and a few tiny drops of water, but that's where I've found the worms hiding. These are all new sprouts, there are no previous season stalks, it's a recently purchased new rooted cuttings bought from Santa Rosa. They were perfect when they arrived, the worms have attacked them since they've been at my house.