Mysterious pest munching urban rocket/arugula

[jitsuryoku]

Hi all.

I have been putting a shady corner of my urban balcony in Tokyo to good use by growing lettuce and rocket/arugula in pots on a platform raised about 150 cm above the cement floor.

For the entire summer the rocket has been doing well -- that is until the day before yesterday.

Overnight about half of my crop of 20 or so plants literally disappeared. I can see munch marks on a couple of fallen leaves that appear to be about 2 mm in diameter. Moreover, it appears that many of the plants were shorn off at the stems before being devoured.

I searched the pots and remaining plants for signs of caterpillars and other obvious pests with no luck.

This morning I awoke to find two of the remaining plants had also disappeared.

My garden was planted using soil that mostly came directly from the garden shop. I live on the second floor in a concrete building with concrete floors and the nearest natural greenery is a tree 10 meters away that doesn't even touch my building.

In the past I was troubled by aphids that attacked a basil plant, and recently, my lettuce has been under assault from tiny red bugs that appear to be spider mites.

However, none of my plants have ever been "munched," and this is the first time I've had any problems with the rocket, which has always been very robust and vibrant.

Any thoughts?

[DavidT]

Could it be Keel slugs? They are black, about a half inch long and live in the soil. They come out at night, do the damage then return under the soil.