I guess I was waiting for the inevitable, the arrival of insects.. Now, I need to identify it and try to get the predator insect for it, as I can't use insecticides. Believe it or not, I thought of using a vacuum cleaner to suck them from the hydroton, hehe :)
The only source I could think of is the arugula seeds, the eggs were there somehow; It's really puzzling.
Update 6PM: I wrote the post early in the morning. But now, not a single insect can be found in the grow bed. Where did they go? Did they drown?