Just to report back. The yellow strips had even more on them today. A good 20 on each I think.
Potato skins work well. I found these minute black mini-knats (?) in the skins. I didn't check every single one after I saw this. I then put the potato skins straight out of the house and cut up more slices. I worked out that the grater is better than trying to peel them as you have a lot more use and they are finer and bigger.
I'm going to bleach the drains and plugholes..