Chris Hogg brought next idea :
On Tue, 10 Sep 2019 10:52:35 +0100, Harry Bloomfield, Esq.
wrote:

Green (recycle) and black (general) bins sit side by side in a shelter
outdoors. This morning was green bin day and after emptying I spotted
no less than 12 slugs inside the bin. None at all on the black bin.
Anyone know why so many, why none on the black bin?

What's the difference in the contents? Food waste? Garden waste? Or
perhaps the black bin, being black, gets hotter than the green one
when the sun shines on it (or vice-versa, whichever slugs prefer).

As I said green recycle is cardboard, plastics, tins and paper. Black
is everything else, food and general waste. Both under shelter, so no
direct sun and the same temperature.