I just finished reading your article about slug control and thought I’d pass this tidbit along.
I’d like to suggest that a generous sprinkling of fresh (unbrewed) coffee grounds has, for the past two years, allowed me complete control of grubs.
The first year I had tried ferrous phosphate and achieved partial control. However, a few weeks later, the problem returned. I did not want to continuously add this chemical to my soil because I did not want to have a build up of it. So, on a hunch, I tried the coffee grounds and got control that lasted the balance of the season. This spring, I waited for the first signs of trouble and re-applied the coffee grounds.
I have not seen any slug damage or trails since.
This builds on research conducted by the University of Hawaii into the effect of caffeine on amphibians.