Much has been written in various coffee forums over the years, regarding
research done by Robert Hollingsworth's at the USDA facility in Hilo Hawaii.
The killer effects of caffeine on slugs and snails appeared in 'Nature' on
the 27 June 02 :-
http://www.nature.com/news/2002/0206...s020624-8.html
http://preview.tinyurl.com/6f54hoq

They used a drench containing 1 - 2% caffeine. This is quite strong, perhaps
20 - 40 times the strength found in a cuppa Joe, which only contains about
0.05% caffeine. However it did state that only "0.01% caffeine puts the
pests off their dinner". This being only one fifth the
amount found in a cup of instant.

On the downside is the comment by Notts Uni about the likely harmful effects
on beneficial insects. This has to be weighed against the possible adverse
effects of Metaldehyde (blue pellets) added to the soil.

If anyone wishes to experiment with coffee, I would suggest making up a
strong solution of say, 10 rounded teaspoonful's of instant to a lire of
warm water. If you are about to buy instant coffee, go for the cheapest
brands available. These are likely to be Robusta coffee from Vietnam. Avoid
Arabica coffee, it tastes nicer, but it has a lower caffeine content.

I don't drink instant, but since November, I've been saving the spent
grounds and pucks from my cafetiere and espresso machine. I boiled up about
2 kilos and added to 5 litres of water. The day before spraying (in late
June) I found under a piece of slate) a medium sized light brown slug, a
giant dark brown and a garden snail. Here's what happened:-

2 slugs and one snail get woken with coffee
http://www.flickr.com/photos/7913357...in/photostream

Rapid exit of all 3 but slugs trail their innards
http://www.flickr.com/photos/7913357...in/photostream

Pot overlaid to keep off sun
http://www.flickr.com/photos/7913357...in/photostream

Next morning, slugs have dissolved, but snails survived in roof of flowerpot
http://www.flickr.com/photos/7913357...in/photostream

Ok so 6 weeks ago I sprayed my lettuce, courgettes and dwarf beans. So far
no slug damage despite the torrents of rain of late. Snails have been
deterred, but I noticed one or two visiting this pm, so I will make up
another batch tomorrow. Enjoy your supper.
Bertie