Beer traps for slugs

The claim

A sunken container of beer attracts slugs, which fall in and drown, reducing the population.

What the evidence actually shows

The first half is well established. A two-year field study of Arion vulgaris and Limax maximus found beer traps captured significantly more slugs than water or 10% ethanol controls, and identified the attractants as fermentation volatiles - isoamyl acetate, limonene and ethyl esters - rather than the alcohol itself. So the mechanism is real and it is about smell, not intoxication.

The critical limitation is what that study measured: capture rate. Not population reduction, not crop damage. "This trap catches slugs" and "this trap protects my lettuce" are different claims and only the first is demonstrated.

University of California IPM is blunter, stating beer traps are not very effective for the labour involved, because they attract slugs only within a radius of a few feet and the bait must be topped up every few days to stay deep enough to drown them rather than simply feed them.

The part that should give you pause

If a trap attracts slugs from a few feet away, placing it among your plants draws slugs toward those plants. Some will be caught. Others will encounter your seedlings on the way. Traps are better placed at a distance from what you are protecting, which is the opposite of how most people use them.

What to do instead

Use traps as monitoring rather than control - they are genuinely useful for telling you whether you have a slug problem and how bad it is. For control, iron phosphate baits, evening hand collection, and removing daytime shelter.