It can be a heartbreaking experience. One day, you have beautiful rows of young seedlings. The next morning, many plants are defoliated or completely missing. The only clue to the source of their demise is a shiny trail of slime.
Slugs and snails can be voracious garden pests but are a bit picky. They prefer younger plants over older plants. They like certain species over other ones, and perhaps most puzzling, they like certain individual plants over other ones. It’s not uncommon to see several slugs and/or snails on one plant while surrounding plants are untouched.
British researchers investigated how snails choose their food. Do they take a bite and then decide if they like the taste? It turns out that this doesn’t happen often. Instead, the snails tend to choose plants depending on volatile organic chemicals (VOCs), basically by smell.
In many plant species, once an individual plant is attacked, it releases VOCs in the air that enable communication with neighbouring plants. Surrounding plants then initiate their defences, which can involve increasing the release of other VOCs, called by one researcher “protective perfumes” or increasing levels of unpalatable or toxic alkaloids in their tissue. Tiny seedlings (in their cotyledon stage), however, are often incapable of mounting such defences.
Tibor Kiss (2017) concluded that slugs and snails avoid plants high in alkaloids but only when given a choice. He cites many studies that found molluscs can learn to avoid certain foods but if there is no choice, they can learn to adapt to unpalatable foods. Moreover, they can detoxify poisonous chemicals, including methiocarb, the pesticide used in slug pellets, if there are no other food choices.
What can the gardener do? You can try to confuse molluscs by intercropping with unpalatable plants or perhaps spray crops with essential oils of aromatic (and hopefully unpalatable) plants.
Sources: Mick E. Hanley, Roger W. R. Shannon, Damien G. Lemoine, Bethan Sandey, Philip L. Newland and Guy M. Poppy. Riding on the wind: volatile compounds dictate selection of grassland seedlings by snails. Annals of Botany. 2018. Volume 122. Pages 1075–1083,
Tibor Kiss. Do terrestrial gastropods use olfactory cues to locate and select food actively? Invert Neuroscience. September 2017. Volume 17. Issue 3. Page 9.