These herbivores feed on the young emerging shoots of herbaceous plants in spring and have a potentially significant impact on species’ persistence. As is discussed in the section ‘Establishment by planting’, species vary considerably in their palatability to molluscs as adult plants and there is much variation in the degree of damage experienced by individuals of the same species within a planting. It is not clear whether this is due to the feeding preferences of individual slug species, their spatial distribution or varying palatability of individuals of a species. The author’s research into the long-term persistence of prairie plant communities in slug-rich environments suggests that there is a relationship between weed density in spring and the degree of damage to the prairie plant species. Prairie plants on weedy plots suffer more competition from colonising weeds, they also have to contend with more intense predation. In the moist, shady environment generated by dense weed-cover, molluscs are present in higher densities and feed for longer. Where this type of vegetation is subject to this intense competition/predation regime, the most palatable species decline and disappear over a number of years, resulting in low densities of the most robust/least palatable species. Although there has been very little research into these ecological relationships in other types of naturalistic vegetation, this is probably the most common reason for the disappearance of many native and non-native species from designed herbaceous vegetation.
Until an inexpensive, effective, yet low-toxicity molluscide is developed, the most satisfactory means of dealing with this phenomenon is to select less palatable species, and/or manage to reduce spring weed-cover. The biological control of slugs with nematodes can be effective, however it is currently too expensive to use on a large scale. The practice of sand mulching to aid weed management also appears to reduce mollusc damage.
