Slugs as Generalist Herbivores: Tests of Three Hypotheses on Plant Choices

Three hypotheses explaining plant choices by generalist herbivores were tested by measuring leaf acceptabilities to three slug species, each of which lives in a different successional stage of vegetation. The acceptabilities of 61 plant species were determined in laboratory feeding trials by offering individual slugs leaf discs of a test and a control plant (lettuce) and recording consumption. The plant—defense hypothesis, that plants predictable to herbivores should have greater defense commitments and be less acceptable than unpredictable plants, was contradicted when predictability was judged by successional status. The acceptabilities of climax forest plants and early—successional plants were not significantly different for any of the three slug species. This was also true within woody species and within herbs. However, the plant—defense hypothesis was supported when predictability was measured as seasonal leaf persistence. As seasonal leaf persistence of forest herbs increased, acceptability declined. As alternative explanation based on growth rate is discussed. Alternatively, feeding choices were hypothesized to be determined by adaptations of these generalist herbivores to plants available in their local habitats (the herbivore—adaptation hypothesis). Because slugs are generally restricted to feeding on herbs, woody species were predicted to be less acceptable than herbs and this was strongly supported. Tree leaves were seldom eaten by any of the three slug species. In addition, each slug preferred a few herbaceous species unique to its specific habitat, suggesting local adaptation, but the actual mechanism remains to be determined. If both plants and slugs are coevolving, plant choices would depend upon where we examine the coevolutionary spiral, but they should be unique for each slug species as it coevolves with local plants (the coevolution hypothesis). In these tests, feeding responses of the three slug species were significantly positively correlated and unique responses were not evident. These hypotheses depend upon the probabilities of plant—herbivore encounters, which can be difficult to determine. For these slug species stem heights of only 10 cm acted as effective feeding barriers. Plants were vulnerable during emergence, but they can escape slugs if they emerge early in the spring. Because such subtle factors may determine encounter rates, predictions of feeding choices and plant defenses may be difficult to make. Specific adaptations may not be evident from these tests of general patterns. Field observations suggest that these introduced slug species are exerting significant selective pressure on the morphologies, phenologies, and defenses of native plants.