Experimental Approaches Testing the Herbivore Defense

Removal of secondary compounds from lichen thalli with the acetone-rinsing technique allows testing of herbivore-deterrent functions for these compounds (Gauslaa 2005; Poykko et al. 2005). Gauslaa (2005) gave common garden snails (Cepaea hortensis) the choice to graze on two halves of one lichen thallus, one was acetone rinsed and one served as control. Grazing after one night was quantified as loss in dry weight. Such a protocol works well with a number of lichens such as H. physodes and Evernia prunastri. The experiment is simple and useful in ecology laboratory classes for students, and visualizes how powerful secondary compounds can be in deterring herbivores. Other lichen-feeding gastropods can also be used like Arion fuscus, Cochlodina laminata, and Helicigona lapicida, and the method functions also for woodlices (Isopoda) (Wolfgang Bilger, pers. comm.). The method has been used with the lichenivorous larvae of the moth Eilema depressum (Poykko et al. 2005), and a modified method has successfully been used with bank voles (Nybakken et al. 2010). Some snails also feed on filter paper and digest cellulose. By adding the acetone extracts with compounds to filter paper, the deterring role of the extracted compounds can be quantified. However, tests of pure compounds individually are necessary to elucidate which compounds are responsible for the deterrent effect (see Table 2).

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