FIGURE 5.20 The effect of arbuscular mycorrhizal inoculation on the population of the root burrowing nematode Radopholus similis. Source: Data from Elsen et al. (2001).

Lawrey (1986; 1989) made acetone extracts from lichen species (Aspicilia gibbosa and Lasallia papulosa) that were shown to be readily eaten by the slug Pallifera varia, and from lichens that were avoided by the slug (Flavoparmelia baltimorensis and Xanthoparmelia cumberlandia). The extracts were added to cultures of the bacteria Bacillus megaterium, B. subtilis, Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa. The lichens that were avoided by the herbivore had the greatest antagonistic effects against the bacteria Bacillus megaterium, B. subtilis, and Staphylococcus aureus, whereas the growth of both Escherichia coli and Pseudomonas aeruginosa were unaffected by any lichen. Defensive chemicals produced by the two lichens avoided by the herbivore included norstictic, stictic, usnic, constictic, connorstic, gyrophoric, caperatic, and protocetaric acids. In a controlled experiment, Lawrey (1989) also showed that vulpinic, usnic, and stictic acids reduced the growth of the same susceptible bacterial species. There thus appears to be a universal defensive role of these chemicals, before against herbivory or microbial attack and as an allelopathic agent (Table 5.18).