Lichen Secondary Compounds: Evidence for a Correspondence between Antiherbivore and Antimicrobial Function

Laboratory tests were done to determine the antimicrobial activity of lichen secondary substances known to influence the feeding behavior of the lichen herbivore, Pallifera varia. Antibiotic test disks were prepared using acetone extracts of two lichens preferred (Aspicilia gibbosa and Lasallia papulosa) and two lichens avoided (Flavoparmelia baltimorensis and Xanthoparmelia cumberlandia) by the herbivore. These disks were placed on nutrient agar plates inoculated with various bacteria. The lichen extracts consistently inhibited gram-positive bacteria used in the assays, and the order ofeffectiveness was the same as it had been against the herbivore. Pure lichen substances were also found to vary in effectiveness in these assays. These results suggest that lichen secondary substances can defend lichens against attack by both herbivores and microorganisms; however, they are apparently not designed to defend specifically against either.