Defense of Bracken Fern by Arthropods Attracted to Axillary Nectaries

The phenotypically variable bracken fern, Pteridium aquilinum (L.) Kuhn, is an economically important plant that establishes dense monocultural stands by spore dispersal and by spreading subterranean rhizomes throughout the world, except for hot and cold desert regions (Page, 1976). Bracken produces a number of so-called "secondary plant compounds" that have been shown to protect it from some nonadapted insects (Cooper-Driver et. al., 1977). These compounds include the cyanogenic glucoside, prunasin (CooperDriver and Swain, 1976; Cooper-Driver et. al., 1977), lignins and silica (Lawton, 1976), sesquiterpene pterosins (Jones and Firn, 1979a), phytoecdysteroids (Jones and Firn, 1978), and the protein thiaminase (Evans, 1976). Tannins, flavonoids, and phenolics have also been implicated as possible defensive compounds in bracken fern (Cooper-Driver et. al., 1977; Jones and Firn, 1979b). Despite bracken’s well-developed biochemical arsenal, adapted and nonadapted herbivorous insects in experimental plots located in Michigan and Massachusetts often destroy up to 30 percent of a frond’s biomass after the pinnae are completely expanded. In addition to these herbivores, stands of Michigan bracken also support a diverse community of ectoparasites, parasitoids, and predators of bracken herbivores, including nearly 20 species of ants and spiders that form temporary symbiotic relationships with the bracken croziers.