1983] 
Douglas — Defense of bracken fern 
315 
“...bracken-feeding herbivores face progressive deterioration in 
“food quality” as the growing season progresses.” Yet, we have 
found that herbivorous species diversity and abundance, particu- 
larly of adapted herbivores, increase dramatically after the second 
week of June, and peak during late July and August. 
Bracken’s palatability early in the spring may also be affected by 
the production of thiaminase and the cyanogenic glucoside, pru- 
nasin (Jones and Firn, 1978). But bracken in England is polymor- 
phic for the production of HCN: some bracken clones contain the 
B-glucosidase enzyme and not prunasin, while others contain 
neither enzyme nor the glucoside (Cooper-Driver and Swain, 1976; 
Cooper-Driver et al., 1977). Likewise, Zavitkovsky (1979) found 
uniformly negative results for cyanogenesis in Massachusetts brack- 
en fern. 
Thiaminase may be the only known chemical deterrent in bracken 
with any potential for disrupting normal insect development, since 
thiamine is essential to insect development (Dadd, 1973). However, 
bracken does contain thiamine (Berti and Bottari, 1968), and thiam- 
inase activity in English bracken drops from a high of nearly 30 ug 
to 7 ug thiamine destroyed per min/g dry weight between the last 
week of April and the second week of May (Evans, 1976). Even so, 
populations of adapted bracken herbivores increase only after the 
second week of June in Michigan, perhaps weeks after cyanogenic 
glucosides (if functional) and thiaminase activity have fallen to low 
levels. By contrast, most other diapausing and temperature sensitive 
insects such as butterflies have eclosed from the pupae by early May 
in Michigan, and their larvae can be found by the second and third 
weeks of May. 
Our study in Michigan suggests that bracken has evolved another 
line of defense that complements its biochemical defense system, 
and protects it from serious herbivorous damage during the rapidly- 
developing crozier stage. This second line of defense consists of 
predaceous arthropods, particularly ants and spiders attracted to a 
sweet, viscous fluid secreted by a number of axillary nectaries. The 
nectaries are dark oval enlargements that appear in the axils where 
the pinnae and major pinnules branch off from the rachis. Large 
numbers of “nonassociated” arthropods which utilize bracken for 
purposes other than food are also attracted to the developing 
bracken canopy and their oozing nectaries early in the spring. In 
