Abstract:
Virtually all plants employ direct and
indirect defenses against herbivores. While it is
known that plant defenses can be affected by below-
ground symbiotic microbes under controlled condi-
tions, studies showing these multitrophic interactions
in nature are surprisingly scarce. Here we tested for
effects of rhizobia on insect attraction and direct
defense (cyanogenesis) in wild lima bean (Phaseolus
lunatus) plants in Costa Rica. We performed bioas-
says with rhizobia-inoculated (R+) and rhizobia-
free (R-) potted plants distributed among native
lima bean communities at two spatially separated
field sites (450 km apart) and in two field seasons.
Without affecting overall plant size, rhizobia altered
leaf chemistry (cyanogenesis and soluble leaf nitro-
gen) and ultimately insect communities visiting the
plants. Natural herbivorous chrysomelid beetles were
strongly attracted to R + plants, while natural ene-
mies, ants and parasitoid wasps, preferred R- plants
resulting in a particularly high herbivore:carnivore
ratio on R + plants. This suggests that symbiotic
microbes mediate trophic interactions by influencing
