Responses of generalist predators fed high-Ni Melanotrichus boydi (Heteroptera : miridae): Elemental defense against the third trophic level

The recent discovery of herbivores that feed on Ni-hyperaccumulating planta and contain elevated Ni concentrations in their bodies suggests that Ni ma y be transferred to their predators. We tested this hypothesis using tile h igh-Ni herbivore Melanotrichus boydi and four predator species: thr spiders Pholcus phalangioides and Misumena valia, the mantid Stagmomantis californ ica and the lacewing Chrysoperla carnea. Survival of each predator species was compared when individuals were fed either M. boydi or low-Ni prey. No s ignificant survival difference between diets was observed for P. phalangioi des or S. californica, although individuals fed M. boydi contained signific antly elevated Ni concentrations (470 mug Ni g(-1) for P. phalangioides and 460 mug Ni g(-1) for S. californica). No significant difference in surviva l to adulthood was observed for C. carnea: adult production was 45% for hig h-Ni and 55% for low-Ni diets. In contrast, survival of M. vatia was signif icantly decreased when fed high-Ni prey: only 32% of those fed M. boydi sur vived after 20 d, in contrast to 89% of those fed low-Ni prey. Misumena vat ia collected from Ni-hyperaccumulating Streptanthus polygaloides plants in the wild contained some Ni (a minimum of 110 mug Ni g(-1)), but significant ly less than the 420 mug Ni g(-1) measured in those fed M. boydi. This indi cated that M. vatia, which is native to S. polygaloides sites, consumes som e M. boydi under natural conditions. We concluded that: (1) Ni can be trans ferred from a high-Ni herbivore to its predators: (2) predators varied in t heir sensitivity to the Ni contained in M. boydi, and; (3) Ni can poison pr edators which specialize upon high-Ni herbivores and thus may act as an ele mental herbivore defense.