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 plants and contain elevated Ni concentrations in their bodies suggests that Ni may be transferred to their predators. We tested this hypothesis using the high-Ni herbivore Melanotrichus boydi and four predator species: the spiders Pholcus phalangioides and Misumena vatia, the mantid Stagmomantis californica and the lacewing Chrysoperla carnea. Survival of each predator species was compared when individuals were fed either M. boydi or low-Ni prey. No significant survival difference between diets was observed for P. phalangioides or S. californica, although individuals fed M. boydi contained significantly elevated Ni concentrations (470 mg Ni g21 for P. phalangioides and 460 mg Ni g21 for S. californica). No significant difference in survival to adulthood was observed for C. carnea: adult production was 45% for high-Ni and 55% for low-Ni diets. In contrast, survival of M. vatia was significantly decreased when fed highNi prey: only 32% of those fed M. boydi survived after 20 d, in contrast to 89% of those fed low-Ni prey. Misumena vatia collected from Ni-hyperaccumulating Streptanthus polygaloides plants in the wild contained some Ni (a maximum of 110 mg Ni g21), but significantly less than the 420 mg Ni g21 measured in those fed M. boydi. This indicated that M. vatia, which is native to S. polygaloides sites, consumes some M. boydi under natural conditions. We concluded that: (1) Ni can be transferred from a high-Ni herbivore to its predators; (2) predators varied in their sensitivity to the Ni contained in M. boydi, and; (3) Ni can poison predators which specialize upon high-Ni herbivores and thus may act as an elemental herbivore defense.