title: Testing the joint effects hypothesis of elemental defense using Spodoptera exigua.
abstract: Metal hyperaccumulation may be an elemental defense, in which high concentrations of a metal in plant tissues decrease herbivore survival or growth rate. The Joint Effects Hypothesis suggests that a combination of metals, or a combination of a metal with an organic compound, may have an enhanced defensive effect. The enhancement may be additive or synergistic: in either case the concentration of a particular metal necessary to provide a defensive benefit for the plant is lowered. We tested the Joint Effects Hypothesis using Spodoptera exigua (beet armyworm) neonates fed artificial diets. Metal + metal experiments utilized diets amended with metal pairs, using four metals commonly hyperaccumulated by plants (Co, Cu, Ni, and Zn). We also conducted metal + organic compound experiments, pairing each metal with nicotine, mustard seed powder, or tannic acid. We tested for joint effects using both lethal (LC20 levels) and sublethal concentrations (10-25 % reduced larval weight) of the chemicals tested. For all experiments, either additive or synergistic effects were found. Of the metal + metal pairs tested, three (Co + Cu, Cu + Zn, and Ni + Zn) were synergistic in lethal concentration tests and only Co + Cu was synergistic in sublethal tests. For metal + organic combination lethal tests, synergism occurred for all combinations except for Co or Ni + nicotine, Ni + mustard seed powder, and Zn + nicotine. For sublethal tests, Zn + all three organic chemicals, Co + mustard seed powder or tannic acid, and Cu + nicotine, were synergistic. These results support the Joint Effects Hypothesis, suggesting that metals combined with other metals or organic compounds may be more effective against herbivores than individual metals.
authors.affiliation: Brookstone School, Columbus, GA, USA.
authors.forename: Dorothy J
authors.initials: DJ
authors.lastname: Cheruiyot
authors.forename: Robert S
authors.initials: RS
authors.lastname: Boyd
authors.forename: William
authors.initials: W
authors.lastname: Moar
mesh_terms: D000821:Animal Feed / Q000032:analysis; D000818:Animals; D004032:Diet; D020387:Food Chain; D060434:Herbivory; D007814:Larva / Q000187:drug effects / Q000254:growth & development; D019216:Metals, Heavy / Q000633:toxicity*; D018411:Spodoptera / Q000187:drug effects* / Q000254:growth & development
chemical_list: D019216:Metals, Heavy