Investigation of elemental defense and trophic transfer of metals using beet armyworm, Spodoptera exigua, Hübner
Type of Degreedissertation
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Elemental defense is a phenomenon in which high concentrations of a metal in plant tissues can defend a plant from herbivores. Elemental defense is one of the hypotheses proposed to explain the evolution of metal hyperaccumulator plants. In this dissertation, I evaluated the toxicities of Co, Cu, Ni and Zn against a generalist herbivore, Spodoptera exigua, using artificial diet. First, I determined the minimum lethal (MLC) and minimum sublethal concentrations (MSC) for each metal against S. exigua larvae. I showed that MLC values (dry mass basis) for three metals (90 µg Co g-1, 570 µg Ni g-1 and 280 µg Zn g-1) were below published minimum hyperaccumulator levels, whereas that for Cu (530 µg Cu g-1) was above. MSC levels (dry mass basis) were all recorded at concentrations lower than published minimum accumulator levels: 15 µg Co g-1, 95 µg Ni g-1 and 200 µg Zn g-1, with the exception, again of Cu (175 µg Cu g-1). Furthermore, both MLC and MSC values for Zn were within normal tissue concentrations (20-400 µg Zn g-1). Second, I evaluated the type of joint effect that occurs between Co, Cu, Ni and Zn when fed in combination to S. exigua larvae and between the four metals and three organic compounds (nicotine, mustard seed powder and tannic acid) when fed in combination to S. exigua. The Joint Effects Hypothesis suggests that combinations of metals, or a combination of a metal with an organic defense compound, may have increased defensive effects (these may be additive or synergistic). I tested for joint effects using both lethal and sublethal concentrations of the chemicals tested. I showed that all lethal and sublethal joint effects were either synergistic or additive (none antagonistic). Third, I evaluated the survival, growth and development of a generalist predator (Podisus maculiventris) when fed herbivorous prey (Spodoptera exigua) reared on artificial diets amended with MSCs and MLCs of Co, Cu, Ni and Zn as an example of potential tritrophic interactions. Additionally, I determined if metals were biotransferred or bioaccumulated from diet to herbivore and to predator. Survival of P. maculiventris was not affected by feeding them larvae raised on diet amended with any of the four metals. Predators fed herbivores reared on Cu-amended and Zn-amended diets took a significantly longer time to mature to adults compared to control treatments (P = 0.0001 and P <0.0001, respectively). Adult weights were significantly reduced only for predators that were raised on herbivores reared on diets amended with the MLC of Cu (P = 0.018). The presence of metals in artificial diets resulted in biotransfer of Ni, and bioaccumulation of Co, Cu and Zn, from the artificial diet to the herbivore and from the herbivore to the predator. Overall, my research showed the potential ecological importance of elemental defenses at relatively low metal levels, the role joint effects may play in lowering threshold levels for sublethal and lethal effects of the metals studied, and potential sublethal effects of metals on predators that feed on metal-containing herbivores.
- Dorothy Jelagat Cheruiyot.pdf