Examining top-down and bottom up effects on aphid abundance on Ligusticum porteri

Abstract

Top-down and bottom-up effects can be seen in tritrophic systems of plants, herbivores and predators. Understanding what factors have a significant impact on herbivores can explain patterns of herbivory in the field. In this study, I report on the aphid Cavariella aegopodii on the plant, Ligusticum porteri and investigate which effects impact aphid populations. In the observational study, I censeused plants weekly in shade and sun light environments to establish patterns of aphid (Cavariella aegopodii) and aphid predator abundance. In an experimental study, I applied aphid populations to plants in a variety of light environments as a bioassay of plant quality. I also used sticky stem traps on experimental plants to examine abundance of predators and mutualists among light environments. The observational census demonstrated light was a significant predictor of aphid presence (χ2 =3.88, P < 0.049). The presence of a facultative predator (Hemiptera: Lygus spp.) was greater in shaded environment (F = 4.654, P < 0.0334). On experimental plants light also demonstrated a significant effect - lighter environments indicated larger relative growth rates of aphids (F = 4.714, P < 0.0359). I found that the presence of predators on sticky traps did not vary with light environment. Bottom-up effects can be seen with aphid presence and light environment, while predator presence did not show significant effects on aphid presence. However, abundance of the aphid predator Lygus varied with light environment, and this could be a light-influenced top-down effect.

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