Interspecific ant competition over novel aphid resources and changes in plant chemistry due to ant-aphid mutualisms on milkweed plants.

Abstract

Ants and aphids have a mutualistic relationship in which ants tend the aphids for honeydew, a sugary substance that aphids excrete, and in turn, aphids receive protection from predators that the ants provide. We looked at this relationship on milkweed plants in the UVB field at the University of Michigan Biological Station in Cheboygan County, Michigan. We investigated how this mutualism affected carbon to nitrogen ratios of the plants, and how the ratio differed between the original and novel plants. We also looked at how the carbon to nitrogen ratio was affected by aphid density on the plant. Finally, we investigated interspecific ant competition over a novel aphid resource. To do this, we clipped leaves with aphids from an original plant, and moved them to a novel plant without aphids. We took leaf samples of the original plant and the novel plant that aphids were being moved to. We then recorded aphid colonization of the novel leaf and ant recruitment and tending on that plant. At the end of four days, we took a final sample of leaves from each novel plant. We used these to determine carbon to nitrogen ratios of the original plants, and before and after of the novel plants. There was no significant relationship between ant and aphid densities on the plants to effectively determine that a mutualism is present. There was no significant difference in the carbon to nitrogen ratios in the original plants compares to the novel plants before aphid transfer. There were no differences in the changes of plant chemistry of the plants with only aphids, plants with ants tending aphids, and plants with no aphids. There was no relationship between the carbon to nitrogen ratio and the aphid density on the plant. We also saw only 5% of plants being tended by two ant species, so we could not conclude anything from this. The reason that none of our statistical analyses were significant might be because we did not give enough time for the aphids and ant-aphid mutualism to change plant chemistry.