32-Phosphorus Tracer Studies of a Horseweed-Aphid-Ant Food Chain

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Abstract
Earlier studies with 32P have suggested that the old field ant (Dorymyrmex pyramicus), supposedly a predator and scavenger species, was feeding on horseweed (Erigeron canadensis). This supposition was based on an unusual pattern of 32P uptake in the ant population. The present investigation showed that Dorymyrmex is indeed a consumer of horesweed, accomplished not through herbivory, but instead by tending aphids present on these plants and ingesting honeydew. Ants confined for 7 days to tagged horseweed plants harboring aphids had over 4 times as much radioactivity as ants confined on tagged plants without aphids. The study suggests that the rapid rise and subsequent decline of 32P in the ant populations in the field is characteristic of species with this food source. It may be a result of changes in the level of 32P in the phloem sap together with a bias in the sampling caused by collecting ants only from the vegetation.

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