Abstract: Complex species interactions support ecosystem functioning in a variety of systems from natural rainforests to managed agricultural fields. Ecological studies have often focused on untangling the factors
that shape the direct interactions between important focal species. Fewer studies have investigated the
multi-trophic indirect effects of a diversity of species. In tri-trophic plant-insect systems, plant host
diversity can lead to increases in parasitoid attack rates of herbivores. In a biological control context, this
increase in parasitism rate as a result of plant diversity is known as the enemies hypothesis. We tested the
enemies hypothesis by examining the effects of plant diversity on parasitoid-herbivore interactions in
asteraceae flower heads. Many species of these flower heads support the larvae of tephritid flies (Tephritis
signatipennis and Campyglossa dupla) which are often parasitized by chalcid wasps. In the summer of
2017, we collected flower heads of Erigeron speciosus in 14 meadows near the Rocky Mountain
Biological Laboratory (Gothic, CO, USA). Within each meadow, we measured the presence or absence of
40 other species of Asteraceae. We preserved E. speciosus flower heads in cups to identify, count, and sex
the insects that emerged.
Using this data, we specifically asked: What is the effect of meadow-scale asteraceae biodiversity on
parasitism rates of floral herbivores? Using a generalized linear model, we found a significant positive
correlation between the richness and Shannon diversity and the parasitism rate of the tephritid flies by the
wasps. This analysis supported the enemies hypothesis. Our dataset also allowed us to explore the other
factors beyond this hypothesis that might shape the parasitism rate of tephritids in this system. To address
this second exploratory objective, we analyzed the sex ratio of the surviving flies and the environmental
conditions of each meadow. Our data indicated that male tephritid flies were more likely to be reared out
of the collected heads than female tephritids. This male biased sex ratio suggests a parasitoid preference
for female larvae. Future research is needed to understand the mechanism behind this biased sex ratio.
The elevational gradient of these sites also positively influenced the parasitism rates of these tephritid
flies. Our study found support for plant biodiversity, host sex ratio, and environmental factors as
important drivers of the parasitism rate in these flower heads. These findings illustrate patterns of
parasitism in this system that support the enemies hypothesis and provide interesting future directions in
