Population regulation of an aposematic organism and underlying host-plant interactions

Background/Question/Methods

Aposematic herbivores may be primarily bottom-up regulated due to their apparent toxicity, making them ideal organisms for studying the mechanisms of bottom-up regulation. Yet, few studies have examined bottom-up regulation of aposematic species. In this experiment we specifically examined Chrysochus auratus, the dogbane beetle, a specialist herbivore that consumes and sequesters toxins from Apocynum cannabinum, the dogbane plant. We conducted a field experiment to examine if C. auratus populations are regulated through density dependent survival or reproduction, and whether survival and reproduction are sensitive to bottom-up forces including host-plant quality and plant chemical defenses. We caged the beetles on stems of A. cannabinum at four different densities with eight replicates per treatment. These cages were monitored every three days to track beetle survival and collect egg capsules. We measured plant quality traits before and after the experiment to establish the change in plant quality over the course of the experiment.

Results/Conclusions

We found evidence for density dependence in the reproduction, but not the survival, of C. auratus. There was an effect of beetle density on egg mass production, which varied over time. The rapidity of the effect on reproduction indicates that an induced defensive response of the plants may be responsible. We also found strong effects of the beetles on its host plant with significant reductions in plant growth and leaf area with increasing beetle density, and increasing C:N ratio in the lowest three beetle densities. The strong effects on plants likely explain the beetles’ density dependent reproduction.