Development time and host-parasitoid stability: An experimental test

The duration of specific life stages and relative lengths of the predator and prey life cycles can, in theory, exert strong influences on population dynamics and may be important considerations in managing pest outbreaks. However, rigorous experimental tests of this theory do not exist. We conducted a microcosm experiment using the cowpea weevil (Callosobruchus maculatus) and its parasitoid (Anisopteromalus calandrae) to assess how a 60% increase (long-duration treatment) or 60% decrease (short-duration treatment) in the invulnerable (late larval to mid pupal) host stage affected host-parasitoid population dynamics. We predicted that stability is most likely for microcosms in the short-duration treatment, whereas generation cycles should occur for the long-duration and control treatments, with a longer period in the long-duration treatment. Our results largely support these predictions. In the control treatment, persistent oscillations occurred with a period of about one generation, consistent with theory. The long-duration treatment produced cycles similar in period and strength to the control, rather than predicted longer period ones. As expected, cycles disappeared in the short-duration treatment and host densities were suppressed by 41%-49% relative to the other treatments. Variability in host densities in the short-duration treatment was high, likely a consequence of demographic stochasticity in small populations, whereas variability in parasitoid densities was 24%-27% lower than in the other treatments. Complementary simulation models that closely matched the biological details and protocols used in the experiment reproduced the observed dynamics in the short-duration and control treatments. The oscillatory period observed in the long-duration treatment was also replicated when stronger, scramble-type competition in the weevil attack process was incorporated into the model. We conclude that age structure and development time are critical determinants of host-parasitoid population dynamics and should be explicitly considered in the selection of biological control agents and in evaluations of program success or failure.