Duration and variability in host development time: Effects on parasitoid functional response

Background/Question/Methods Recent theoretical studies have demonstrated that parasitoid-host stability is strongly enhanced by either a decrease in the duration or an increase in the variability of the duration of the vulnerable host stage. The underlying cause for stability is not well known but is likely linked to the parasitoid’s functional response. The cowpea weevil Callosobruchus maculatus and its parasitoid Anisopteromalus calandrae were used as a model predator-prey system. We characterized the distribution of development times of the different host and parasitoid stages, and conducted a short-term experiment to quantify the functional response of A. calandrae in response to changes in the mean or variability of the duration of the vulnerable host stage (T_H2).
Weevils and parasitoids were reared under controlled laboratory conditions. Host life cycle was divided into 4 stages, with the late larva-early pupal stage (H₂) being vulnerable to parasitism. Parasitoids were divided into a juvenile and adult stage. For each stage, we determined the minimum, mean and distribution of development times. For the functional response experiment, we manipulated the mean (short, medium, long) and variance (low vs. high) of T_H2 by altering the exposure period or age of the H₂ stage. We also manipulated host abundance and parasitoid density.

Results/Conclusions

There was considerable variability in the development time of host and parasitoid life stages. Development times were well fit by a gamma distribution in accord with a number of other published studies. The H₂ stage, in particular, lasted a minimum of 4 d and averaged 6 d. The functional response of A. calandrae was asymptotic (type II or III) and females exhibited either pseudo- or mutual interference. As expected, hosts parasitized per parasitoid increased with the duration of the H₂ stage. Trials involving the manipulation variability in T_H2 are underway, but we predict that an increase in the variability of T_H2 will cause increased heterogeneity in the distribution of parasitism risk; a source of stability in host-parasitoid models. Finally, we describe a stage-structured model developed specifically for this system that incorporates gamma-distributed development times for each host and parasitoid stage and the parasitoids functional response. This model reveals that a reduction in the mean or increase in the variability of T_H2 can stabilize inherently unstable weevil-parasitoid population dynamics. Our future directions involve testing these model predictions by experimentally manipulating the mean and variability in T_H2 and quantifying its effects on the long-term population dynamics of the system.