What limits predation rates by the specialist seed‐feeder Penthobruchus germaini on an invasive shrub?

Abstract

Summary Specialist seed‐feeders are widely used in weed biological control, but seed predation rates are frequently insufficient to cause the required impacts. Understanding the underlying reasons is prerequisite to predicting efficacy. We conducted continental‐scale surveys of an introduced, multi‐voltine seed‐feeder [Bruchidae: Penthobruchus germaini (Pic.)] on an invasive legume (Caesalpinaceae: Parkinsonia aculeata L.). We tested three hypotheses as to what limits seed predation; namely, seed escape through egg aggregation, mortality of immature beetle stages, and failure to track temporal fluctuations in resource availability. We also tested how these factors interacted with the environment and each other. Mean seed predation was relatively low (2–30%), despite mean egg densities of between 0·55 and 3·2 eggs per seed. Eggs were slightly aggregated (negative binomial, k = 1·87). Unexplained egg mortality (6–44%), egg parasitism (10–70%) and larval/pupal mortality (62%) were high, but egg parasitism was the only mortality factor that was density‐dependent and that varied across climatic regions. Egg densities responded poorly to rapid within‐season increases in seed availability. All examined factors dampened seed predation rates. However, we developed a deterministic mathematical model which showed that seed predation would still have been relatively low (5–56%) at the observed egg densities, even without direct effects of immature beetle mortality. Also, to achieve a benchmark seed predation rate of 80%, egg densities would need to be over 8·5 eggs per seed with no parasitism, and an unrealistically high 27·8 at 70% parasitism. Available data suggest that seed predation by specialist seed‐feeders will often be constrained by one or more of the factors we identified. Synthesis and applications. Selecting effective biological control agents is an important challenge for weed biocontrol. Our results suggest that many seed‐feeders will not regulate plant populations, but that predictions of their efficacy can be greatly improved by quantifying the way eggs are distributed across seeds and the various mortality factors that affect immature beetle stages. The ability of seed‐feeders to track resource fluctuations may also be predictable. For example, multi‐voltine insects appear better at tracking between‐season and between‐site variation in resource availability than sharp changes in within‐season availability.