An abundant biological control agent does not provide a significant predator subsidy

Classical weed biological control agents, regardless of their effectiveness, may provide subsidies to predators and parasites. The chemically defended weevil Oxyops vitiosa Pascoe is a successful agent that was introduced to control the invasive tree Melaleuca quinquenervia. Two consecutive small plot experiments that lasted two and three years, respectively, examined the population dynamics of O. vitiosa while subjected to predation by Podisus mucronatus, a native generalist predator. During this time the estimated mean (±SE) percent predation of O. vitiosa larvae was 7.2 ± 1.7% per sample date in the two year study and 8.4 ± 0.8% in the three year study. There was no relationship between the number of larvae per tree and the number that were predated in either experiment. Consumer losses from predation did not cascade down to the producer level and influence any plant variable in either experiment. Time series analysis found no autoregressive processes for predation in either experiment while there were strong first through fourth-order auto-correlations for live larvae in both experiments, indicating the presence of strong trends in prey density. If longevity was a gauge of the relative importance of a predator subsidy, then any provided by O. vitiosa was negligible because predation was unlikely to increase over two consecutive sample periods despite increasing prey populations. The benign presence of sustained populations of a biological control agent provides a tailored counter argument to studies that imply inevitable and perilous linkages between introduced agents and community food webs