Biological control by multiple natural enemies; key roles of behavioral plasticity of pests and natural enemies

Background/Question/Methods

Whether introduction of multiple natural enemies results in more effective suppression of pests than that of a single one has been debated for a long time. Intra-guild predation (IGP) among natural enemies preying on pests has been recognized as an important agent disrupting effective pest control. Classical theoretical studies on IGP have shown that introduction of multiple natural enemies is always unfavorable for pest control, whereas empirical studies have showed both positive and negative results. To fill the gap between empirical and theoretical studies, we extended the classical model on IGP and considered a dynamical system model including adaptive defense by the pest and switching predation by the omnivorous natural enemy (omnivore) and examined separate and combined effects of them on efficiency of biological control. We assumed that the pest can adaptively allocate efforts toward two kinds of defense against each predator to increase its own fitness, at the cost of reduction in its own reproduction. We also examined effects of switching predation by the omnivore using the Holling’s type III functional response to the pest and another natural enemy (intermediate predator). Stability of equilibrium states and pest density as an index of success in biological control were investigated.

Results/Conclusions

Adaptive defense by the pest increased the pest density when attacked by a single enemy but enhanced coexistence of the omnivore and intermediate predator because the pest allocated more defensive effort against the more threatening predator. Since it was difficult for the pest to effectively defend against two predators, introduction of two natural enemies were favorable for reducing the pest density when the pest defended itself, unless IGP was too severe. Switching predation strongly promoted coexistence of two predators but could not improve biological control by two natural enemies. However, the type III functional response tempted the pest to abandon defensive effort against the omnivore since the population pressure was negligible at low pest density and saturated at high density. Since the intermediate predator was suppressed by the undefended omnivore, defensive effort against the intermediate predator also diminished. Thus, switching predation could offset the effects of defense and the prey density could be lowered by two natural enemies even under severe IGP. Consequently, types and combination of behavioral plasticity might cause qualitatively different outcomes of biological control introducing two natural enemies. Optimal biological control should be determined by considering behavioral traits of pests and natural enemies.