Synergistic and antagonistic interactions between plant defences and biological pest control

Plants have evolved various defence mechanisms against herbivorous pests, ranging from physical barriers (constitutive defences) to the release of toxins (induced defences) in response to an attack. Plant defences are not always target-specific and can also act against natural enemies of these pests. This could potentially become problematic when considering both plant defences and predators as part of an Integrated Pest Management (IPM) scheme. Here we used a predator-prey model to capture the population dynamics of a biological control system to investigate the conditions where both plant defences and biological control act synergistically, leading to better pest control, and under what condition antagonistic outcomes could be expected. Our results demonstrate that both antagonistic and synergistic interactions are observable under small changes in key parameters, such as predation and growth rates. We then compared the qualitative model predictions against a set of population dynamic experiments using the Orius laevigatus (Hemiptera: Anthocoridae) - Frankliniella occidentalis (Thysanoptera: Thripidae) predator-pest system on tomato plants. In particular, we investigated the effect of plant defence-induced changes in pest growth rates and predation rates. For this we compared wild-type tomato with a mutant unable to synthesise jasmonic acid, an induced defence mechanism against pest attack, whereas changes to predation rates were realised by using O. laevigatus predator strains that are, due to differences in size, differentially affected by the plant's trichome-based constitutive defences. In agreement with model predictions, the qualitative dynamics confirmed the existence of both antagonistic and synergistic outcomes. Our results highlight the importance of considering multi-trophic interactions for the development of effective IPMs. •Plant defences can also act against natural enemies of herbivores and become a problem for Integrated Pest Management (IPM).•· Predator-prey models are useful to investigate the conditions where plant defenses and biological control help each other•Model predictions can be tested in experiments where key parameters of predators and prey are artificially modified•The qualitative dynamics confirmed the existence of both antagonistic and synergistic outcomes.•Multi-trophic interactions need to be considered for the development of effective IPMs.