  Main goal: The aim of this note is to propose a modeling approach for
assessing the effectiveness of pest biocontrol by natural enemies in
diversified agricultural landscapes including several pesticide-based
management strategies. Our approach combines a stochastic landscape model with
a spatially-explicit model of population dynamics. It enables us to analyze the
effect of the landscape composition (proportion of semi-natural habitat,
non-treated crops, slightly treated crops and conventionally treated crops) on
the effectiveness of pest biocontrol. Effectiveness is measured through
environmental and agronomical descriptors, measuring respectively the impact of
the pesticides on the environment and the average agronomic productivity of the
whole landscape taking into account losses caused by pests.
  Conclusions: The effectiveness of the pesticide, the intensity of the
treatment and the pest intrinsic growth rate are found to be the main drivers
of landscape productivity. The loss in productivity due to a reduced use of
pesticide can be partly compensated by biocontrol. However, the model suggests
that it is not possible to maintain a constant level of productivity while
reducing the use of pesticides, even with highly efficient natural enemies.
Fragmentation of the semi-natural habitats and increased crop rotation tend to
slightly enhance the effectiveness of biocontrol but have a marginal effect
compared to the predation rate by natural enemies.
  This note was written in the framework of the ANR project PEERLESS
"Predictive Ecological Engineering for Landscape Ecosystem Services and
Sustainability"(ANR-12-AGRO-0006).
