Higher and more stable biological control of multiple herbivore species in diversified strip cropping systems

Context: Agricultural intensification has simplified agroecosystems,
reduced biodiversity and weakened natural pest control. Effective
biological control requires reliable, long-term control of multiple
herbivores. While studies investigated biological control in diversified
cropping systems, there is little information to what extent pest
suppression is sustained throughout the growing season and whether it
provides satisfactory control of several herbivores. Objectives: Here we
assessed how increasing crop diversity and an “attract-and-reward”
strategy influence effectiveness and temporal stability of biological
control. Methods: We compared predation, parasitism of eggs and
caterpillars in a cabbage system with four levels of crop diversity:
cabbage monoculture, cabbage-oat strip cropping, strip cropping with six
crops, and an attract-and-reward system in which a strip cropping system
was enhanced by inclusion of a parasitoid-attractive cabbage cultivar and
nectar-providing buckwheat. Experiments were conducted at two
organically-managed farms in the Netherlands in 2023 and 2024.
Additionally, a full-factorial experiment was conducted in 2023 at one
location to assess the effects of the attract-and-reward components.
Temporal stability was quantified as the coefficient of variation and the
proportion of sampling weeks in which each treatment exceeded the mean
biological control level. Results: Across locations and years, strip
cropping enhanced parasitism rates of Pieris brassicae and Plutella
xylostella compared with monocultures. Pieris brassicae parasitism
increased from 28% in monocultures to 58% in strip cropping systems, while
P. xylostella parasitism rose from 10% to 25–30%. Parasitism of M.
brassicae eggs by Trichogramma spp. reached up to 30% in the strip
cropping system with six crops, compared with 1% in monocultures.
Attract-and-reward provided no improvement beyond standard strip cropping.
Monocultures showed the highest coefficient of variation and exceeded the
site-year mean in only 13% of weeks, compared to more than 60% in strip
crop systems. Synthesis & application: Our results highlight that
strip cropping can enhance and stabilize biological control. Even simple
two-crop designs provide clear benefits over monocultures, while
increasing crop diversity to six crops offers additional advantages.
Because the temporal stability of biological control increases with crop
diversity, designing strip cropping systems with more crops can help
maintain consistent and resilient pest regulation throughout the growing
season.