Primary tillage sequence and weed seed placement strategies to conserve carabid seed predators and enhance predator efficacy

Background/Question/Methods

Regulation of weed seed banks and impending weed interference in agroecosystems is typically accomplished by reducing weed seed inputs, or by stimulating weed seed germination with tillage. Strategies to enhance seed mortality are frequently overlooked. Seed predation by invertebrates such as carabid beetles is an important source of weed seed mortality in some cropping systems. The potential to enhance carabid abundance and weed seed predation potential with tillage rotational systems and other cultural practices is poorly understood, and is the focus of this research. Tillage increases carabid mortality, destroys habitat, and positions weed seed so that carabids may not have access. Studies were placed in irrigated vegetable row crop fields in both high and low rainfall areas of Oregon to measure the effect of crop rotation and associated primary tillage sequences and cultural practices on carabid habitat, activity-density, and weed seed loss to carabid predators. Carabid activity-density was measured with pitfall traps placed in fields during the usmmer months of 2006 to 2008. Seed predation was measured by placing a 5 cm Petri dish containing 50 pigweed seeds set flush with the soil surface and surrounded by 1.2 cm mesh screen to exclude rodents and birds. Crop specific insecticides were applied in large subplots within each field to assess direct effects of insecticides on carabid activity-density, recolonization intervals, and weed seed predation.

Results/Conclusions

Pterostichus melanarius and Harpalus pensylvanicus were the primary species captured in all high-rainfall sites. The primary species in low rainfall areas was Harpalus pensylvanicus, followed by Agonum melanarium and Pterostichus melanarius. Carabid activity-density was typically greater in August than July and September, but varied greatly among sites. Insecticide treatments applied to plots reduced carabid activity-density most at the center of the plot, and caused seed loss to drop from 1.5 seeds/day on the perimeter of the insecticide treated sub-plot to 1.06 seeds/day at the center of the treated area when averaged over the three months and all sites. There was no effect of spring primary tillage treatment (conventional vs. strip tillage) on beetle activity-density or species in the first year of the three year rotation. Weeds seeds were seeded into plots in a pulse-chase experiment that will measure the impact of primary tillage sequence, pesticide use, and weed seed burial depth on carabid activity-density and seed predation throughout a three year vegetable crop rotation.