Let them work! Harvester ant seed predation leads to higher seed losses than seed burial
More so than in natural systems, timing and frequency of disturbances can be manipulated in arable systems to enhance weed seed mortality. Such disturbances (e.g. tillage) can bury weed seeds to a depth where successful germination is impaired or where they are prone to seed decay; however in doing so seed predation may be compromised, as buried seeds may be rendered inaccessible to seed predators. In this work we compared seed mortality related to seed predation by the harvester ant Messor barbarus to rates of seed decay for Lolium rigidum and Galium aparine, two commonly occurring weeds in winter cereal fields in north-eastern Spain. Seed predation rates were measured and ant searching activity was estimated for 48h intervals every 3 weeks from July to September 2014. Seed availability on the soil surface was also measured. Mortality from burial was measured by burying twenty nylon bags with 30 seeds of either of the weed species 6 cm deep for up to 93 days. Seeds from retrieved bags were classified as non-viable (dead or germinated) or viable.
Mortality rates due to predation were higher (99 and 97% for L. rigidum and G. aparine respectively) than those due to burial (55 and 32% respectively). Seed availability measures revealed that there were seeds available for predators on the soil surface even after three months following their release from the mother plant. Analysis of ant searching activity showed that M. barbarus ants explored the area more intensely and were less selective of seed species as background seed density declined. Delaying disturbance proved to be a better strategy to reduce the flux of weed seeds entering the soil seed bank than inducing seed decay or burial with tillage and should be considered as a useful strategy to achieve sustainable weed management within arable systems.