Carabids as weed seed control agent : impact of soil cultivation on carabid weed seed regulation

Abstract: As weed seed predators, carabid beetles can regulate the weed seed bank within agricultural fields. Little is anyhow known about how weed seed bank regulation by carabids is affected by different tillage treatments with decreasing intensity, why this was here examined. Data on weed seed bank size, weed seed availability and carabid activity density were collected from field trials in Västergötland, Sweden. A weed seed bank regulation was estimated using the weed seed bank sizes from March and August. Weed seed predation strength was estimated using activity density and data on preferred prey choice. To study any differences over treatments were ANOVA and Tukey’s test used and a piecewise structural equation model (pSEM) was formed and tested for all three tillage treatments separately to see whether any differences within the different treatments were present. Weed seed bank size and weed seed availability increased significantly in the ploughed treatment, but not in fields with reduced tillage or direct seeding. The pSEM for direct seeding showed that more weed seed resources increase the carabid activity density in July which in turn increases the weed seed bank regulation. For reduced tillage treatment, a correlation between higher carabid activity density in June and lower weed seed availability in July, as well as lower weed seed availability in July compared to ploughing was found. Despite the indications of a greater seed build-up in ploughed and fewer weed seeds in reduced tillage was no difference in weed seed regulation from March to August, nor in weed seed predation strength was found. The lack of differences in weed seed bank regulation could be caused by a lack of input data to the fields, where the interaction between chemical applications, chiefly herbicide use, would assist in disentangling the effect of the interaction between tillage and chemical herbicide control. Here, weed seed bank regulation was only assessed during the cropping season, while other outcomes would be possible if assessing annual weed seed bank regulation. To be able to use results about weed seed bank regulation by carabids to give clear advice to farmers should long-term effects be examined.