Can obligatory omnivore carabids be useful for the biocontrol of weeds?

National audience; Many thousands of carabid individuals exist in farm fields, in communities of carnivore and opportunistic (e.g. granivores) and obligate omnivore guilds. As carabids can eat a substantial amount of weed seeds they are considered as credible biocontrol agents for the regulation of weeds in arable fields. Some studies have suggested that “granivores” are more important biocontrol agents than obligate omnivorous species. Yet, in some instances, obligate omnivores are very abundant and appear to drive the predation of weeds leading some authors to question whether specialist natural enemies are better. Moreover, niche complementarity, facilitation and interference (i.e. intraguild predation), can occur in carabid communities and hinder the power of our prediction pest pest-control. Studying the foraging behaviour of obligatory and opportunistic omnivorous individuals and their sensitivity to biotic factors such as competition and predation would increase our understanding of predator interference, facilitation and niche complementarity in carabids communities and, hence, the predictability of predation. In this study we compared the foraging behaviour and weed consumption pattern of two common carabid species, one known as obligatory omnivore (Poecilus cupreus) and the other as a “granivore” (Harpalus affinis), for four species of seeds of weeds. Consumption was scored for 13h using no choice tests. The effect of biotic interactions, was also assessed by looking at the foraging behaviour of the two carabid species under 4 different experimental conditions: i) predation risk; ii-iii) intra- and interspecific competition; and, iv) control. Our results show that the consumption pattern of the two carabid species differs markedly. P. cupreus eat faster but express strong feeding preferences while H. affinis feed slowly but eat all species of seeds equally. Moreover, while the consumption pattern of P. cupreus remains stable between the experimental conditions, H. affinis change their behavior, eating more and more rapidly when faced with a risk of predation.