Laura Riggi

and 2 more

Identifying and quantifying crop stressors interactions in agroecosystems is necessary to guide sustainable crop management strategies. Over the last 50 years, faba bean cropping area has been declining, partly due to yield instabilities associated to uneven insect pollination and herbivory. Yet, interactions between pollinators and a key pest, Bruchus rufimanus (florivorous and seed predating herbivore), on faba bean yield have not been investigated. Using a factorial cage experiment in the field we investigated how interactions between two potential stressors, lack of pollination from Bombus terrestris and herbivory by B. rufimanus, affect faba bean yield. Lack of insect pollination reduced bean weight per plant by 15%. Effects of B. rufimanus herbivory differed between the individual plant and the plant-stand scale (i.e. when averaging individual plant scale responses), likely due to high variation in the level of herbivory among individual plants. At the individual plant scale, B. rufimanus herbivory increased yield but only in the absence of pollinators, possibly due to plant over-compensation and/or pollination by B. rufimanus. At the plant-stand scale, we found no effect of B. rufimanus on yield. However, there was a tendency for heavier individual bean weight with insect pollination, but only when B. rufimanus herbivory was absent, possibly due to a negative effect of B. rufimanus on the proportion of legitimate flower visits by B. terrestris. This is the first experimental evidence of interactive effects of B. terrestris and B. rufimanus on faba bean yield. Our preliminary findings of negative and indirect associations between B. rufimanus and individual bean weight call for a better acknowledgment of these interactions in the field in order to understand drivers of crop yield variability in faba bean. This study showed that herbivory can increase yield, but this effect is only detectable when investigated in combination with lack of pollination.