Abstract
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.