Bee on Boron—Sufficient Boron Supply of <i>Brassica napus</i> Is Crucial for Attracting Pollinating Insects to Ensure Seed Yield

A sufficient supply of the micronutrient boron (B) is crucial for plant development and fertility. Boron limitations in crops, such as rapeseed (<i>Brassica napus</i>), cause deformed or infertile flowers and may consequently affect yield both directly and indirectly via reduced flower fertility or insect pollination, respectively. This latter point is particularly relevant for rapeseed, as its yield increases via insect pollination, which relies on healthy-looking flowers to attract pollinators. This study aimed to quantify the direct and indirect effects induced by B limitation on yield. Thus, we determined the extent to which B-deficiency in rapeseed affects plant-pollinator interactions by cultivating B-inefficient (<i>Daktari</i> and <i>CR3153</i>) and B-efficient (<i>CR2267</i>) cultivars at B-sufficient and B-deficient substrate levels in pot trials and exposing them to a natural pollinator community in a cafeteria experiment. All cultivars displayed phenotypic B-deficiency symptoms at B-deficient levels with 9% of the total flowers appearing deformed in B-efficient <i>CR2267</i> compared to 34% and 18% in <i>Daktari</i> and <i>CR3153</i>, respectively. Both B-inefficient cultivars were visited by a higher number of pollinating insects at B-sufficient levels (<i>Daktari</i> by 59%, <i>p</i> < 0.001; and <i>CR3153</i> by 37%, <i>p</i> = 0.002) than under B-deficiency. Compared to B-inefficient cultivars, pollinator visits to <i>CR2267</i> were less abundant and not affected by B-deficiency (<i>p</i> = 0.252). Across cultivars, pollinator visitation decreased with an increase in the number of deformed flowers (<i>p</i> < 0.001). Insect pollination increased seed yield compared exclusively to self-pollination only for the B-inefficient rapeseed cultivars by around 20% on either B-deficient or B-sufficient supply levels (<i>p</i> < 0.05), but not for <i>CR2267</i>, suggesting a cultivar-dependent effect of insect pollination on yield. Sufficient B supply is crucial for successful rapeseed flower growth and fertility as well as for attracting pollinating insects, thus securing yield in a pollinator-dependent crop.