Bee visitation, pollination service, and crop yield in commodity and hybrid seed canola

Insect-mediated pollination of crops is an important service to agriculture, as increased insect visitation can increase fruit production by increasing pollen deposition. Unfortunately, pollination is often treated as a “black box”, and pollination management suffers from key knowledge gaps that hinder its greater utility, particularly the specific mechanisms underlying the processes of visitation, pollination, and fruit production. We present a causal model that links insect visitation to pollination to three separate components of yield, using field data from two types of canola (Brassica napus) production systems. Our results demonstrate that yield in commodity canola fields is primarily determined by plant size, and we found no relationship between honey bee (Apis mellifera) visitation and pollen deposition, or pollen deposition and seed yield. In contrast, yield in hybrid seed canola fields was similarly controlled by plant size, but there was also a strong relationship between alfalfa leafcutting bee (Megachile rotundata) visitation and pollen deposition, as well as pollen deposition and seed yield, meaning that leafcutting bee visitation in increased pollen deposition in seed canola fields, while honey bee visitation did not. This model serves as a step towards a dynamic model of pollination services, and highlights the relative importance of bee pollination in canola production.