Review of methods to investigate pollinator dependency in oilseed rape (Brassica napus)

Abstract

Insects contribute to the pollination of about 75% of crop species. Among such insect-pollinated crops, oilseed rape (Brassica napus) is increasingly cultivated worldwide. However, the degree to which seed set in B. napus depends on insect pollination is still under debate. To compare research approaches for estimating pollinator dependency, we reviewed the relevant literature published between 1956 and 2018. The majority of the studies were conducted in Europe (75%) on winter cultivars (77%) and monitored honeybees (52%). Our main findings are: (1) Dependency on insect pollination differs among cultivars and regions. (2) Field observations provide more realistic and comparable results than observations on caged plants. (3) Field conditions, soil nutrient availability, and pest management practices all influence insect dependency and subsequent yield. (4) Plot size conditions pollinator behaviour, as pollination success can be related to insect density or diversity depending on the size of the plot. (5) Insect numbers vary according to the observation distances within a field. (6) Comparison between studies is complicated by the use of different proxies to assess final yield, which may not be equivalent. We provide several recommendations to improve the reliability of future studies of oilseed rape pollination and of insect pollination dependency such as choosing open field observations compared to bag, cage or greenhouse studies, registering field conditions (pests, pesticides, fertilisation level) and defining yield as total seeds per plant to take into consideration compensation mechanisms.