26 September, 2022

Consequences of pollinator availability and effectiveness for pollen transfer in a gynodioecious seed crop system

 by R. Waytes, R. Cartar, S. Hoover

A leafcutter bee on canola

Pollen limitation can constrain the number of seeds that a plant sets, and therefore its reproductive success. For plants that depend on animal pollinators, the availability of pollen is determined in part by the availability of pollinators and their ability to effectively deposit pollen (i.e., their efficacy).

We studied pollinator visitation and efficacy in hybrid seed canola (Brassica napus) production fields in Alberta, Canada. These fields consisted of 1 m wide bays (lines) of ‘male’ flowers (hermaphrodites which act as pollen-donors) alternating with 6 m wide bays (lines) of ‘female’ flowers (hermaphrodites with induced male sterility). Honey bees (Apis mellifera) and alfalfa leafcutting bees (‘leafcutter bees’; Megachile rotundata), both introduced European species, are typically placed in the crop for pollination. Native pollinators, including bees and flies, may also contribute. We used a GoPro® camera targeted at an inflorescence of female flowers placed at 1 m distance from the observer to offer flowers to insects visiting the canola and record their responses. We examined how insects responded to female flowers and what behaviours affected pollen deposition. Additionally, we measured pollinator visitation to both male and female bays.

Most flower visitation in this system was by managed pollinators (honey bees and leafcutter bees), who were placed and maintained in the canola fields. Flies and native bees were present, but at much lower numbers. Honey bees and leafcutter bees tended to individually specialize on male or female flowers, and their unwillingness to move between bays represents a potential barrier to pollen transfer.

Female leafcutter bees deposited more pollen than honey bees or flies, although male leafcutter bees and honey bees deposited similar amounts. Bumble bees were similar to female leafcutter bees in pollen deposition, but their low presence in the canola fields implies low contributions to overall pollination. Pollen deposition increased with the amount of time a pollinator spent on a flower.

So what mattered in pollen deposition? Far and away, it was the sex of flower from which the pollinator moved: moving from a male flower transferred more pollen. Also important were pollinator identity (female leafcutter bees were the best) and time on flower (more time meant more pollen deposited). Pollen receipt is a confluence of three factors: from where, who, and how long.

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