Visiting a flower is not the same as pollinating it

What has to happen

Pollination is the transfer of pollen from anther to stigma, and for most plants it must be from a different individual of the same species. A visitor pollinates only if it picks pollen up, carries it, and deposits it in the right place on the next flower — and each of those steps can fail while the visit still looks purposeful.

1. 1Nectar foraging without contactA small bee working a large flower can reach the nectar without touching the anthers at all. The visit is real and the pollination is zero.
2. 2Nectar robbingSome bees bite a hole at the base of a corolla and drink through it, avoiding the reproductive parts entirely. Short-tongued bumblebees do this on long tubular flowers, and honey bees readily use the holes once made — secondary robbing, which spreads the effect.
3. 3Pollen theftA bee can groom pollen efficiently into its scopa and deposit almost none, particularly where the pollen is packed tightly and the bee is a careful groomer.
4. 4Effective pollinationContact with anthers, imperfect grooming, and contact with the stigma of the next flower. Slightly untidy bees are often better pollinators than fastidious ones.

Why the honey bee is a mixed case

Honey bees are highly abundant, extremely flower-constant within a trip, and forage over kilometres — all of which makes them valuable pollinators of many crops. They are also efficient groomers, unable to buzz-pollinate, short-tongued, and quick to use robbing holes. For some plants they are the best pollinator available and for others they are close to a bystander.

How this should change what you read

• 'Attracts bees' on a label describes the visitor, not the service, and often not even a real observation.
• A busy plant is not necessarily a well-pollinated plant, and a well-pollinated plant is not necessarily busy.
• Studies that count visits measure something different from studies that measure pollen deposition or seed set. The second is what matters and the first is what is usually available.
• Adding hives to increase visit counts can leave the pollination unchanged while adding competition for wild bees.