Close Encounters

Do flowers removed of nectar and pollen attract fewer bumblebee pollinators?

Do pollen thieves and nectar robbers reduce flower attractiveness to bumblebee pollinators in Impatiens oxyanthera?

Pollen and nectar are the primary rewards offered by flowers to insect pollinators. The roles of pollen and nectar in attracting potential pollinators has been recognised for hundreds of years. Bees are particularly special flower visitors because, almost uniquely, they use both nectar and pollen as food and completely rely on them for both adult and larval nutrition. Adult bees consume nectar and usually some pollen as well, whereas larvae consume large quantities of both pollen and nectar.

In some plant species, flowers offering pollen and nectar rewards attract not only pollinators, but other types of visitors, including pollen thieves and nectar robbers. While pollinators forage and provide a pollination service, pollen thieves and nectar robbers acquire the tasty reward without pollinating the plants they visit. Previous studies have indicated that this may have negative effects on plant fecundity by reducing pollinator attraction. However, experimental evidence for the effects of absence of either pollen or nectar in a given flower on pollinator attraction remains scarce.

A flower of Impatiens oxyanthera with a curved nectar spur. The long corolla tube matches with the bodies of bumblebee pollinators. Honey bees have smaller bodies and act as pollen thieves “stealing” pollen in the flowers, without pollination. Image credit: D. Li.

In their new study published in AoBP, Li et al. investigate whether flowers removed of either nectar or pollen would attract fewer pollinators in the protandrous species Impatiens oxyanthera. They conducted floral reward manipulation experiments to explore how the removal of either nectar or pollen from flowers influences pollinator behaviour by comparing their visitation rates and visit duration.

In their study they found that, in general, the flowers removed of pollen attracted significantly more bumblebee pollinators, but the flowers removed of nectar or those removed of both pollen and nectar attracted significantly fewer bumblebee pollinators. The visit duration of bumblebee pollinators to control flowers or flowers removed of pollen was also longer than that to flowers removed of nectar or those removed of both pollen and nectar. This is likely because during the later female phase of flowering, where pollen isn’t present, the flowers of I. oxyanthera contain higher amounts of nectar than the earlier male phase flowers. Li et al. conclude that nectar seems to be the main reward, and bumblebee pollinators mainly used the absence of pollen as a visual signal to locate flowers with a potentially higher amount of nectar.

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