Nectar robbery by short-tongued bees is throwing off delicate pollination cycles
Nectar-robbing by bees leads to fewer visits by other pollinating insects, impacting flower reproductive success
Native bees are some of the world's greatest insect thieves.
Bees have evolved to become extremely successful pollinators, and generally have a mutually beneficial relationship with plants. They forage in search of nectar and pollen, drinking the nectar in flowers and collecting pollen to bring back to their offspring. Bees look for nectar because it provides energy in the form of simple sugars, which they use to fuel their flights. Pollen, on the other hand, is generally only consumed by young bee offspring, and contains necessary proteins and amino acids for growth and development. When bees collect pollen, they also accidentally transfer pollen between flowers — a process that allows many plants to reproduce.
Because these relationships are long established, some flowers have evolved to cater to certain bee species. For example, long-tongued bees are attracted to flowers with deep corollas, the term for all the petals that make up a flower. But though these plants may prefer long-tongued bees — those with a long proboscis that allows them to burrow deep into the flower, and pick up and deposit pollen along the way — short-tongued bees sometimes still visit these flowers anyway. These less-endowed bees use a process called "nectar-robbing."
Nectar-robbing is a behavior in which an insect lightly bites a small hole in the a flower's tissues at the base of the petal to access nectar, without performing the act of pollination. Once a hole has been made in the petal, other pollinators may take advantage of the easy access to nectar, bypassing the floral opening. Even bees with tongues long enough to reach the nectar in a very deep flower may perform secondary nectar-robbing, using a hole cut by a small-tongued bee — reducing pollinator visits inside the flowers for the plant.
This might not sound serious, but nectar-robbing can have a profound impact on a plant's ability to reproduce. In a study published in the scientific journal Oikos in February 2020, researchers counted how many robbed flowers there were in 35 sites in northeast India. Over the course of five years, the scientists studied the behavior of pollinators at these sites to try to better understand how robbing affected pollinating insect trips to flowers — and what impact that had on the plants.
The scientists observed three categories of flowers: those naturally robbed by pollinators, unrobbed flowers, and flowers cut with a small scissors, mimicking nectar-robbing cuts. To make sure they didn't miss any insect activity, the researchers put bags over the flowers until they were being observed, and removed the bags only during scheduled flower watches.
The study found that small-tongued bees were more likely to be nectar-robbers, likely because these bees had a harder time accessing nectar without robbery. With shorter tongues, these bees can't reach the center of deeper flowers, where the nectar-secreting organ called the nectary can be found. Due to the mismatch in flower depth and tongue length, these bees were more prone to nectar-robbing.
Once flowers were robbed, all bees — even those with long tongues — were less likely to visit. Robbed flowers were also visited by fewer types of insects; out of 77 species studied, 42 species visited unrobbed flowers, and only 26 species of insects visited robbed flowers. As a result, the researchers found nectar-robbed flowers produced fewer seeds.
Nectar-robbing is a common activity across all known bee species, and occurs in healthy populations. It can also be a learned behavior, passed from one species to another. But the outcomes of nectar-robbing can cause stress on bees and other pollinators. Nectar-robbing can deter pollinators from entering the corolla of a flower, reducing the number of flowers pollinated by a "legitimate pollinator" — the term for an insect that connects compatible pollen from a flower's anthers to its style, two important floral reproductive organs. Previous studies have also found this results in plants producing fewer fruit.
By reducing pollination, nectar-robbing can also cause rippling impacts, reducing available food sources. That's bad for bees: food stress in honey bees has been shown to cause bees' failure to identify flowers from new smells, a behavior that is essential to finding new food sources in their habitats. If young bees are forced to start foraging before they are fully developed—an activity caused by stress applied to the colony — their learning ability is also much lower than a healthy mature bee.
Generally, neuroplasticity and cognitive performance are lower in stressed-out bees. Like in humans, bee stress can come in many forms, whether that's pathogens or parasites, pesticides, or human degradation to the environment—including a lack of food sources. But bees are particularly vulnerable to stress damage, because their complex behavior requires a high level of brain function within such a small brain.
That said, nectar-robbing may benefit bees in stressed situations in the short-term, providing them access to nectar for quick energy for flight—even at the detriment of the plant's ability to pollinate and produce seeds.
This tension makes nectar-robbing a fascinating habit. While it may cause less fruit to be produced in many plants, it also represents a fascinating evolutionary behavior to circumvent natural systems. All in all, it's worth a second look.