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

Lila Westreich

Pollinator Ecology

University of Washington

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.

The head of a plasterer bee, a short-tongued bee

A plasterer bee, a short-tongued variety

Diego Delso / CC BY-SA 

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.

Comment Peer Commentary

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This is a very interesting article about bees circumventing the  mutualism of “legitimate pollination”. I really like that it addresses different aspects of bee behaviour, especially food stress and neuroplasticity. I have always found it interesting to see how bees can have complex foraging bouts, where they switch between robbing and accessing nectar legitimately.  Your article is incredibly engaging!

I have some comments, but they are more about the Oikos 2020 article you reference.

First, you mention that the study found that the seed set for robbed  flowers is lower than that for unrobbed flowers, but this doesn’t seem to be the case. I might be mistaken, but I think that the researchers  found that self-pollination reduces seed-set. Robbed and unrobbed flowers on the other hand, appear to have the same seed-set. I suppose that this would mean that even robbed flowers receive some degree of  legitimate pollination, leading to similar seed-set rates as unrobbed  flowers.

The study also finds that the species richness of pollinators reduces for robbed flowers. However, this decline appears to be driven by bees (small and large) rather than the other visitors. This isn’t surprising to me, as the flowers are labiate, and seem to be adapted to bees. Even though the researchers consider butterflies and moths as a part of the pollinator community, I wonder if they aren’t nectar thieves instead.

Nectar thieves are flower visitors that don’t “fit” the morphology of the flower. Owing to this morphological mismatch, they can access nectar in the same way that a legitimate pollinator would, but without touching the anthers or stigma. In this example, a butterfly with a long proboscis could access nectar just as well as a long-tongued bee could  without touching any of the reproductive organs of the flower.

Studying this is difficult, needing a lot of behavioural  observations, morphological measurements and perhaps exclusion experiments. But I wonder if considering the effect of nectar thieves as a separate category of flower visitors would lead to different insights. Surely, as you said in your article, it is worth a second look!

Lila Westreich responds:

These are awesome questions, thanks for taking a deep dive. I see what you’re saying - it’s not explicitly the seed set is lower, and I see that the seed sets do appear to be similar. I think it would make sense, physiologically, that the robbed flowers still receive somepollination - potentially from insects other than bees that don’t mimic  the robbery by using the same hole at the base of the flower. But that’s definitely an unknown - maybe I was too strong with that statement, based on the evidence!

Yes - species richness was lower for robbed flowers, and I really have no idea about whether moths/butterflies are nectar robbers! Since it’s not my exact area of study, I can’t be sure of anything. But it’s a great question. And yes, a butterfly with a long proboscis could access  without touching reproductive organs, but the robbery process seems unique in bees, who COULD enter the flower but choose to bite a hole at the base (especially in the case of secondary robbery). Studying this is  definitely really, really difficult. I think you brought up some fantastic points.

Sam Zlotnik

Ecology & Evolutionary Biology

University of Florida

Nectar robbing is a super fascinating bee behavior, and as you point out, it is also important for us to understand in relation to the ongoing biodiversity declines of both plants and insects.

I was curious about how different types of insect pollinators respond to plants that have had their nectar ‘robbed’ in this way. Are all insect pollinators equally adept at identifying and avoiding the flowers that have been robbed of their nectar? And if pollinator species vary in how well they detect prior nectar robbing, could that put certain species at a competitive disadvantage in terms of finding food?

I was also wondering whether some plants have specialized adaptations for avoiding nectar robbing. And on the flip-side, are some plants especially susceptible to nectar robbing? I was wondering how plant population dynamics might change in different ways depending on their unique relationships with different groups of pollinators, especially considering that many insect pollinators populations are declining due to climate change, pollution, and other human activities.

Lastly, do you know if there have been studies specifically  investigating whether nectar-robbing behavior has increased over time in bee populations that have been experiencing higher stress in recent years? You alluded to this possibility in your article, but I wasn’t sure if there was definitive evidence for this potential change in bee behavior over time. It would be very interesting if we could link increased nectar robbing to specific environmental stresses, such as pesticide pollution.

Lila Westreich responds:

Fantastic questions! Most of them are outside my specialty, and Sailee actually brought up similar ideas in terms of your first questions. I don’t know if other insect pollinators are adept at identifying or avoiding robbed flowers, but that could throw some really interesting variables into the mix of a study like this. I think you’d  need much longer sampling time, early morning and evenings, to capture as many species as possible in your observations. It could definitely give advantage or disadvantage to different species, based on their ability to detect robbed flowers. Then again, nectar regenerates, so it may not be a big deal - if the flower has been nectar robbed, it will still develop new nectar, it will just be easier to access for robbers through the hole at the base of the flower. So maybe this is a moot point, after all!

I don’t know of any plants that have specialized adaptations for avoiding nectar robbing. Flowers with shallow corollas are always going to have an advantage, because it’s equal work for a bee with a short proboscis to enter a short flower than it is for them to bite a hole and rob the flower. I think shallow corolla flowers have the ‘advantage,’ if you call it that. I can see bees visiting these plants more, especially those with a short proboscis, thereby supporting those populations and their next generation in place of deep corolla flowers.

I haven’t seen studies looking at nectar-robbing behavior over time, but I wouldn’t be surprised to see more of that in the future. Pesticides have been linked to lowered cognitive ability, so I would assume that leads to some funny behavior. Whether nectar-robbing is one of those behaviors, I’m not sure!