A floral ‘map’ to nectar discourages bumblebee robbers

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In the business of survival, the bright colors of blooming flowers mark a serious transaction. Their nectar, color and fragrances are all designed to attract pollinators to come hither and transfer pollen to help plants reproduce but occasionally, these plans go awry. Some bees choose to avoid the pollen and tunnel into flowers to steal nectar instead. A study published in PLOS ONE last week explains how plants deter these robbers by providing them a map to reach nectar more quickly. Author Anne Leonard explains their results:

How did you become interested in studying floral guide patterns? 

I think many people are intrigued by the fact that bees see the patterns on flowers differently than we do. I was studying color learning in bumble bees, and as I looked through the literature I realized there were still many unanswered questions about how these patterns affect bees’ behavior. Living in Tucson, I started to photograph the dazzling variety of nectar guides on Sonoran desert wildflowers, slowing down many a hike in the process. Between all the reading and photography, I clearly had nectar guides on the brain.

Why do flowers have nectar guide patterns?

The patterns of nectar guides appear to be very attractive to many bee species. Bright colors, high color contrasts and star-like outlines could simply help a plant increase visits from pollinators. It’s even been suggested that these visual features might have evolved to mimic rewards, for example bright yellows and oranges might resemble protein-rich pollen to the insects. Secondly, plants that produce distinctive and memorable patterns might also benefit because they provide an identifying feature for pollinators to learn, remember, and return to.

Third, a nectar guide may reduce the overall time the bee spends on the flower. If bees are sensitive to the time costs associated with visiting different flowers, then they should prefer to visit flowers they can handle quickly. Finally, our research suggests a novel benefit: the pattern can reduce a bee’s tendency to rob nectar. In this case, the pattern benefits the plant by incentivizing the bee to access nectar “legitimately,” in a way that is most likely to transfer pollen.

Can you explain what nectar robbing refers to and what a ‘legitimate’ way of getting nectar looks like?

If you take a moment and imagine a bee visiting a trumpet-shaped flower like a morning glory, what you’re picturing is most likely what we call a “legitimate” visit. The bee lands on a  petal, and walks forward to probe down to the nectar located in the tube-like part of the flower. In the process, she is likely to pick up pollen or transfer pollen from her body to the flower. This exchange of nectar for pollen transfer forms the basis of the relationship between plant and bee. We refer to this type of nectar for pollen transfer via the floral opening as a “legitimate” visit, from the plant’s perspective.

In  a second type of visit,  the bee lands on the flower but instead of going the legitimate route, it  bites a hole in the side of the flower to access nectar, without necessarily depositing pollen or picking up new pollen. Because the plant has lost nectar to the bee without gaining pollen transfer, this type of visit is termed ‘nectar robbing’.

Why do bees indulge in ‘nectar robbing’? Is this behavior seen with other insects and birds?

Although observations of bees nectar robbing date back to at least the 18th century writings of Sprengel, we are still studying why bees do it. Some species, like carpenter bees, have a reputation as frequent robbers. Others like honey bees and the bumble bee species I study, Bombus impatiens, are better known as opportunistic nectar robbers. They’ll rob some plant species but not others; propensity to rob seems to also vary somewhat across individuals. Some studies show that bees may be more likely to rob if a previous visitor has already created the access hole. Likewise, our research suggests that if the flower doesn’t have a nectar guide pattern to direct the bee to the floral opening, they are more likely to stray and encounter an access hole left by a previous robber.

In your paper, you found that when a flower had a guide pattern, bees were less likely to rob nectar. How did you test the bees’ behavior?

We use an array of specially designed artificial flowers that my co-author Josh Brent spent many hours trouble-shooting. These flowers had nectar available in two different ways. The bee could either land on the top of the flower and access nectar “legitimately” from a small central well, or she could land on the underside of the flower, and “rob” nectar from a small well located on the side of the floral tube.

We kept bee colonies in the lab so they were naïve with respect to experience with real flowers. We let bees into the arena one at a time, and recorded their visits to the flowers on the array. Half the bees were given blue flowers with yellow star-shaped guides, and the other half saw only plain blue flowers with no patterns. We noted whether the bee robbed or visited each flower legitimately, and we were also able to measure how quickly she located the nectar after landing in each case.

We found that bees robbed less frequently when the flowers had nectar guides and also landed more quickly on flowers with guides than those without them. This suggests the bees indeed found the nectar guide more attractive to land upon than the plain flower top, and that the  guide helped them find nectar faster.

Does this discovery have applications for bee-keepers or horticulturists?

We’d need a few more of the pieces of the puzzle before claiming that our research on floral patterns might yield better honey or healthier honeybees, but our research suggests that the stripes and dots that provide color patterns pleasing to the human eye can also affect the way the bee interacts with the flower.

Typically, varieties of nursery plants are bred for human aesthetics. Given a choice, planting a variety with a dramatic nectar guide pattern might allow an observant gardener the satisfaction of seeing more pollen transferred by bees. On the other hand, those gardeners eager to see nectar robbing in action might select a relatively plain variety. The committed backyard scientist might be inspired to plant varieties with different types of patterns, sit back, and watch what happens. Of course, flowers of different plants can also differ in many other aspects that might affect a bee’s propensity to rob nectar (such as floral scent or nectar chemistry) and keep in mind that some may have UV patterns that the human eye can’t see.

Citation and images: Leonard AS, Brent J, Papaj DR, Dornhaus A (2013) Floral Nectar Guide Patterns Discourage Nectar Robbing by Bumble Bees. PLoS ONE 8(2): e55914. doi:10.1371/journal.pone.0055914

 

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