This week the Guardian posted a reader’s question to be answered by other readers:
“Why do bees not appear to compete aggressively over territory when gathering pollen – either among themselves or with, say, butterflies?”
It’s an interesting question. But answering it means starting with one important bit of context: There are over 20,000 species of bees in the world, and around 4,000 in North America. Asking why “bees” don’t seem to be aggressive with each other is painting with some pretty broad strokes; bee species differ tremendously in their behavior, life cycles and habitat use. So I reached out by email to bee experts in the United States, Canada and the United Kingdom for answers. Collectively, their responses focused on two main points.
Some bees are aggressive… sometimes
“The truth is some bee species are quite territorial about forage,” said Sheila Colla, assistant professor of environmental studies at York University.
Joe Wilson, associate professor of biology at Utah State University (Tooele) and co-author of the book “The Bees in Your Backyard”, concurred. Males can be particularly territorial, patrolling a flower patch and chasing away anything that comes too close — even larger animals like humming birds. “Males can’t sting but they sure act aggressively,” said Wilson. “Some even have spines at the end of their abdomen that are pretty threatening and I assume can do some damage.”
Colla, Wilson and others referred to the European wool carder bee (Anthidium manicatum) as a primary example. Males of this species will defend a patch of flowers and only allow females of their own species to forage on it; they then mate with the females while they forage. In their aggressive defense of the flowers, the males will dive bomb other species of bees and can even break their wings.
But the wool carder bee certainly isn’t the only species that shows pugilistic tendencies.
“There are a few bees that guard patches of flowers,” said Dave Goulson, professor of biology at the University of Sussex and founder of the Bumblebee Conservation Trust in the United Kingdom. “Stingless bees sometimes do this in the tropics, usually guarding flowering trees.” Trigona spinipes is one such tropical bee. In seeking to exclude a natural competitor (Melipona rufiventris) from resources, T. spinipes will employ “finely tuned hostility” ranging from “threat displays to prolonged grappling and decapitation”.
It’s also not uncommon for honey bee colonies to pick on each other when resources are scarce. They can start robbing pretty quickly when there is exposed honey or there is a weak colony nearby. According to one historical account from Virginia, this aggressive behavior can “cost thousands of the winged creatures their lives”.
“Males of large carpenter bees, Xylocopa virginica, are also quite territorial,” said Colla. “They will fly right up to a human and look them in the eye if they trespass!”
And spend just a little time observing the flowering plants around your home and you’re sure to see the occasional brief tussle that can happen between bees of the same or different species.
“I swear I saw aggressive interactions between bumble bees at a flowering bush at one of my prairie sites,” said Karin Gastreich, associate professor of biology at Avila University, who is in the beginning stages of a baseline survey of bees around Kansas City, Mo.
It’s not economical
However, the consensus of these experts is that it generally isn’t economical for bees to get into fights over food resources.
“By and large, as foraging adult females, the majority of wild bees have a very short amount of time in which they are able to collect food for their offspring and a bit for themselves,” said Scott MacIvor, assistant professor of applied conservation biology at the University of Toronto Scarborough. “Conflict with others is a waste of time!”
“A bee visits thousands of flowers a day, and it cannot hope to guard that many,” said Goulson.
“Generally, pollen is not a limited resource,” said Wilson. Flowering plants produce an abundance of pollen and they often bloom in batches, so there are typically plenty of flowers around at any given time. Also, bees have evolved to emerge at different times throughout the year, and they have diversified to make use of different microhabitats and different flowers, which can also help avoid competition. “There are floral generalists, and specialists, and those in between” said Wilson.
And the reason bees don’t tend to mix it up with butterflies? Butterflies are generally after a different resource: nectar. “Bees do collect nectar, but they’re mostly interested in pollen,” said Gastreich. Also, many flowers have evolved to attract different kinds of pollinators. “So what brings in butterflies generally doesn’t appeal to bees and vice versa,” she said.
And then consider this…
Scott McArt, assistant professor of pollinator health at Cornell University, agrees that it might be rare to see bees behaving aggressively with each other. But that doesn’t mean they’re not competing.
“Just watch bees visit flowers later in the day compared to early in the day,” said McArt. You’ll notice that these later bees briefly sniff and then skip over the majority of flowers they encounter. That’s because they can recognize when a flower has already been visited and no longer contains the resources they’re after. “That’s competition,” he said. “They’re competing with the bee that took the pollen/nectar earlier in the day!”
McArt said there’s also another form of competition that’s a bit more indirect and insidious: pathogen spillover. This occurs when one bee visits a flower and incidentally deposits pathogens of some kind on that flower while foraging; the next bee to come along picks up the pathogens and eventually becomes sick too. How common is spillover like this?
“We’re finding bee pathogens on about 20 percent of flowers on average!” said McArt. “So, potentially very common.”
Answering the question
The question that got this story started is based on two key ideas. The first is aggression, which Webster’s New World College Dictionary defines as: “forceful, attacking behavior, either constructively self-assertive and self-protective or destructively hostile to others or to oneself.”
Clearly, such behavior can and does occur between bees to varying degrees. But this brings us to the second key idea: the appearance of the aggression. The reason we don’t see such behavior very often is because it’s generally not economical in terms of energy spent versus rewards gained; resources tend to be relatively abundant, and bees have evolved into different niches. However, with an interaction like spreading pathogens, the behavior that’s “destructively hostile to others” is not easily apparent to the casual observer’s eye.