Monday, August 16, 2004

Yet more proof that insects are cool

(Or: A look at bee pheromones)

Let's say you and your family are living in a nice home that you've built from the ground up with your own sweat and tears, and then all of a sudden some punk kids come around the corner and start pelting your home and family members with rocks. What would you do? You'd grab all your buddies and rush those punk kids to scare 'em off.

Well, that's exactly what some bees did recently in Santa Ana after a few kids started throwing rocks at the hive the bees had built in an apartment complex wall. What the kids didn't expect, however, is that the hive contained an estimated 120,000 bees, 40,000 of whom were found inside the hive while the other 80,000 were out foraging (or defending the hive) at the time and were only caught later (LA Daily News, LA Times, CNN). The hive weighed more than 500 pounds. When firefighters arrived on the scene they tried to hold the bees off with water but were unsuccessful (seven firefighters were stung, according to the LA Times), and everyone had to wait for professional beekeepers to arrive before the bees were finally "controlled".

Bees are an amazing example of a social insect, and there's a lot of neat biology contained in this little event. Probably the most interesting, though, is how 120,000 bees were able to communicate with each other and coordinate a multi-hour defense of their hive without cell phones, radios, GPS systems, or verbal or print-based communication of any kind.

Bees communicate primarily through pheromones, which are organic molecules released by one individual that affect the behavior or physiology of another individual. Moths are a good example of an animal that uses pheromones: male Bombyx mori moths (silkworm moths) respond behaviorally to one molecule of female Bombyx mori pheromone (bombykol) in 10^17 molecules of air. The males, of course, are trying to find females so they can mate, and pheromones help a lot when individual moths may be miles apart.

Bees use pheromones to coordinate virtually all of their hive activities, including mating, kin recognition, defense, orientation (to food, nests, and swarms), and many colony-level functions (maintenance of nectar stores, inhibition of worker bee ovaries, inhibition of queen rearing). More than 18 individual pheromones have been identified, and it's hypothesized that more than 36 exist (note: this information comes from a 1987 bee book, so is probably out of date).

The firefighters and other people around this particular bee colony should have been most concerned with the alarm pheromones of the bees. Bees produce multiple alarm pheromones, one of which is isoamyl acetate (the first to be discovered; others include 2-nonanol, (Z)-11-eicosen-1-ol, and 2-heptanone). Isoamyl acetate can be released by bees voluntarily, but is also released involuntarily when a worker bee stings.

When bees sense alarm pheromone in the air around them, they immediately begin showing defensive behaviors. If alarm pheromone is put onto a marble that is rolled into a hive entrance, the bees become more agitated, assume aggressive postures (the bee equivalent of "grrrr"), and once they determine that the alarm pheromone is coming from the marble, they'll charge the marble and group around it, evaluating the threat that it poses. If the marble stays put they generally won't try to sting, but if the marble moves the bees will begin stinging.

Alarm pheromones also act as an orientation pheromone, meaning that bees can follow the gradients of alarm pheromones in the air to determine the point source of the pheromone. Thus, as soon as one bee has stung an object, that object is tagged as a threat and all surrounding bees can use the alarm pheromone released by the bee's stinger to find the location of the threatening object.

So we now can understand how the bees were defending their hive. Betsy and Barbara Bee, along with some of their sisters, were stationed as guards at the entrance to the hive. When the kids rounded the corner, Betsy and Barbara had just gotten back from sipping some honey during their break, and were focusing on releasing Nasonov pheromones (involved in orientation) to ensure that all the foraging worker bees would be able to find their way home. As soon as the kids started to hurl rocks at the hive, however, Betsy and Barbara both started voluntarily releasing alarm pheromone. This alarm pheromone alerted the bees in the hive that there was some danger at the entrance to the hive, and thus they began to congregate around the hive entrance. Betsy, Barbara, and their sisters then began to search for the source of the danger, focusing on anything that moved near the hive. Since they were already pumped up on alarm pheromone, once they found something that was moving (e.g. a human flailing wildly at them) they stung it, which released more alarm pheromone, which then caused more bees to become aggressive and fly towards the person they had just stung. Since the pheromones persist in the atmosphere even after the bees have stung, newly arriving bees immediately knew that there was a threat, and even knew who specifically their sisters had determined was posing the largest threat.

Betsy and Barbara, along with 119,998 of their sisters, died valiantly defending their home.

Note: Some reports have stated that there were three hives, not one, in the wall. I've written this as though there were one hive to simplify things.

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