Much ado is being made of the fact that Africanized honey bees have recently been found in the San Francisco Bay Area. Most of the articles I've read on the subject have disseminated information that is good, but can be confusing to the average person who isn't a beekeeper. Most people who don't understand bees fear them, and only want to know: (1) Should I be worried? and (2) How much should I worry?
First, some background: The European honey bee (Apis mellifera) was introduced to North America with the first European settlers on the continent. It is a docile bee, easy to work with, and generally a good honey producer. Several strains, or subspecies, of A. mellifera have been bred over the years, resulting in stocks that beekeepers refer to as Italians (A. mellifera ligustica), Carniolans (A. mellifera carnica), Russians, and others. Beekeepers choose strains of bees that suit their preferences, in terms of temperament, honey production, speed of colony build-up, and disease or parasite resistance.
We began our beekeeping adventures with two packages of Italian bees, which proved to be very sweet and extremely productive. Most of our mentors told us not to expect to harvest any honey our first season, as the bees would be busy growing the colony and finding enough food to feed themselves over the winter, and yet we harvested over 100 pounds of surplus (i.e., beyond what the bees needed to overwinter) honey. Since then we've not had to buy packages again and have acquired colonies by either catching swarms (fun!) and splitting our existing hives.
Occasionally a beekeeper has to re-queen a hive, to replace one that has gone missing or is failing to lay well. Sometimes the bees take matters into their own hands(?) and rectify a situation that they feel is lacking; they will build a new queen from one of their sister larvae, who will supplant their collective mother and take over the egg-laying duties. We have re-queened hives that are bitchy, the ones in which the bees fly up at us the moment we crack the hive open and bang into our veils. I don't like to work with pissy bees, and while I know I shouldn't be afraid of our bees, I've had a bad enough sting reaction to warrant allergy tests that determined I have a mild-moderate allergy to honey bee venom. So I'd much rather work with sweet bees, like Italians or Russians that just look up at us from between the frames or keep going about their business as we tear apart their home.
How and why does a colony of mild-mannered, easy-to-work-with bees become a nightmare to deal with? What happens probably goes something like this. Worker bees may decide, over the course of a season, to supersede their mother and re-queen their colony. The new queen, who is the sister of the workers, flies out and mates with a dozen or so drones from other colonies, then returns to her natal hive to begin laying. If she mated with drones who carry Africanized alleles, then some of her offspring will possess those alleles. A beekeeper with a hive that has become more defensive can change its overall temperament by introducing a new queen that comes from a lineage known for its gentleness.
More about the Africanized bees: Africanized honey bees are the result of inadvertent hybridization between strains of the European honey bee, including the Italian A. mellifera ligustica, and the African bee, A. mellifera scutellata. The African bee was imported to a lab in Brazil in the mid-20th century, when beekeepers were attempting to increase honey production. It escaped from quarantine in 1957 and began hybridizing with the European honey bees that had been established in the New World for centuries. It has been expanding its range northward since; the first reports of Africanized bees in southern U.S. states were in the early 1990s. They have been in southern California since 1994.
An Africanized honey bee has a sting that is no different from that of a European honey bee, and she will still die when she stings someone. The difficulty, as far as humans and livestock are concerned, is that Africanized bees are much more defensive of their colonies and are generally easier to piss off. When they perceive a threat they usually emerge from the hive in great numbers and attack the intruder. They have also been known to chase people long distances and keep attacking. People who accidentally upset a colony of Africanized bees tend to get stung dozens or hundreds of times, and the accumulation of that much venom can be fatal.
The only way to know for certain that a bee is Africanized is to examine her genome for African alleles. We know now that Africanized honey bees are in the Bay Area. Whether or not they become permanently established remains to be seen, but if there's one Africanized colony surely there must be others.
So, should you be worried? In my opinion, there are lots of things that are more worrying than Africanized honey bees. Then again, where I live they haven't been around very long and I've never encountered a purely Africanized colony. If we have bees in our hives that are pissy we re-queen the colony, so it's unlikely that any of our colonies will be taken over by Africanized bees. However, this is a biological system we're talking about, so nothing is guaranteed.
Here in northern California, most beekeepers aren't too worried about Africanized bees because we think they won't survive our winters. That said, we're heading into a pretty strong El Niño event and may not have a cold winter this year, although we all hope it's a rainy one (and a snowy one in the Sierra Nevada). Plus, with climate change and a generally warming planet, conditions that favor survival of the Africanized bee may soon prevail in much of the U.S. Residents of the southern U.S. should probably take care not to disturb a colony of feral bees because they may be Africanized. Call a beekeeper (not an exterminator!) and let a professional deal with it. Bees (both European and Africanized) that are foraging, though, tend to be focused on the job at hand and won't bother you unless you bother them first. Just leave them alone and watch from a safe distance.