A less-frequently spotted Vespula in our area: the forest yellowjacket (Vespula acadica). (Tony Iwane via iNaturalist, CC-BY-NC)

Ask almost anyone about yellowjackets; they will have a harrowing tale to share. There’s a lot we humans don’t see, however, and once the insects leave their buzzing at late summer picnics behind, they return to their dark underground nests—a hidden, complex world of their own.

There, they tend to a thriving population of at least 4,000 to 5,000 hive-mates, including a busy queen and thousands of squirming larvae that require continual feeding and tending. These insects are tireless matriarchal society-builders, whose efforts come with an array of ecosystem benefits that most people don’t consider: they spread native seeds, hunt pest insects, and feed some of our bolder predators. 

Every yellowjacket in the colony plays a particular role within its social structure, much like honeybees or ants. The queen gives birth to workers, and the workers build an insect metropolis that crumbles down to just a few survivors, all within six months. What we see in the late summer and early fall, when yellowjackets are at their most aggressive, is the collapse of those underground societies. That roast-beef sandwich at your Labor Day picnic is just too tempting to hangry insects struggling to eke out a living. 

A western yellowjacket at the entrance to a kingdom (or shall we say queendom?) near Sebastopol. (Darvin DeShazer via iNaturalist, CC-BY-NC)

In casual conversations about yellowjackets—of which I’ve had many lately—curious folks usually ask if they are native to our region. While there are hundreds of yellowjacket species worldwide, we have a few common species in the Bay Area. The notable nonnative is the invasive German yellowjacket (Vespula germanica). Then there are a handful of native Vespula—the western yellowjacket (Vespula pensylvanica) is the species you are most likely to see while out and about. The next most commonly spotted (at least on iNaturalist) is the American yellowjacket (Vespula alascensis). The native California yellowjacket (Vespula sulphurea) is present throughout our region, but is a little more shy around humans and therefore seen less frequently. Occasionally seen are the prairie yellowjacket (Vespula atropilosa) and forest yellowjacket (Vespula acadica) most commonly found in open fields and forests, respectively. At the end of this article, we’ve posted some identification tips for the bold.

Now that we have the lay of the land of yellowjackets, let us (cautiously, metaphorically) sidle up to a sleeping western queen, and see how her life unfolds …

Life cycle

The annual saga of the western yellowjacket begins in the early spring, as our single solitary pregnant queen emerges from a quiet and safe place—often a hole in the ground or under a flap of bark—to begin building her empire. She first finds an appropriate location—sometimes in a cavity of a human dwelling, but usually underground. (On walls or buildings in our area, such nests are probably not our two native species but the homes of of German yellowjackets, paper wasps, or a species of aerial yellowjackets that only feeds on other insects. ) Having chosen her real estate, she begins to build a nest. She births a few smaller female workers, which she conceived the previous year, to help her build her castle. These workers strip tree bark, chewing it down to manufacture a sort of concrete out of wood pulp and saliva. They use this stuff to make large, elaborate nests that are much like papier-mâché, with hexagonal cells inside for nurseries. These nests are notably strong and are even reinforced with trace amounts of minerals like titanium. If you’ve ever been blessed (or cursed) enough to see these shimmery, spherical wafery paper masses, then you know how large, ornate, and beautiful they can be—they could hold their own in SFMOMA. 

A western yellowjacket. Apparently they chew wood. (masteralgae via iNaturalist, CC0)

Throughout the spring and early summer, the worker population grows as the queen settles into her main role: egg-laying machine. Her offspring quickly hatch into squirmy larvae that are steadfastly tended by the female workers, which deliver them to the comfortable hexagonal cribs within the paper kingdom. The growing babies need protein, which the workers collect and serve in the form of little meatballs. The queen is hungry, too, since she’s too busy building the next generation to hunt for sugar, so the workers feed her. The matriarchal lineage continues to build until late summer, when the next generation of queens and males are born and reared. Birthing males requires just the right conditions, and they come from unfertilized eggs. That’s right—all males are born through parthenogenesis, and are direct genetic copies of the queen. The difference between males and females is the number of chromosomes: males have just one copy (“haploid”) while females have two (“diploid”). 

By early fall, when temperatures begin dropping at night, the society begins its collapse. The future queens and males leave the nest to mate with individuals from other nests. Males die soon after mating: job done. The next generation of queens then prepare for winter hibernation, usually underground or somewhere protected, just like their queen-mother before them. The queen herself eventually dies, too. At this point, the workers are mostly in it for themselves and begin aggressively searching for sugar until cold nighttime temperatures kill off every last one. Only the future queens survive to the next year to begin the cycle all over again.

A secretive California yellowjacket (Vespula sulphurea) in the mud of Mount Diablo State Park, near Pine Creek. (Ameet Zaveri via iNaturalist, CC-BY-NC-SA)

The original protein craze

Even if you’ve never seen the hefty queen, which is usually about 25 percent larger than her workers and roughly the size of a large paperclip, I’m certain you’ve observed her smaller, steadfast workers, scanning the landscape for food to bring back to the nest. They are seeking two kinds of food. Larvae require protein—a bite of the aforementioned roast-beef sandwich, other insects, or perhaps some roadkill—in order to grow. The adults rely on more of a “rocket fuel,” as Lynn Kimsey, a UC Davis entomology professor, calls it: sugar. That explains the buzzing around your soda can. 

Yellowjackets: They’re after your ribs because they have babies to feed back at home. (Western yellowjacket; Lauren Robertori via iNaturalist, CC-BY-NC)

While the western yellowjacket colony continuously builds through spring and summer, the chaotic and hungry times of colony collapse are when you’re most likely to have an aggressive encounter. There isn’t enough food to go around as autumn rolls in—nectar food sources dry up just as the population of hungry insects peaks. Workers begin searching more aggressively for something to eat (I get it, yellowjackets, I feel the same way when I’m hungry at a hot late-summer picnic).  

Much like in human society, famine causes conflict and aggression in this insect world, and I can’t help but think the workers are just storming the Bastille of our barbecues while the queen munches on the cake they’ve collected. Until, that is, her demise. 

Defenders of the nest

Yellowjackets usually stick close to the nest, within about 1,000 feet, so when you see a lot of them at once, a nest is likely nearby. The true danger is in disrupting a nest, which can happen when gardening, landscaping, or just casually minding your business on a stroll, like my dog did last week.

If you uncover a nest, slowly walk away. Quick movements can cause them to make chase. Do your best not to swat or squish yellowjackets, as squished venom sacks located in the head can release a pheromone that signals their kin to attack.  A modified ovipositor in the workers’ abdomen makes for a very potent stinger that releases both a toxin and an enzyme (Phospholipase A1 for those of you keeping track) that causes a massive inflammatory response in humans and pets. And unlike honeybees, yellowjackets can keep on stinging. Yellowjacket stings should always be taken very seriously, as up to 8 percent of the population may have an anaphylactic response. Treat the sting with an ice pack and ibuprofen, and seek medical attention quickly if you have been stung more than 10 times. 

A German yellowjacket (Vespula germanica), photographed in Germany. They’re seen in the Bay Area, too. (Nico Blüthgen via iNaturalist, CC0)

Some non-human critters, however, can withstand the stingers due to very thick skin and fur, and conveniently, most of these sting-resistant animals love a yellowjacket larva snack. Bears, raccoons, and badgers all munch them. Skunks prefer them as a midnight snack, when workers are cold and slower to attack.

Their good works

While the stings are painful, these buzzing creatures provide myriad benefits for local ecosystems and human communities. According to Kimsey, one colony can hunt hundreds of thousands of pest insects that plague food crops. Anecdotally, I’ve seen (from a safe distance) a yellowjacket hovering around my tomato plants, popping back aphids like tiny green potato chips. 

Western yellowjackets (and actually all yellowjackets) are not the most effective pollinators, because they lack body hairs to catch pollen grains. Nonetheless, they have been known to pollinate some of our native orchids, according to Kimsey. And they have been documented as somewhat prolific seed spreaders for our native spicebush (Calycanthus occidentalis) and some trillium species. The fatty and protein-rich seed coating of these plants, called the elaiosome, is the perfect snack for growing yellowjacket larvae. The workers remove the nutritious snack from the seed, thus dispersing the plants. Colleagues of mine in the native plant growing trade refuse to collect spicebush seeds for this very reason—they’re usually swarming with yellowjackets. 

Climate change winners?

The good news for us is, or has been, that yellowjackets can’t fly under 60 degrees Fahrenheit, so in winter they make themselves scarce. The bad news is that the climate is warming, which means that we could start to see yellowjackets year-round. In fact, Southern Californians are already seeing perennial nests that don’t pause for the winter and instead continue growing into supercolonies—reaching numbers of up to 100,000 individuals. In Hawaii, our own native western yellowjackets have invaded in recent decades and created perennial pest colonies. 

Yellowjackets are hated by many, but it’s not their fault that our picnics and playgrounds are so alluring. “We’ve set up the perfect conditions for them,” says Kimsey. They are remarkable creatures, building intricate societies that grow and crumble in less than a year. What else in this world is smaller than a quarter but sends 225,000 people to the ER every year while simultaneously spreading native seed and keeping pests in check? They are terrifying but fascinating creatures that have found a place in our human-made landscapes, and they aren’t going anywhere soon. A toast to the yellowjackets—but check your glass, before you take a sip.

AT YOUR NEXT PICNIC

How to distinguish Bay Area yellowjackets

Many buzzing insects can easily be mistaken for yellowjackets to the untrained eye, so let’s set the record straight. We have, of course, honeybees. They are covered with fine hairs and will never ever go after meat, while yellowjackets are bald and will lurk around your sandwich. 

Paper wasps are slenderer and longer than any yellowjacket species. Then there are the graceful mud-daubers, aptly referred to as “thread-waisted.” They have a long, thread-thin midsection, which yellowjackets do not have.

Once you’ve got the relatives ruled out, it’s remarkably easy to differentiate between our main local yellowjacket species—if you’re feeling brave enough to get close. German yellowjackets are a honking bold school-bus yellow throughout, while native western yellowjackets are a softer, more muted yellow, like a lemon. Zoom in even closer, and you can tell by their faces: 

German yellowjacket: Has three black dots on the face (where a nose would be if it were a human face). 

American yellowjacket: Sports yellow eye rings and a thick, black band from the antenna to the top of the head.

Western yellowjacket: Has yellow rings circling its orb-like eyes, and some yellow extending up through the first half of its antennae (unlike the other two, for which the first part of the antenna is completely black). 

California yellowjacket: Look for a pinched waist, yellow around eyes, and a notably thick black band from the antenna to the top of the head.

Clockwise from top left: German yellowjacket; western yellowjacket queen; California yellowjacket queen; American yellowjacket. (Nico Blüthgen via iNaturalist, CC0; Brenda Dobbs via Flickr, CC BY-NC 2.0; Tony Iwane via iNaturalist, CC-BY-NC; Alan King via iNaturalist, CC-BY-NC)