Yellowjacket

The wasp that ruins a picnic in August is not being difficult for the sake of it. It has run out of sugar at home, and there is a reason for that.

A yellowjacket colony runs on a trade. Workers bring meat and insects to the larvae, and the larvae pay them back with a sweet secretion that the adult workers feed on. For most of the summer it balances.

Then late summer comes, insect prey gets scarce, the larvae produce less, and a few thousand workers suddenly need to find sugar somewhere else. Ripe fruit, soda cans, garbage, your plate.

Yellowjackets are the wasps of the genera Vespula and Dolichovespula, in the family Vespidae.

• Kingdom: Animalia
• Phylum: Arthropoda
• Class: Insecta
• Order: Hymenoptera
• Family: Vespidae
• Genera: Vespula and Dolichovespula

Because the name covers a number of species with very different circumstances, there is no single conservation status. Common ones include the German wasp, the common wasp and the southern yellowjacket.

One member of the group is routinely filed in the wrong place. The bald-faced hornet is black and white rather than yellow, which is why nobody calls it a yellowjacket, but it belongs with the yellowjackets and not with the true hornets at all.

This is the mistake worth clearing up, because it changes what you should do about one.

• A honey bee is covered in dense tan-brown hair. A yellowjacket is smooth and hard-looking.
• A honey bee has flattened hairy hind legs for carrying pollen. A yellowjacket has nothing of the kind.
• A yellowjacket has yellow or white markings on the face.
• A yellowjacket flies rapidly side to side just before it lands.
• Yellowjackets are only ever found in colonies.

Workers are about 0.47 inches (12 mm) long and queens about 0.75 inches (19 mm). Most species are black and yellow in alternating bands across the abdomen, though some are black and white and a few have a red background.

Despite lacking any pollen-carrying equipment, yellowjackets do still pollinate flowers while visiting them for nectar. They are just not built for the job the way a bee is.

Yellowjackets make paper. Workers chew wood fiber into a pulp and build the nest out of it, which is the same basic process a paper mill uses, several million years earlier.

The two genera build differently, and it matters if you are trying to avoid one. Vespula species conceal their nests, usually underground, in soil cavities, stumps or abandoned mouse burrows. Dolichovespula species build the exposed aerial nests that hang from a branch or an eave.

An eastern yellowjacket nest is fully enclosed except for one small entrance at the bottom, and a mature nest can reach roughly the size of a basketball. Most last a single season.

The whole colony is an annual project, and almost all of it dies.

Only fertilized queens survive the winter, tucked into some sheltered spot. In late spring a surviving queen builds a small nest alone and lays eggs in 30 to 50 brood cells, feeding the larvae herself for 18 to 20 days until they pupate into the first workers.

From then on the workers take over, raising the brood and enlarging the nest, and the colony grows fast. At its late-summer peak it can hold 4,000 to 5,000 workers and 10,000 to 15,000 cells. New males and queens are produced, the males die after mating, and the new queens go looking for somewhere to spend the winter. Everything else, including the old queen, dies.

There is one exception that puts the scale in perspective. In the southeastern United States the southern yellowjacket can keep a colony going year after year, and a perennial nest has been recorded holding 100,000 adult wasps.

Adult yellowjackets live on sugars and carbohydrates: fruit, flower nectar and tree sap.

The larvae need something else entirely. They need protein, so workers hunt insects and scavenge meat and fish, chew it up, and feed it to the grubs. In return the larvae secrete a sugary substance that the workers consume, so the nest runs as a two-way exchange: protein in, sugar out.

In late summer the supply of insect prey falls off. Fewer insects means less food for the larvae, which means less sugar for the workers, which means thousands of adults going out to look for sweetness wherever they can find it. That is why yellowjackets appear at outdoor tables in August and September and not in June, and it is a shortage inside the nest rather than any change of personality.

A honey bee stings once and loses its stinger. A yellowjacket does not.

Its stinger is lance-like, with only small barbs, so it pulls back out cleanly and can be used again, and a disturbed yellowjacket will usually sting repeatedly. Occasionally the stinger does lodge and tear free, but that is the exception rather than the rule. All females can sting.

The venom is a serious problem mainly for people who are allergic to it or who are stung many times over. Yellowjackets are also genuinely aggressive when the nest is disturbed, which is the main reason to find out where a nest is before mowing near it.

Here is the part that usually gets left out. Yellowjackets are significant predators of pest insects, and a colony of several thousand workers hunting all summer to feed its larvae removes an enormous number of caterpillars, flies and other insects from a garden or a field.

Many of the insects they take are agricultural pests, which makes them useful to farmers in a way almost nobody credits them for. They also clean up carrion, and they pollinate a little on the side.

So the wasp at the picnic is a predator the size of a paperclip that spent three months killing the insects eating your vegetables, and has arrived in August because the grubs back home stopped paying it. It is not an argument for letting one sting you. It is a reason to destroy a nest only when it is genuinely in the way, which is a different decision from destroying it on sight, the same calculation that applies to the spider in the corner of the room.