Wasp Years

Canada has an enormous variety of wasps, and most have very little interest in stinging people. The ones responsible for most of our unpleasant encounters are social wasps, particularly yellowjackets, paper wasps and bald faced hornets.

“Social” means that they live cooperatively in colonies with queens and workers. It does not refer to their manners around potato salad.

Health Canada describes social wasps as the most common stinging problem in many Canadian cities. They can also sting repeatedly, unlike a honey bee worker, which normally leaves its barbed sting behind.

The enormous nest you see in August did not contain an enormous colony in May.

In our climate, the workers, males and old queen of most of these social wasp colonies die as winter approaches. Newly produced, mated queens leave the nest and spend winter sheltered elsewhere.

In spring, a surviving queen starts the process again. In yellowjackets and bald faced hornets that normally means a queen establishing a new nest. Paper wasp colonies can sometimes be started by more than one female, but the principle is the same: last summer’s workforce is gone.

Late in the season, new queens leave their natal nests, mate and find protected places to overwinter.

This may be the most important answer to why one year has far more wasps than another.

Researchers followed common wasp, Vespula vulgaris, populations for 39 years in England and 23 years in New Zealand. Spring weather was an important predictor of how many wasps appeared later in the year, probably because it affected queens while they were establishing colonies and producing their first workers.

Another 13 year study found that greater spring rainfall was associated with fewer common wasp nests.

So by the time you start complaining about wasps in August, much of the story may already have been written in May.

Starting a colony is the dangerous part.

At first, the queen has to build the nest, lay eggs, obtain food and care for the first larvae herself. Once those daughters become workers, they begin foraging and maintaining the nest while the queen concentrates increasingly on reproduction.

That changes the arithmetic dramatically. Each new group of workers helps support the production of still more workers.

A queen that fails early produces no late summer colony at all. One that successfully gets through the founding stage can eventually preside over hundreds of workers, and some yellowjacket colonies can become considerably larger.

That helps explain why fairly small differences in spring survival can become very obvious differences by August.

You might expect lots of wasps this year to mean lots of queens and therefore even more wasps next year.

Long term research says it is not that simple.

The 39 year common wasp study found strong density dependence. That means population growth tended to slow when wasp numbers were already high. The abundance of wasps in the previous year was actually one of the strongest predictors of population change the following year.

Researchers suspect that competition among queens during the vulnerable colony founding period may be part of the explanation.

Nor did the long term data show a dependable alternating boom and bust cycle. There are wasp years, but there is no simple rule saying a good year must be followed by a bad one.

A backyard can contain several little paper wasp nests while the wasps pestering you at dinner are yellowjackets from somewhere else.

Paper wasps make the familiar exposed comb of hexagonal cells, often tucked under eaves, decks and other sheltered places.

Yellowjackets frequently nest underground or inside cavities, although nest location depends on the species. Their nest may be completely out of sight.

Bald faced hornets make the large enclosed grey paper nests usually suspended from trees, shrubs or structures.

So counting visible nests is not necessarily a good way to count your neighbourhood wasps. Suitable nest sites can affect how many colonies become established. In one New Zealand study, disturbed pasture containing suitable underground cavities had an estimated 137 common wasp nests per hectare, while adjacent unmodified pasture had none.

All three make nests from plant fibres chewed into a papery material, but the architecture gives away the builder.

Paper wasps leave their comb exposed. If you can look directly into the open cells, paper wasps are a good bet.

Bald faced hornets surround their combs with a grey paper envelope, producing the classic hanging “hornet’s nest.”

Many yellowjackets construct enclosed paper nests underground or inside existing cavities. That is why discovering one with a lawn mower can be considerably more exciting than identifying it from a photograph.

Health Canada notes that social wasp nests range from open paper combs to completely enclosed structures and may be either conspicuous or hidden.

Paper wasps certainly visit flowers and food, but yellowjackets are especially notorious scavengers around garbage, patios and outdoor meals.

Their workers collect both protein and carbohydrates. Research on German yellowjackets has shown workers foraging for protein as well as sugary foods and carrying carbohydrate resources back to the colony.

By late summer, colonies are also large, meaning there are simply a great many workers out looking for food.

That combination helps explain why a yellowjacket problem can appear to arrive suddenly in August and September.

The wasp carrying off a chunk of your sandwich has another occupation.

Social wasp workers capture flies, caterpillars and other insects and bring animal protein back to their larvae. Adults themselves rely heavily on carbohydrate rich foods such as nectar and other sugary liquids.

That makes these wasps predators as well as nuisances. Health Canada specifically notes their role in catching flies and caterpillars and their contribution as pollinators when adults visit flowers.

A healthy respect for a nest is still warranted. It just turns out that the creature threatening your lunch has probably been doing some free pest control as well.

By autumn, the colony begins producing males and new queens rather than simply building its workforce.

The new queens mate and leave. They seek protected places in which to overwinter, while the workers, males and founding queen eventually die.

The paper nest itself is therefore usually a one season establishment. A large nest hanging empty after freeze up is not normally waiting to fill with the same colony next spring.

The important survivors are somewhere else.

And that brings us right back to the beginning: how many of those queens survive, and what happens to them when they try to establish colonies next spring, will help determine whether next August is pleasantly wasp poor or another year when lunch has wings.