Wasps along rivers and tracks in late summer

By late summer, anyone who walks river flats or beech-forest tracks in New Zealand knows the sound: a steady hum in the trees, and wasps around every sweet drink at lunch. They are not just an annoyance. Introduced wasps harm native birds and insects, and a sting on a remote track can turn into a real problem.

Why there are so many

The Department of Conservation’s wasp page says New Zealand has some of the highest densities of German and common wasps in the world, because they have no natural predators here, the winters are mild and there is plenty of food. New Zealand also has native wasps, which have never become a nuisance. The pest species are introduced: DOC lists five social species accidentally introduced since the 1940s, the German wasp, the common wasp and three species of paper wasp.

German and common wasps look very similar, both black and yellow. According to DOC, the German wasp arrived in the 1940s and the common wasp in the late 1970s, and both now live in large colonies, with nests often about the size of a soccer ball. German wasp nests are grey and common wasp nests are brown. Nests that survive the winter can grow much larger.

Nests beside tracks and rivers

Wasps often nest in the ground, in banks, under tree roots and in hollow logs. Riverbanks and the low banks beside tracks suit them well. The first sign is usually a steady stream of wasps flying in and out of one spot. Walking past is usually fine; standing on the entrance, or a group stopping for lunch right beside it, is not.

On a trip:

• Watch for traffic at ground level in late summer: a line of wasps going in and out of a hole means a nest. Walk around it, widely.
• Don’t stop for breaks near a nest, and warn people behind you.
• Keep sweet food and drinks covered, and check cans and bottles before drinking.
• Move steadily, don’t swat. Flailing at wasps tends to make things worse.
• Pack your antihistamine if you use one, and carry any prescribed adrenaline auto-injector where you can reach it fast.

If someone is stung

Most stings are painful but not dangerous. Move away from the nest first, because disturbed wasps sting repeatedly. Wash the area and use a cold cloth to ease the pain and swelling.

Multiple stings, stings to the mouth or throat, and stings to young children also need medical advice. Anyone who has had a severe reaction before should talk to their doctor before heading into the backcountry in wasp season.

Wasps and the forest

Wasps compete with native birds and insects for food, and they prey on native insects. In beech forests they take a large share of the sweet honeydew that many native birds rely on. That is one reason conservation groups control them over large areas in summer.

DOC describes the main large-area method: a protein bait called Vespex, laid in bait stations. Worker wasps carry it back to the nest and the colony dies. It is targeted at wasps, isn’t attractive to bees, and only works in the warmer months when wasps are hunting protein. Users have to be approved before they can buy it. It is used on private land and on public conservation land.

A nest near the house

Wasp nests turn up in gardens, wall cavities, sheds, roofs and the banks of streams at the bottom of sections. DOC describes a way to treat a single nest yourself with insecticide dust at the entrance after dark. Before deciding to do it yourself, think about:

• Where the nest is. Ground nests in the open are one thing. Nests inside walls, roofs or ceilings are another: they can break through into rooms, and treating them often means opening up the building.
• How big it is. Large nests, especially ones that have survived winter, hold enormous numbers of wasps.
• Who is around. Anyone with a known allergy, young children or pets nearby changes the risk.
• Access. Nests up ladders or in confined spaces are risky to reach.

If the nest is in or on the building, very large, hard to reach, or anyone nearby is allergic, call a professional pest controller rather than tackling it yourself; waspari.co.nz sets out which nests are reasonable to treat yourself and which are not. If you do treat a simple ground nest, follow the product instructions exactly, work after dark when the wasps are in the nest, wear protective clothing, and don’t shine a torch straight into the entrance; DOC notes that wasps will fly up the beam.

Paper wasps

Paper wasps build small, open, umbrella-shaped nests under eaves, in shrubs and on fences. They are less aggressive than German and common wasps but still sting when disturbed, often when someone is pruning or picking fruit. Look before you reach into a hedge in summer.

Timing

Wasp numbers build through summer and peak in late summer and early autumn. That overlaps with some of the best tramping and river weather of the year. Plan for it: take the kit, keep an eye on the ground, and give nests a wide berth.