Paper wasps-guests or pests?
Adroit as a hummingbird, agile as a gymnast, these builders of elegant paper mansions are becoming a familiar sight in many northern areas. Can they be the same insects that savage some of our favourite garden residents and mete out painful stings to unwary gardeners?
Danger! Black-and-yellow, black-and-yellow. Danger!
As universal as a stop sign, as recognisable as a flashing neon, this colour scheme is a potent warning employed and respected by crea-tures of water, land and air. It gives notice to both competitors and predators that they risk venomous bites or stings, or a poisonous meal, if they attack the wearer of these bold signals. So effective is the signage that even we humans have adopted the device—look on the rear ends of many road construction vehicles, on shunting trains and wharf container movers.
So when the Asian paper wasp, Polistes chinensis (right) arrived in New Zealand in the late 1970s wearing the same black-and-yellow livery as its European relatives the German and common wasps, it boded no good for either people or other insects. And so it proved: the new arrival settled quickly into the suburbs of Auckland, and is now responsible for the majority of wasp stings there that result in visits to the doctor.
The Asian paper wasp resembles the Australian (sometimes called the Tasmanian) paper wasp Polistes tasmaniensis humilis, a black and red-brown wasp with yellowish legs (facing page, below) which has been part of the bush and garden scene of northern New Zealand for over 100 years.
The two species have very similar habits, and are frequently seen hunting around the same plants without any obvious mutual aggression. They may even site their very similar nests within a few metres of each other. Yet there is apparently either competition between the two species or differing tolerance to some environmental factors, because their relative abundance changes from year to year. Lately, the Asian wasps have been more common throughout the greater Auckland and Northland regions, but it is still not difficult to find the Australian species.
Whereas the German and common wasps are found throughout New Zealand, tile two species of paper wasps are largely restricted to north of a line from Kawhia to Whakatane, although the odd record has come from the southern North Island, Nelson and, this year, Blenheim.
The Asian species hails from Japan and parts of China, and occurs on Hokkaido, the northern island of Japan, which can get as cold as the southern South Island. So perhaps it will eventually spread throughout the entire country.
In the north, Asian paper wasps are now very abundant in gardens, around farms, and in scrub and shrubland. One researcher counted 210 nests—each containing 50 to 80 wasps—in a one hectare study area near Doubtless Bay.
Neither species is normally aggressive towards people, and they generally ignore us, even when we are working within a few inches of their nests. However, brush inadvertently against a nest while trimming the hedge and, understandably, the wasps will defend their castles-or rather, maternity homes-and stings result.
Most people are not badly affected by the sting, which, after the initial sharp pang, is more irritating than painful. Perhaps one in ten people will be mildly allergic to wasp stings, including those from paper wasps, suffering swelling around the injection point and discomfort for several days. In rare instances, victims go into anaphylactic shock, their breathing and blood pressure being seriously affected. This condition requires speedy treatment by adrenaline injection.
A disturbing feature of wasp and bee sting allergies is that they can develop in people who have never reacted badly before, though they may have been stung many times. Once this new sensitivity shows up, sufferers need to be especially careful about future stings, for they remain highly allergic thereafter.
Paper wasps are easily distinguished by their habit of flying rather slowly with their long back legs hanging down looking somewhat like a helicopter hovering and cruising slowly on a search mission.
As they patrol the shrubs, fruit trees and vegetables of our garden: tile wasps make what trainee aircraft pilots would call “touch and go landings” in their tireless search for insects to feed to their developing larvae. Nectar is also taken, some supply their own energy needs, an, some to be stored in cells of the nest (above).
Paper wasps take their name from the papier mache nests they construct as nurseries in which to raise their young. The nest is a pendulous bell-shaped structure consisting of many hexagonal cells with very thin paper walls. It is usually secured to a near-horizontal support by a black stalk-like peduncle constructed from plant fibres and resins splayed out against the attachment surface (right). Small at first, the support is strengthened and its attachment area expanded as the nest is enlarged. Nests are not restricted to branches of small trees and bushes, but are often cosily positioned out of the weather under the eaves of houses or where corrugated iron overlaps guttering.
The nest hangs more or less vertically, so that rainwater cannot drip into the open brood cells, and the umbrella-shaped top and outer cells are waterproofed with plant resins. The larvae hang head down the opening, bracing themselves against the walls so they do not fall out, awaiting food brought by adult embers of the colony.
Nest paper is made from finely chewed plant fibres that the wasps strip from dried flower stems or soft wood. Friends of photographer Brian Chudleigh—who followed paper wasps around his garden for two seasons to capture the images in this article—had the surface of weathering cedar boards on their house chewed away, producing unsightly patches of differently-coloured timber.
When gathering wood fibre, a wasp first dribbles saliva on to the surface to moisten and soften it, then nibbles and tears off fine strips with its hard-edged mandibles. The fibres are chewed to a pulp and, back at the nest, are woven and glued in place with saliva to form the hexagonal cells. Like a blocklayer raising a wall, the wasp circles the lip of a cell, extending it by a millimetre or two each round (above).
At night, the nest is always crowded with wasps. Some enter vacant cells head first, seeking shelter from the cold and dew (facing page), and others cluster on the back or settle on’ stems and foliage nearby. On overcast and rainy days most of the colony stays on or close to the nest, and many again shelter in empty cells.
Each new paper wasp colony is started by a single fertile female in spring. (Unlike the founders of German and common wasp colonies, these females are not normally referred to as queens). The nest consists of just a few cells—perhaps half a dozen—which is as much as she can manage by herself. In each cell she lays a single egg, attaching it to a side wall nearly half way down. The eggs usually hatch within a week, and after passing through a number of moults, reach full size in two to three weeks.
Mature larvae spin a silken cap to close off their cell (facing page, below left), and during a pupation period that lasts for about a week they steadily change from a white sausage-shaped grub to a handsome winged adult. (Eggs, larvae at different stages of maturity and capped pupae can be seen in the main photograph.)
The speed at which all the stages progress depends very much on temperature and how much the larvae are fed. At one study site in Auckland, the first eggs were laid in late October, and the first batch of brood emerged at Christmas. But the time from egg to emerging adult may be as little as a month at the height of summer. On hot days nests risk overheating, and workers can be seen fanning larvae with their wings to keep them cool (facing page, below right). Mortality of nests and females is very high in this early stage: 60 to 80 per cent perish while the first brood is developing.
Bursting through the cell caps, the adult wasps emerge to begin short lives as busy workers, helping to build more cells, lengthen existing ones and catch food and feed it to the next batch of larvae. While they go about their tasks their mother lays more eggs in the vacant chambers and in new ones built while she waited for her first progeny to emerge.
By the autumn, a nest typically grows to about 7 5 mm in diameter and has some 200 cells. Of these, about 50 will be under construction around the edge of the bell, and up to a third of the remaining cells in use may be sealed over by the pupae within. When food has been plentiful through the summer, and the rainfall light, nests may reach 100mm in diameter and have up to 350 cells.
Paper wasps do not have the same social order as German and common wasps, whose colonies consist mainly of sterile female workers that support and feed thousands of larvae, at times a few male drones and the single queen. Instead, paper wasp colonies have both male and female workers, with all of the fertile females capable of laying eggs.
Males seem more active than the drones of bee hives, but just how much they contribute to the well being of the hive is unclear. Females are fertilised in autumn, and large individuals survive the winter in woodpiles or other protected sites. Occasional Australian paper wasp nests have been seen to survive the winter, but this has not so far been observed for Asian paper wasps, which start to die out in cooler areas as the cold of May seeps in.
The sexes of Asian paper wasps are easy to distinguish. A female is usually larger and darker, and her face is black with two distinct horizontal, scalloped yellow bars. A male’s face is plain yellow in the shape of a triangular shield without any black markings, and the underside of the body is much more yellow than in the female. Antennae in the male have a tight curl—almost a loop—at the tip; female antennae are straight. The wasps in all the photographs in this article, except the opening shot, are females.
It is possible that there may be two classes of females. Small individuals (13 – 16 mm long) have mottled black eyes and small mandibles, while those of a larger size class (16- 18 mm long) have shiny black eyes and noticeably larger, black mandibles. Both classes appear to help with nest expansion and food capture, but it may be that only the large specimens are fertile egg-layers.
If a nest is removed while some of the residents are away foraging, those wasps left suddenly homeless cluster together in a tight ball on a nearby stem and may still be found huddled together a week later. That they appear lost and lacking in leadership is an indication that, although there is no obvious queen, some of the wasps on the nest are at least lieutenants. Without direction from these leaders, the workers appear to be completely lacking in motivation.
Paper wasps may fairly be regarded as the gardener’s friends, for wherever they are found in good numbers they control garden pests, especially caterpillars and aphids. Yet this helpful behaviour has its downside. A much more warmly welcomed immigrant insect, the monarch butterfly, is also targeted by the wasps, and for many people a love for the butterfly has engendered a hatred of the wasps.
Not many animals are interested in eating monarch caterpillars. Like wasps, they wear black-and-yellow-striped jerseys to warn off potential predators. The threat they advertise is not a sting or bite, but poisons absorbed from the milky sap of their natural food, the swan plant, which make vertebrate predators such as birds, rats and mice vomit and retch.
Unfortunately for the monarch caterpillars—and monarch butterfly lovers—predatory insects are not affected by the poison, and do not respect the warning signs. The soldier bug, South African praying mantis and, most seriously, both species of paper wasp all take their toll of the much-admired monarch caterpillars.
Some people watch monarchs laying eggs on the leaves of their swan plants, and then wonder why so few caterpillars emerge. In truth, they nearly all hatch and start feeding, but within a day or two are abducted by paper wasp baby snatchers that feed them to their own young.
Careful examination of swan plants reveals leaves with chewed scars on just one side of a leaf, about the size of the end of a pencil, representing the hatchling’s first one or two days’ feeding. It is at this stage that most of the caterpillars are killed.
The few that escape notice appear to be able to cope with wasp attacks when they become larger. It is not unusual to see a cruising or hovering paper wasp casually deliver a hit-and-run sting without actually settling on the caterpillar.
The victim writhes and throws the front of its body about with evident pain for about half an hour, and if a wasp comes near, it rears up to repel the attack. After one to two hours the effects of the sting appear to have completely worn off, with the caterpillar feeding voraciously as normal.
The caterpillar becomes vulnerable once again when preparing to pupate, because in starting to loosen its skin in preparation for its final moult, it loses the ability to rear up and repel attackers. One of its last acts as a caterpillar is to spin—from a “chin” gland just below the mouth—a silken support plate over the surface of the leaf or twig from which the chrysalis will hang.
The attachment mesh is drawn together over and around the last pair of abdominal legs to make a short, stiff suspension rope which the pupa will use after shedding its striped body stocking for the last time.
This suits the wasps very well. Once the caterpillars have suspended themselves securely from an overhanging support, the wasps attack, and with sharp mandibular knives slice sizable “steaks” to take back to the nursery nest (facing page and left). There they are cut into portions that are shared with other nursing workers before the smaller servings are minced to a pulp that the larvae can easily swallow.
Paper wasps do not confine their diet to imported insects. Since they are the most abundant type of wasp in Northland, there has been concern that they may reduce numbers of native insects substantially. They have been seen taking caterpillars of plume moths, kowhai moths and various owlet moths.
However, they are not found under the dark, damp canopy of dense bush—perhaps it is too cool—confining themselves to margins and clearings. This preference reduces the impact they are likely to have on native insects, but concerns remain.
Common wasps in the beech forests of the South Island are considered a serious problem (although numbers of wasps per hectare are not that different), but no funded research is being carried out on paper wasps and their effects on native fauna.
Because they are common in domestic gardens, and not aggressive, paper wasps can be readily observed without hazard. One of the interesting behaviour patterns displayed by Asian paper wasps seems to relate to the family bond that exists between colony members. On bright, clear days the wasps, males especially, are often seen patrolling and basking on sun-warmed surfaces, often wood (especially unpainted fences, gates and patios), but sometimes brick, tarpaulin or pipe. T hey often crouch low so that their bodies are pressed hard against the warm surface, 74 typically resting for less than 30 seconds before crawling or flying to another hot spot.
When a wasp lands where others are already sunning themselves, it invariably settles first on the back of another wasp before moving on to the warm surface alongside. The wasp that was landed on may then jump on to the new arrival, or another wasp, and these exchanges continue for hours as wasps come and go. It looks for all the world like a well-organised game of waspish leapfrog.
Some behaviour is less peaceable. When a lone female is away seeking food for her developing brood, another female has been observed to kidnap larvae to nourish offspring in her own nest. This photograph of a “larvanapper” in the act was taken early in the season, when food was less plentiful. A later attempt to remove a second larva was repelled by the outraged mother. Even wasps have maternal instincts.
Indeed, like us, they like a dry roof over their heads, enjoy a varied diet of sweet and meaty foods, and when their private space is invaded, can get testy to the point of showing that their black-and-yellow jerkins are no idle warning.