Down Garden Services

Wasp

Scientific name: Paravespula vulgaris

Treatment

An over-wintering 'queen' wasp which was found in some folded fabric in the potting shed.

There are over 30,000 different species of wasps, some are used in horticulture as biological controls. The best known is the Yellow and Black Wasp, Paravespula vulgaris which although feared by many, is also useful to the gardener, eating harmful flies and aphids as well as caterpillars. They have a 'sweet tooth', also eating nectar, refuse and fruit.
There are three distinct body divisions; the head with strong mandibles; the thorax with two pairs of transparent wings; and the abdomen which has the sting at the rear.

Wasp severing a caterpillar of the Small White Butterfly to take back to the nest.

Living in groups or colonies, they work together in three social types - the queen (a large female which starts the nest and lays eggs), workers (small females which build nests and feed the young), and drones (males). At the end of autumn, all of the wasps die except mated queens, which burrow into leaves or soil to spend the winter.
In the spring the queens emerge and start the nest building process. They are built underground, in buildings, in trees or in bushes. The wasps will prune small twigs to clear a flight-path through shrubs which is a clear indication that a nest is present. The nests are complex with many six-sided cells made from chewed wood and plant fibres which are mixed with saliva and dry to form a paper-like substance - it is said to have inspired the invention of paper. The gnawing to obtain raw material can cause superficial damage to wooden fences and buildings, it is often seen on dead branches or fence posts where the edges are smoothed off - the rasping noise of the mandibles can be heard if you are close enough to the process.
The pictures below were taken as a queen was starting a new nest attached to the ceiling of a wooden garage. The flash from the camera must have startled her and in the right hand picture she had moved up behind the first cells which revealed that she had already attached some eggs in some of them.

An old wasp nest showing the internal labyrinth.

After the first eggs hatch and the larvae mature into workers, the nest is enlarged for further generations forming an intricate labyrinth. Each cell may be employed several times to rear larvae and by the end of the summer the nest may contain 20,000 or more wasps. The final generation of the season has drones and queens which mate for the next year's population.

Treatment

The wasps are more likely to be a nuisance in the autumn when they no longer have larvae to feed and go in search of ripe fruit and other sweet things. However they usually sting in response to a perceived attack which can be accidental if they become trapped in clothing or a nest is approached - button up clothing while picking fruit in the late summer to prevent a curious wasp crawling in while drowsy from eating a sugary meal. Also flapping wildly at their presence is likely to provoke a response so it is best to stay calm and move away slowly or wait until they fly away. However if you are close to a nest they will come straight for an attack and the first you may know about it is the strike. When they sting it causes a range of reactions from localised irritation for a few hours to a severe anaphylactic shock where the mucus membranes swell up causing the airways to close and can be fatal if not treated immediately. Anyone with a known allergy or if a previous sting caused a prolonged reaction, should be very careful as a subsequent sting will always be more severe. In normal situations the venom, which is alkaline, can be treated with vinegar or another mild acid to neutralise it. If uncertain how to deal with a nest, leave it to a professional. Do not flood or damage it without first killing the occupants, this is best done in the evening when the wasps have settled for the night, using a powerful knock-down insecticide recommended for the purpose. They are attracted to food and refuse so if eating outdoors keep food covered until ready to eat; remove over-ripe fruit from trees and pick up windfalls; keep rubbish bins covered. Jars of sweetened water can be placed around as traps to attract and drown them. In the late summer the queens leave the nest to find a new home for the winter, so the other occupants no longer have rearing tasks to keep them and they go out to seek nourishment before dying off. This is a good time to kill them before they leave, to avoid the chances of encountering a drowsy wasp which could become trapped in clothing. Insecticides are usually supplied as a powder which is puffed into the entrance of the nest. Apply it with a swift motion and have a clear path to retreat quickly to a safe distance as the wasps will emerge to fend off the attack. There is also a foam spray available that can be aimed from about 2 metres away, which is safer to use. Both types can be purchased at hardware shops or a garden supplies stockist and should be used late in the evening or the early morning when the wasps are relatively inactive. Also you may need to repeat the treatment a day or so later as new wasps could hatch out in the meantime. Nests inside buildings such as a in garage roof, can be dealt with using a fume insecticide containing permethrin. It is supplied in a can and is lit to release the fumes while the area is left for a few hours. It will kill most other creatures in the enclosed area as well, such as spiders and flies. Some Hoverflies can look similar and their larvae feed on larval wasps so may be used as a biological control in places like New Zealand where they have no indigenous predators.

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