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Wasps - Pest Control Fact File

Size: Head to Abdomen tip 10-20mm. Wingspan 25 - 40mm
Markings: Adults wasps have characteristic yellow and black bands on abdomen, with a distinct waist between thorax and abdomen. Head features are commonly used to identify different species.
Location: Country-wide. Nests found in hollows in trees, roof spaces, voids and cavities. Adult wasps found around sweet foods and drinks, and near skips and bins etc.
Detection: Visual sightings of adults, complaints of a 'suspected' nest. Stings.

Eleven species of true wasps are found in Europe, while only two, Vespula vulgaris and Vespula germanica, can be described as widely distributed pest species. The hornet, Vespa crabro, also attracts a considerable amount of interest but is seldom sufficiently common to constitute a true pest. Various species of bees are often confused with wasps on the basis of their black and yellow colouring. Owing to their ability to sting and their fondness for sweet material, feral honey bees occasionally achieve a similar pest status to wasps.

Biology:

Both wasps over-winter only as queens. Common wasps are commonly found in houses in the spring and autumn as they are leaving and seeking a hibernation spot. German wasps typically over winter under the bark of dead trees. Having emerged from hibernation the queen will start to construct the nest in a hole in the ground, a hollow tree or similar artificial structures such as birds nest boxes, attics etc. She will lay 20 or 30 eggs and raise these first few workers by her own efforts and these workers will then take on enlarging the nest and caring for the immature stages to follow. Nest construction normally starts in earnest during June and the nest will reach a peak size in September of five to ten thousand workers. These workers will normally forage up to 400m from the nest in search of food (insects, spiders and carrion), fluids (sap, nectar and fruit juices) and nest building materials (wood pulp). The number of wasp colonies will vary considerably from year to year and from place to place. Peak densities of two nests per hectare have been recorded in 'wasp years'. In the autumn new queens seek somewhere to hibernate while the rest of the colony dies out. Unlike honey bees, wasps never swarm.

The hazard of wasp stings is universal knowledge - very few people manage to avoid being stung at some time or another. In the UK around three people die each year from wasp stings, either as a result of respiratory obstruction following a sting in the mouth or throat or from 'anaphylactic shock'.

Damage:

Wasps chew through the skin of apples, pears, plums and other fruits in order to reach sweet pulp. When wasps are abundant, such damage can be severe to the commercial grower as well as the private gardener. Wasps and particularly hornets have been known to enter bee hives and eat the brood and honey, although this is fortunately not a widespread problem. Wasps chew wood to make pulp for their nests, and where this occurs on. for example, garden furniture, timber cladding on houses, etc it is sometimes seen as a problem.

Treatment:

Although killing foraging wasps using contact sprays or baited traps, jam jars etc. will kill many workers, it will seldom solve the problem. For maximum effect it is essential to find and treat the nest. Following wasps leaving a food source may provide an indication of the direction of the nest. Ideally, nests should be treated in cool weather or early or late in the day when most wasps are resting quietly in the nest.

To attempt to treat an active nest on a warm day is likely to provoke an aggressive counter attack. The simplest technique is to apply 10-20 grammes of a proprietary wasp killing dust directly into the nest hole using a pneumatic duster, gas powered blower or manually-operated blower.

When it is impossible to get the dust into the hole, then an application around the hole may be considered, but this may result in accidental contamination of non-target insects. Alternatively, nest entrances may be treated using a liquid insecticide applied from a sprayer using a pinstream nozzle to soak the nest entrance and the area immediately surrounding it. Carbamate insecticides are particularly effective against wasps. A purpose-designed aerosol spray is another good way of applying the insecticide. Most products are fast acting; knock-down will occur within minutes of treatment. The majority of the wasps will be dead within an hour, while nests are normally completely eliminated within 24 hours of treatment.

If the wasp's nest is found to be in a chimney, then an insecticidal smoke generator is often used. It is important to restrict the airflow up the chimney flue to avoid all the smoke vanishing up the chimney in a matter of seconds.

If you are working in a roof space or other dark area, try to position your torch away from yourself (e.g.: rest it on a beam), as any wasps you disturb will fly angrily towards the light source. You should also advise your customer not to stand below the loft hatch for the same reason!

There is no real need to remove a treated nest once it is dead, although it is more professional to do so if you can gain access. It is unlikely that it will be re-used in subsequent years, but any access points to the cavity or void could allow re-infestation and therefore some proofing work is always wise. This may include the use of mastic or mortar, or the proofing of air bricks and chimney pot tops with metal fly screen mesh.

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