With wasp season is upon us, pest managers should be ready to identify the most commonly encountered species and have a fast-acting solution to hand.
There are over 15,000 species of native and introduced Hymenoptera (wasps, bees, ants and sawflies) in Australia, including over 3000 species of ants. Excluding the ants, although there are many species of Hymenoptera that are of economic importance (mostly in agriculture), there are only a few that can truly be considered urban pests, worthy of control measures. Although, on occasions the European honeybee (Apis mellifera) can nest in inappropriate situations and warrant removal to avoid painful stings when disturbed, it is the wasp species that are the main Hymenopteran flying pest.
Native paper wasps (main picture, above) are found throughout Australia. The common paper wasp (Polistes humilis) has some yellow/ orange bands on the abdomen, but is mainly black. There are two pairs of brown-tinted wings, with the first pair larger. Recently, the introduced Asian paper wasp (Polistes chinensis) has been reported from several inner city suburbs of Sydney. This closely related species is larger than the native Polistes and tends to have more distinctive yellow and brown bands.
Paper wasps form small colonies, and make paper nests in sheltered sites under tree branches and the eaves of houses. The nests are shaped like inverted cones, and consist of a cluster of hexagonal cells made from wood fibre mixed with saliva.
Nests start up in the spring in one of three ways:
- A new nest is started by overwintering queens
- An overwintering queen will utilise an old nest
- An overwintering colony can continue to use their nest from the previous season.
Paper wasps are generally beneficial to the environment, collecting caterpillars as food for the larvae and feeding on nectar and acting as pollinators themselves. Whilst they are not normally aggressive and only attack humans if their nest is disturbed, accidental stings are common when humans accidentally encounter nests in vegetation, especially in late summer/autumn, when their numbers are at their maximum.
The European wasp (Vespulagermanica) and the common wasp (Vespulavulgaris) first appeared in Australia mid last century. While the European wasp has been relatively successful, the common wasp is still limited to a small area around Melbourne. The European wasp is now known in most parts of southern Australia with cool and wet climates. Like paper wasps, European wasp fertilised females overwinter and re-emerge to found a new colony in spring with the nest growing in size through to the end of the season. Although not common, wasps can survive successive winters under ideal conditions and can grow to nests with over 100,000 individuals.
European wasp founding queens can establish nests in many diverse places including hollows underground, in roof attics, wall voids, tree hollows and many other small enclosed spaces. Only around 10-20 percent of nests will be in houses. Unlike paper wasps, European wasps are very aggressive and can attack in numbers.
Will it be a big wasp season?
Always a bit of a conversation topic for locals and pest managers alike, but predicting the size of a wasp season can be tricky. However, recent research from the University of Wellington1 determined the key factors that can help predict the size of a wasp season:
- Large numbers in previous year tend to lead to lower numbers the following year. This is thought to be due to larger number, but lower quality, of queens being produced and/or increased competition between queens the following spring
- The number of wasp nests in a given season is dependent on spring rains, when the colonies are starting to form
- The number of wasps in a given nest is driven by favourable environmental and food conditions during the season.
Control measures vary depending on the species and nest location. Treatments for large nests and especially European wasp nests are best carried out in the evening when the wasps are back in the nest and generally quieter.
Paper wasp nests on the outside of buildings can be easily dealt with by rapid dosing with a high volume of liquid insecticide. Specialised wasp aerosols, such as Sumitomo’s WaspJet Pro, are ideal for fast, convenient nest control, from a distance.
European wasp nest control is more dependant on the location, structure and size of the nest. In many circumstances the use of permethrin dust or powder blown into the nest to excess provides the best option. Extra distance can be obtained by inserting a length of ducting into the nest opening and blowing powder down the ducting, whilst block off the nest entrance. Although baiting with protein and sugar-based materials laced with certain insecticides is known to be effective this is not currently an approved means of control other than under special permits
Some species can cause more cosmetic damage to building such as the mud dauber wasp (Sceliphron spp.) and potter wasp (Abispa spp.) and although they can occasionally sting people in suburban environments when disturbed, they are largely beneficial, collecting spiders and caterpillars as food for their larvae. Another prominent wasp observed in the suburbs is the large orange and black spider wasp often seen dragging around huntsman spiders.
Like the mud dauber wasp and the potter wasp, these spider wasps use the paralysed spider as an incubator laying a single egg inside which the larvae feeds and grows. These wasps are all fearsome-looking but largely harmless to people and best left alone to carry out their ecological role.
Garry Webb, General Manager Professional Products, Sumitomo Chemical
1 Lester et al (2017). J. Animal Ecology.