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VESPULA GERMANICA, THE EUROPEAN INVADER

Wasps of the genus Vespula are challenging pests, but how did they become so well established in Australia, and what can be done to control their numbers?

Their colony structure, division of labour and sheer numbers have made social insects very successful invaders and difficult to eradicate or control. Three species of social wasp from the genus Vespula have established outside of their native range of Europe and temperate Asia to become pests: the German wasp or yellowjacket (Vespula germanica), the western yellowjacket (V. pensylvanica), and the common wasp (V. vulgaris). In Australia, it is Vespula germanica, sometimes known as the European wasp, which presents the most issues to homeowners and subsequently the most frequent calls to pest managers. Why are these aggressive invaders so successful in establishing themselves in new continents and what can we do to control them? In a 2019 article published in Annual Review of Entomology, academics from New Zealand shared their insights.

Vespula wasps have proven themselves to be excellent travellers. Of the invasive species, V. germanica is the most widespread, establishing in areas as wide-ranging as southern Patagonia, South Africa and Iceland. V. germanica was first reported as having established outside of its native range of Europe and temperate Asia in 1945 (in New Zealand) and Australian populations were first seen in 1959 in Tasmania and on the mainland in 1976. The invasive Vespula species travelled to these new regions via the international movement of goods (such as sawn timber) which contained mated, hibernating queens.

Vespula wasps typically produce new queens and males each autumn, with queens leaving the nest to mate before finding an overwintering site to hibernate. The queens emerge in the spring to establish a colony, which grows over the summer from the single queen to nests often consisting of several thousand workers. In milder climates, the wasp season can stretch as long as six months. In particularly warm conditions, the nest may survive the winter, becoming large and perennial. Some perennial nests can contain as many as half a million wasps. These huge nests pose a significant risk to human health – particularly to pest managers tasked with destroying them!

The problem with wasps

Vespula spp. emerge in the early summer, with numbers increasing steadily well into the autumn. Known for their aggressive behaviour and venomous sting, these invasive wasps frequently spoil outdoor activities and are dangerous not only for their sting, but for causing accidents as people seek to avoid them. A 2017 study found that of the 199 per 100,000 people admitted to hospital in Australia for envenomation (over a 12-year period) 31% of incidents were caused by contact with wasps or bees.1 Anaphylaxis from insect stings in general occurs in approximately 3% of cases and may even be fatal on the first sting – an Australian study estimated 2-3 deaths occur as a result of wasp stings each year.

Their aggressive behaviour is not only directed at the humans swatting them away; they are aggressive foragers too. V. Germanica has been observed killing bird chicks in nests, presumably attracted by the scent of the eggs. Some social wasps even work together to steal food from ants, with some wasps preventing the ants from leaving their nest while other wasps plunder the food source. With V. germanica having a broad diet that includes honeydew, insects and carrion, it is a sad fact for humans that everything from fizzy drinks to barbecued meats are considered attractive food sources.

Invasive Vespula wasps have a very broad diet, but find soft drinks very attractive

Why have they been so successful?

The predatory success of invasive Vespula can be attributed to what is known as behavioural plasticity (adaptive behaviour) together with their cognitive abilities. They locate food sources and prey using their visual, auditory, and olfactory senses, and can learn to follow pheromone scents to find prey. These wasps quickly learn to associate cues with food resources and can return to food resources even if those cues are moved. Incredibly, the common wasp (V. vulgaris) even has the ability to resolve complex tasks that include the recognition of different human faces.2 Coupled with the fact they are omnivores with a broad diet – honeydew, nectar, other insects and spiders, vertebrates and carrion – they are very adaptable and can exploit whatever food source becomes available. In short, they are excellent foragers.

In their natural habitats, predators would normally keep populations in check. Small wasp nests are very susceptible to predation during this development phase, particularly from parasitic wasps. In the US, the rate of parasitism is estimated at anywhere between 23-65%. Large wasp nests are pretty resilient although they will be attacked by large vertebrate predators such as bears and badgers that are looking to feast on the larvae. But in new locations, they have few, if any predators. Birds have been noted as eating wasps occasionally, but at a rate unlikely to have any significant effect on wasp populations.

Control efforts now and in the future

To date, methods of wasp control have included the killing of hibernating spring queens, destroying nests, insecticidal baiting, trapping, electric grid devices, contact insecticides, preventative measures, biological control, and integrated pest management. Pest managers who receive a call to control wasps at a property will typically reach for an insecticide (dust or aerosol) and treat the nest and immediate area.

Controlling invasive wasps at a national level has proven incredibly challenging. Wasps carry a diverse array of potential pathogens, so biological control – unleashing a particular pathogen to attack wasps – is one option that has been explored. However, attempts at biological control of Vespula have so far been largely ineffective, with non-target effects being high (particularly to ants and honeybees).

Another potential control option is the use of artificial pheromones, in the form mating disruption or attractants. Mating disruption pheromones could be used confuse the males’ ability to find receptive females, but it would likely be expensive to execute this on a large scale. Much more research is needed into wasps’ sex pheromones before this would even be feasible.

What about baits? Insecticidal baits are most likely the answer for future, wide-scale control.

Tests have shown fipronil-based baits to be the most effective method to reduce populations of Vespula and other social insects. Fipronil has been shown to successfully reduce wasp activity by up to 99.7%.3

The largest area study that tested wasp control was undertaken in 2017 in New Zealand. Bait stations were set up on a grid at 300 × 50 m apart in areas of up to 2,000 hectares, using one application of Vespex containing 0.1% fipronil. Wasp tra c declined by up to 97% between 20-38 days after baiting. This indicates that baiting is a highly effective method for controlling invasive wasps over large areas, however reinvasion is likely from queens in the following autumn and spring. Eradication is unlikely where wasps are widespread, but insecticidal baits may be an option for eradicating isolated pockets of Vespula. What’s more, non-target effects can be reduced if the baits have limited carbohydrate content (and are therefore not attractive to bees).

Gene drives are a hot topic and a possible method for eradication but the technology is still in the relatively early stages of development. In the case of Vespula, this could see male wasps being genetically modified to produce non-viable sperm, leading to reproductive failure and control at the nest founding stage. The fact that Vespula have little genetic diversity in areas where they have invaded means that the development of a variation enabling resistance would be unlikely, making gene drives an exciting possibility for future control.

Climate change

There is already evidence that climate change is influencing Vespula distribution and dynamics. Milder temperatures are allowing distribution to expand into areas that were previously too cold (e.g. Finland). In addition, milder temperatures can increase abundance and nest size, with perennial nests becoming more common. On the flip side, some areas will become too warm for nest survival, providing some balance to the overall picture.

Further reading: Lester, P. and Beggs, J. (2019). Invasion Success and Management Strategies for Social Vespula Wasps. Annual Review of Entomology. 64. 10.1146/annurev-ento-011118-111812.

1 Welton RE, Williams DJ, Liew D. 2017. Injury trends from envenoming in Australia, 2000–2013. Intern. Med. J. 47(2):170–76

2 Avargu`es-Weber A, d’Amaro D, Metzler M, Garcia JE, Dyer AG. 2017. Recognition of human face images by the free flying wasp Vespula vulgaris. Anim. Behav. Cogn. 4(3):314–23

3 Harris RJ, Etheridge ND. 2001. Comparison of baits containing fipronil and sulfluramid for the control of Vespula wasps. N. Z. J. Zool. 28(1):39–48