Wasp numbers explode after long, hot summer

Wasp numbers have exploded in New Zealand over summer,  says University of Auckland Professor of biological sciences Jacqueline Beggs.

“New Zealand has the highest recorded densities of common and German wasps in the world. It has been a long, dry, hot summer, so numbers have built up.

“Wasp population dynamics tend to be cyclical, so a low year is followed by a high year – and this year the stars have aligned for wasps to reach extremely high numbers,” says Beggs, who is a director of the University’s Centre for Climate, Biodiversity and Society - Ngā Ara Whetū.

New Zealand has five species of these brightly coloured invaders from Europe and Asia - the German and common wasp, and three species of paper wasp. All have painful stings for humans and nasty effects on native species.

“The harms are huge. They kill lots of insects and feed on young, native birds in nests.

“They are very efficient foragers and can tackle prey that are way bigger than them. They can sting and immobilise weta and large dragonflies, carving them up and taking the pieces back to their nest,” says Beggs.

Introduced wasps compete with native birds, bats, lizards and insects for food.

As well as feeding on protein, wasps eat sugars, such as nectar. Sugary honeydew found in South Island beech forests supports massive numbers of wasps.

“It’s estimated the biomass of common and German wasps in South Island beech forests is greater than native birds and introduced mammals put together.”

Beggs says many predator control programmes do a great job of eradicating rats, possums, stoats, and other mammalian pests, but overlook the swarms of wasps.

“Leaving wasps out of the mix is missing a huge part of the problem.

“If you remove mammalian predators from South Island beech forest, will it restore the ecosystem? It will certainly help, but they have left one of the most abundant predators.”

New Zealand is a long way from being able to eradicate wasps entirely, but control is within reach, she says.

“Numbers will be naturally low in many years, so you need to look out for bumper years like this year and go hard on them then.”

While working at Landcare Research, Beggs helped develop Vespex, a toxic bait that wipes out nests of German and common wasps within days. Vespex poses no threat to honeybees, because it uses animal protein to lure wasps – and bees don’t eat animal protein. Bait stations are used to make the fipronil poison safe for other species.

“Toxins are toxic and we way overuse toxins in agriculture and in home gardens. As a rule of thumb, we should be reducing our reliance on pesticides and herbicides, but judicious use of wasp poison is incredibly effective and safe,” she says.

People sometimes take reckless approaches to wasp control, mixing up their own toxic concoctions. Beggs is particularly horrified when home-brewed poisons contain sugar, which attracts not only wasps, but poses a risk to bees and could contaminate honey.

German wasps and common wasps tend to form nests in the ground or clay-covered nests in trees. Pyrethroid dust can be sprinkled on the nest entrance to kill wasps. Taping a spoon to a long stick and gently sprinkling the dust on the nest can help to avoid stings, says Beggs.

Left unattended, wasp nests can grow several metres wide.

“Some nests survive the winter and are occupied by multiple queens. By the end of the second season, there are millions and millions of wasps. It’s extremely dangerous and does a huge amount of damage to the environment.”

At the moment, short-lived male wasps are forming mating swarms. They look menacing, but pose little threat to people, because males don’t sting, says Beggs.

A queen will mate with several males and store their sperm inside herself, before going into diapause – an insect version of hibernation – over winter. The queen is then ready to pump out fertilised eggs and create a new nest in spring.

Wasp nests are fiercely guarded by female workers.

“The nests are dangerous, especially at this time of year, because larger, older nests have a lot more workers in defensive mode, ready to sting.

“January to February is the best time to kill the colony, but it’s not too late to poison wasp nests.

“The aim is to kill the queen and stop the predation pressure each nest puts on the environment,” says Beggs.

New Zealand also has an estimated 2000 to 3000 species of native wasps, but they are solitary, play important roles in our ecosystems and are not a nuisance to humans, she says.