Wasp warfare

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New Zealand has a wasp problem. If you don’t believe it, take a trip into the beech forests around Nelson. Here the singing of birds is replaced by the buzzing of millions of wasps. Holidaymakers are dismayed to find their favourite tramping, fishing and picnicking sites over­run by these venomous insects. In late summer the wasps are thick in the air, and in some of our prime tourist destinations, like the Nelson Lakes, boaties can­not find a wasp-free beach to go ashore safely. Rubbish bins in picnic grounds swarm with wasps, and getting in or out of your car becomes an ordeal.

The wasp problem is also a conservation issue. Wasps relish honeydew ­the sugary secretion produced by scale insects living on beech trees. This is an important food for nectar-eating birds such as bellbirds, tui, silvereyes. Outcompeted by large numbers of wasps, these birds are forced to leave the forests in search of food.

Most people are all too familiar with the German (or Waikato) wasp, Vespula germanica, which estab­lished itself in the Waikato in 1945 after accidental introduction. It spread rapidly, reaching the South Island about 10 years later. It has now been joined by a close relative, the slightly darker-coloured common wasp, Vespula vulgaris. This species has greatly increased wasp numbers in honeydew forests, where this year wasp nest densi­ties reached as high as 50 per hectare.

In 1987 the Department of Scientific and Industrial Research (DSIR) tried to gauge the size of the wasp problem by appealing to the public for information. Nearly 4,000 people re­sponded, sending in some 50,000 wasps from through­out the country. A few people sent honeybees instead — an understand­able mistake, since both wasps and bees belong to the insect family Hymenop­tera. Also, like honeybees, these introduced wasps live in large colonies containing thousands of individuals. (By way of contrast, New Zealand has a number of harmless native wasps, all of which are solitary insects.)

The most significant result of the survey was that the common wasp was becoming, well, more common. By 1987 it was the most abundant wasp in the honeydew forests, but also occurred in most cities from Auckland to Inver­cargill. Its spread since 1987 has been swift: only one of 3367 wasps collected in the Waitahu Valley in 1987 by DSIR scientists was a common wasp; today the population is 99% common wasps!

The common wasp is not the only recent wasp arrival. In 1979 the Asian paper wasp was first recorded around Auckland. Last summer it appeared to be the most abundant wasp in Northland gardens, and it is still on the move, reach­ing the South Island this year. If this efficient predator takes a liking for our forests, it may pose a threat to our native insects.

To combat the wasp invasion, the search is on for both biological and chemical control methods. DSIR scientists in Nelson are monitoring the success of a recently introduced parasite, Sphegophaga vesparum, which lives entirely within the nests of German and common wasps, feeding on the growing larvae. Sphegoph­aga, a wasp itself, is capable of producing two types of progeny: a flightless form that builds up numbers within a nest, and a winged generation which moves out to find another home, spreading the infestation from nest to nest.

For a more novel ap­proach to wasp control, how about a peacock? A Northland farmer reports that his resident peacocks delight in munching up wasps, taking beakfuls straight from the nest. In some places seagulls have also cottoned on to these tasty (?) morsels.

On the chemical front, 1080 poison (sodium monofluoroacetate) in a fish bait is the best option at present. Worker wasps collect the laced fish from bait stations, take it back to the nest and feed it to the larvae and other adults, effectively killing the whole nest. This is a great system if it all works to plan. However, 1080 is highly toxic to other animals and man, and there are prob­lems with ensuring bait acceptance.

The search continues for more attractive, palatable baits. Success in this field is essential if we are to seriously reduce wasp infestations in recreational areas.

Beekeepers are also keen to see the war on wasps succeed. Wasps, especially the German wasp rob honey from hives, kill bees, and are a nuisance during honey processing.

Wasps do have their good side, though. They benefit gardeners and farmers by killing insect pests such as caterpillars and blowflies. It is impor­tant to control wasps only where and when they are causing problems.

To find out how far the common wasp has spread, and where it can outcom­pete the German wasp, DSIR is mounting another nationwide appeal for wasps this summer.

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