Insects' instincts boost population

By ALAN SAMSON*

Whether it is down to good sex or good science, it is a giant leap forward in the conservation of one of our rarest insects.

A Conservation Department visit to Red Mercury and Double Islands off the Coromandel Peninsula has found that captive-bred, giant tusked weta released there in 2000 and 2001 have multiplied.

The importance of the sexual activity is immense.

The imposing, eight-to-10cm insects (Motuweta isolata) are among the rarest of New Zealand's native insects.

In recent times it has been found only on the tiny Middle Island in the Mercury Island group.

Even on the 13ha island its numbers are minuscule. In 74 nights of searching since 1998, only four have been found.

Before an earlier sighting, in the late 1960s, they had been thought to be extinct.

The tusked weta is believed to have once occupied the entire Mercury group. But in most places they have been wiped out by rats.

Middle Island, their haven, is one of the few islands never to have had introduced mammals.

The weta released on Red Mercury and Double islands were bred in an Auckland laboratory from three of the four Middle Islanders - one male and two females.

The laboratory breeding, by Landcare Research, helped by Massey University and the Auckland Zoo enabled the conservation scientists to release 64 weta on Red Mercury and 82 on Double.

But the scientists were worried about the success of the venture.

The tusked weta were bred individually - they are carnivorous and cannibalistic - then placed in their new island homes under specially prepared "saucers".

Conservation Department invertebrate ecologist Ian Stringer said some of the weta stayed living under the saucers, enabling them to be easily monitored.

"Amazingly, two eggs - laid in the ground - were found in April 2001 and a tiny newly hatched juvenile was found in April last year," he said.

The release areas were visited again early last month.

Eight half-grown to almost fully grown tusked weta were found on Red Mercury and one large juvenile on Double Island.

"These weta develop into adults over about two years and then live for a further three to nine months, so finding juveniles more than a year after the last captive-bred adults were released was exciting because it confirms that they can produce their first generation in the wild," Dr Stringer said.

Breeding programmes for the common tree weta have been successful, but the rare tuskers were much more difficult to manage.

The successes were remarkable, Dr Stringer said.

"Usually when you transfer insects, especially low numbers, it is extremely rare to find that they've laid eggs - and unheard of to find them at the first nymphal stage after being hatched," he said.

"There was an attempt to put 100 or so of a darkling beetle on to an island and they were never seen again."

The good science that has allowed the tusked weta to proliferate has been difficult to monitor.

The weta dig underground chambers to live in, emerging to hunt other insects only during the darkest nights when there is no moon and it is humid and warm. "You hardly ever see them," Dr Stringer said.

"Not only do they come out only on the darkest nights, but they seem to be very choosy about when they appear.

It had been suggested that this was so they could avoid being eaten by tuatara.

"There's a few tuatara on Red Mercury, but only a handful. The thought is that tuatara breed so slowly and insects so quickly ... we do have the advantage."

Concerns that the insects might disappear to the also-reintroduced little spotted kiwi had also proven groundless.

The department's effort to spread the species around the island group follows a drive to rid the islands of mammals.

The intent has been to not only save the weta, but to establish more than one population in case fire or some other accident destroys the original group.

Whether the weta stay active and produce yet another generation will be tested when the conservation scientists carry out another survey of the islands in a couple of years.

In the meantime, science may have to take a back seat to primal insect instincts.

10 things you didn't know about weta

1: There are more than 100 species of weta.

2: The giant weta's scientific name is Deinacrida, which translates as "demon grasshopper".

3: Maori in the Hauraki Gulf called them Wetapunga, after the god of bad looks. In the South Island they were known as Taipo (demon).

4: Essentially, weta are nocturnal grasshoppers with extremely long antennae - usually twice their body length but four times for the cave weta.

5: For insects, weta enjoy long lives - 18 months to two years from egg to adulthood, and then a further six months to two years as an adult.6: Weta can weigh as much as a thrush.

7: They can survive extremes of temperature, such as being frozen in ice, but do not have antifreeze in their circulation system, unlike other insects.

8: Weta have evolved over 80 million years since New Zealand broke free from Gondwanaland.

9: In daytime, weta enter their burrows head first, so their spiny hind legs block the passage to any intruder. In the evening, they back out of the tunnel so sense organs on their rear can assess the weather.

10: Giant weta lead solitary lives but tree weta snuggle together in larger holes. Adult males are in charge, welcoming adult females as companions and tolerating juveniles. But if another mature male enters, a territorial fight ensues. Usually the male with the bigger head wins.

Source: New Zealand Geographic Magazine

* Alan Samson is a freelance journalist writing for the Department of Conservation.