Soft, furry and a danger to native forests the tale of the white-spotted tussock moth

Forest and Bird, Issue 281, 1 August 1996, Page 14

Soft, furry and a danger to native forests

the tale of the white-spotted tussock moth

Sometime in the next month or so, one of the most expensive tracts of this country’s real estate will be sprayed in an aerial eradication attempt without precedent in New Zealand. GERARD HUTCHING explains why conservationists should be worried about the recent arrival of the whitespotted tussock moth.

fo T STAKE IN Auckland’s \ proposed spray eradication is not only the future of roses, plums, peaches and apples of the city’s onan and Mission Bay suburbs, but also native and plantation forests in the remainder of New Zealand. The target of the airborne exterminators is the white-spotted tussock moth (Orgyia thyellina), so called because during its fully grown caterpillar stage the striking animal sports four distinctive patches of hairs resembling tussock on its back. In a light moment one wag from the body responsible for the eradication campaign, the Ministry of Forestry (MoP), christened the moth "orgy" — a reference not only to its Latin name but also its voracious appetite. Birch, oak, maple, willow, Douglas fir and some eucalypts are other species known to be on its menu. MoF would prefer that the moth could be eradicated other than by blanket aerial spraying, but with no natural predators in New Zealand it will otherwise spread

unchecked. Like the closely related, and even more threatening, Asian gypsy moth, the tussock moth also hails from Asia — specifically Japan, Korea, Taiwan, China and the Russian Far East. Being native to that region, it appears to be in balance with its environment; occasionally it goes through population peaks when, in certain areas, it becomes a pest until the normal checks and balances in the way of parasites and predators reassert themselves. Just how the moth arrived in New Zealand is unknown. Possibilities include egg clusters attached to a second-hand Japanese car import, via a ship, in a container or with personal effects. Less is known about the ecology of the tussock moth than the gypsy moth, but sufficient to ring alarm bells among New Zealand authorities. Besides the fact that the moth can go through three generations in a season, and one female can be

responsible for several million moths in that time, laboratory trials have also shown that the caterpillar behaves like a possum — that is, it eats out a favoured food first then moves on to less fancied plants. Forest Research Institute entomologist John Bain found that caterpillars had different diets according to age. The younger preferred blackberry but as they grew older they moved on to pine and other trees. In Auckland gardens it has been recorded feeding on plum, peach, cherry, roses, willows, kakabeak, grapefruit, Acer negundo, apple, birch, oak, wisteria, geranium, coral pea and possibly also on bottlebrush and titoki. While the caterpillar has been found

feeding mainly on horticultural and other exotic trees, that is not a measure of its potential risk to native forests, but more the nature of the urban area it has been liberated in. Feeding trials at the Forest Research Institute have shown the caterpillars will feed on red and silver beech trees, and all expectations are that given the breadth of the animal’s food range in inner Auckland and its ready adaptation over a short time

to New Zealand conditions, it would happily adapt to a wide range of native trees such as kamahi, kohekohe and pukatea throughout the country. Overseas research shows the caterpillar likes to feed on broadleaf hardwoods similar to those that make up the major component of New Zealand forests. If eradication is unsuccessful, and without any natural predators or parasites in New Zealand to keep the moth and its caterpillars in check, the damage to the conservation estate could be of possumlike proportions as the pest inexorably spreads throughout the country.

OF’S GENERAL manager of operations, John Handiside, is under no illusions about the difficulty of the task of eradicating the moth. "There’s been nothing attempted like this in New Zealand before, in fact no-one has tried to eradicate the tussock moth anywhere in the world, but by god we’re going to do it if we can," says Handiside. Authorities first heard about the existence of the moth in New Zealand on 17 April this year. However it is believed to have been in Auckland at least since spring 1995. As soon as MoF went public over the issue, a local doctor came forward to say he had taken a photo of a caterpillar the previous November but had not informed anyone. It is thought to be the first time the

tussock moth has been introduced to any country outside its natural range. By April 1996 the moth had managed to munch its way around seven square kilometres of Auckland’s leafiest suburbs, spreading not only as an adult by flying but also as a caterpillar by being blown on silken threads (known as ballooning). MoF is concerned too that egg masses — a

hundred or more clustered together the size of a five-cent coin or smaller — may have been transported out of the infected area as garden waste. That, however, is a worst case scenario. Assuming the moth has been contained within the seven-square kilometre area, MoF has drawn up an action plan to spray 40 square kilometres (30,000 households)

of Auckland’s eastern suburbs with the organic spray Foray 48B, containing the active ingredient Btk (Bacillus thuringiensis variety kurstaki). A naturally occurring bacteria found in the soil, Btk becomes active only in a caterpillar’s uniquely alkaline gut, posing no danger to plants, animals or insects other than moths and butterflies. All North American studies show no human health problems associated with the spray. As soon as the eggs that scientists are keeping a close eye on show signs of hatching (probably September or October), either helicopters or fixed wing aircraft will swing into action, covering the area by night in a fine mist three to four times over the course of several weeks. Assisting New Zealand’s efforts to get rid of the tussock moth is the experience gained from localised North American eradication operations on the Asian gypsy moth. First accidentally released from a laboratory in 1869 in the north-east of the USA, the Asian gypsy moth has never been completely eradicated from North America. In parts of the west coast where orcharding is important to the economy, aerial spraying has eradicated discrete populations of the moth but in the north-east it has been controlled only. "They've done so much work and we’ve been able to learn from that. We’ve discovered that sometimes spraying hasn’t worked because they haven’t put a large enough application on, or perhaps because they haven't had a large enough buffer area around the critical zone, or possibly it’s rained after they've sprayed and they haven’t sprayed again," says Handiside. Residents of Canadian cities such as Vancouver (19,000 hectares sprayed), Victoria (116 ha), Hope (90 ha), Richmond (430 ha) and United States cities such as Portland (3,000 ha) have all learned to live with low-flying aircraft showering their homes and gardens with Btk. John Handiside says he understands some people’s fears of spraying but believes the alternative — yet another pest attacking native forests and damaging a $2.6-billion export timber trade — is worse to contemplate than the disruption caused to some Aucklanders. "None of us wants to be sprayed and none of us want to be woken in the middle of the night. Some of the people in the area are very opposed to what we dillar

are trying to do and would prefer the moth to be here along with the consequences of that, but the number is minuscule compared to those who are in support," he says. Forest and Bird has strongly supported the planned eradication and has lent its name to MoF’s local poster campaign. While non-target moth and butterfly populations will be damaged by the spraying, MoF expects that the populations will be restored to normal levels within three years. In 1993 Handiside visited Vancouver a year after the largest-scale aerial spraying operation ever carried out in an urban area. "The Vancouver authorities were very pleased with the way the operation went. Technology has progressed since then and we can benefit from newer, quicker and safer ways of doing things. For example, well probably be using much less spray than the Vancouver operation — only about half a bucket to cover an area slightly larger than a rugby field," says Handiside. » EPENDING ON who you talk to, _ the estimates of success of the "ml $5.5-million eradication operation range from 80 percent likelihood to it’s already too late and the moth will have spread outside the targeted area. If eradication does not work, there are a variety of other controls that could be tried, including chemical insecticides, biological control agents, mass trapping, mating disruption and sterile insect techniques. While, in the US, scientists have tried in vain to introduce parasites as biological control agents to control Asian gypsy moth, no biological agent has even been identified overseas to control tussock moth. Even if an agent is found, it will control the moth, not eradicate it. One hope is the knowledge that at least four natural parasites of tussock moth have been recorded, with more likely to be discovered. Male tussock moths could be attracted to traps with a pheromone or chemical attractant. Problems associated with this method are that it will not wipe out the moth, and the pheromone may attract non-target species. As yet, a specific tussock moth pheromone has not been isolated, but sample lures are being trialed. In North America, pheromones have been successfully used to lure male gypsy moths away from females during the mating season. Pheromone flakes or beads are dropped by aircraft, confusing the males and preventing them finding the females.

The s chances of a further hi-tech option — moth sterilisation — working are even more remote. Using this technique, scientists hand-rear large numbers of females, sterilising them before releasing them. They then mate, producing sterile eggs. In small field trials the technique has been shown to work, but it is doubtful if it would succeed on a wide scale. Based on the moth’s spread so far, scientists estimate that in ten years it could cover 400 square kilometres — provided humans do not unwittingly lend it a helping hand by carrying eggs with garden debris. But that is the big risk. Despite MoF’s request to residents not to move uncovered garden waste out of the area, there have been reports of at least six uncovered trailers with garden waste on them seen in one day in the affected zone. Should spraying not eradicate the moth, more draconian measures will be needed

to stop residents from such practices — a small price to pay to stop the pest from spreading. Until the tussock moths eggs begin to stir with the arrival of warmer weather, Ministry of Forestry staff are working overtime to inform residents of their plans. For the moment it is a waiting game, until John Handiside’s "weird looking beast" unsuspectingly ventures out into an Auckland spring to meet its unexpected fate ... Sy

GERARD HUTCHING 1s a freelance journalist based in Wellington and a former editor of Forest & Bird.