Illegal insect immigrants

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In the spring of 1995—maybe earlier—an incon­spicuous moth identified as Orgyia thyellina, the white-spotted tussock moth, arrived in New Zealand from the Orient. Perhaps, like many a sailor before, it jumped ship on the Auck­land waterfront. By the time the gaudily coloured caterpillars produced by the species were brought to the attention of scientists from the New Zealand Forest Research Institute (NZFRI) on April 17, 1996, they were plentiful over several square kilometres of the tasteful Auckland suburbs of Mission Bay and Kohimarama—and causing concern over their possible impact on New Zealand flora.

In its native range (Japan, Korea, Taiwan, China and eastern Russia) the species goes through either two or three generations per year, depending on the severity of the climate. The first two generations are “normal,” in that they consist of fully-winged male and female moths, but in the last generation for the season decreasing day length triggers a change in the morphology of the female: she emerges from her cocoon with tiny stubs of wings, and is incapable of flight.

These flightless females mate and lay their eggs on their own cocoons, or very nearby, and it is these eggs which are the overwintering stage of the insect. Unlike normal summer eggs, which hatch within two weeks, these overwintering eggs are larger and darker in colour.

Local observations so far suggest that, in terms of life history, the moth is behaving here as it does in Japan.

At FRI in Rotorua, white-spotted tussock moths are being reared in quarantine to determine which plants they can feed on. By artificially increasing the apparent day length to which late-season caterpil­lars are exposed in the week or two before they pupate, normal-winged females result, which lay rapid-hatching rather than overwintering eggs.

To date, mature caterpillars in Auckland have consumed a disconcertingly wide range of local broadleafed plants including plums, cherries, grapefruit,geraniums, roses and maples, but no conifers. While Orgyia (despite the sound of its name) is not considered a major pest in its natural environment, predators which may control it mere are unlikely to exist here, increasing tile potential threat.

As expected, young caterpillars are proving more selective in their feeding than older individuals. Preliminary results on timber species indicate that red beech and at least one species of Eucalypt (regnans) support the growth of young caterpillars, but Pinus radiata and douglas fir do not.

It is likely that the infected area of Auckland will be sprayed with a biological control agent when caterpillars emerge in tile spring.

Bacillus thurigenensis strain K, a naturally occurring bacterium, would almost certainly be used for this task. The strain kills only larvae of moths and butterflies, and is harmless to other insects and organisms. It has been successfully used in North America to control the related and voracious Asian gypsy moth.

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