Aerial enemy for hungry worm

By TONY GEE


A Korean wasp and a fly are likely to become the bio-control allies of Far North farmers whose pastures have been devastated in recent years by infestations of the voracious tropical grass webworm.

The wasps, already widespread in New Zealand, confirmed their potential as a bio-control agent after they were found in webworm larvae collected from kikuyu pasture on the Aupouri Peninsula north of Kaitaia, and reared by AgResearch at Ruakura.

Farmers on some peninsular farms lost dozens of hectares of pasture in days when armies of webworm caterpillars attacked in 1999.

More infestations came in 2002.

At densities of around 15 million a hectare, the caterpillars could chew out several hectares of pasture in less than 48 hours.

As well as kikuyu, the pests eat anything green in their path.

The tropical webworm, a recent arrival in New Zealand, is believed to have been blown across the Tasman in adult moth form from Australia.

Each adult moth lays up to 400 eggs that hatch into caterpillars.

Researchers in a task group on the peninsula have found that biocontrol of the webworm is possible through a combination of parasitic insects which attack the pest in its caterpillar stage.

Principal among these is a Korean wasp, Meteorus pulchricornis, which is also found in Asia and Europe.

An entomologist working with the task group, Dr Jenny Dymock, said the wasp was collected from sites around New Zealand where it was found attacking 15 caterpillar species from the moth and butterfly group of insects.

The wasp lays an egg in young caterpillars.

When the wasp larva hatches, it feeds on the caterpillar and eventually kills it.

Dr Dymock said she got the Meteorus wasp from a colony raised by Crop and Food Research at its laboratory in Auckland and tried it against webworm larvae in test tubes and over kikuyu pasture on the Aupouri Peninsula.

Dr Dymock found the wasp attacked and killed the larvae. It also attacked armyworm collected from local maize.

Ruakura scientists are also investigating another parasitoid reared from tropical grass webworm.

This potential agent is from a fly family known as Tachinidae.

Dr Dymock said a combination of the parasitoids could offer farmers a long term and effective way of suppressing webworm.

Task group chairman Eric Wagener said periodic webworm infestations continued on peninsular farms but not on the scale of the "explosions" five years ago.

At that time, Mr Wagener's Houhora farm lost 30ha of pasture in three days.