Hopes tiny insects can make a big difference

The recent releases of two clusters of tiny South American beetles may offer one of Northland’s best hopes in tackling a plant pest despised by conservationists, landowners and gardeners alike.

Tradescantia (also known as wandering jew or wandering willie) is a hardy, shade-tolerant pest plant that forms thick blankets that can rapidly smother and exclude other virtually all other species, especially in Northland’s relatively mild, semi-tropical climate.

However, biosecurity experts are hoping a tiny Brazilian import – the tradescantia leaf beetle (Neolema ogloblini) - may yet prove an effective weapon against the plant, courtesy of the insect’s healthy appetite for tradescantia leaves.

The beetle is native to south eastern Brazil and north eastern Argentina and although it was imported several years ago by Landcare Research, it is only now being released as a biocontrol agent in this country – reportedly the first time it has filled that role anywhere in the world.

Northland has recently seen two releases of several hundred adult beetles – the first in the Pataua area near Whangarei at the end of March and most recently in the Kerikeri Basin in late April. The releases have involved local landowners, conservationists and weed control groups, the Department of Conservation and the Northland Regional Council.

Dr Jenny Dymock, an entomologist who works with the Northland Regional Council, says the adult beetles are about 5mm long and a dark metallic bronze in colour with slight iridescence and were bred at Landcare Research's Christchurch facility.

She says the beetle is host-specific, with extensive research by New Zealand scientists revealing other plants species are not at risk from it.

Dr Dymock says the Northland releases coincide with similar releases in the Manawatu and Auckland areas.

Beetle numbers – and the damage they cause – will be closely monitored over the next few months to ensure the insects are breeding in the wild successfully.

Another two beetle species, one which feeds on tradescantia stems and the other its tips, are also being bred by Landcare in Christchurch to further increase the number of control tools for the plant.

Don McKenzie, Biosecurity Senior Programme Manager, for the Northland Regional Council, says all going well all three species of beetles will be able to be distributed at targeted sites around Northland within the next year or two.

“This is cost-effective pest control which does not require the use of herbicides or other chemicals. While it is definitely aimed at reducing the impact of tradescantia, people need to realise it’s not a ‘magic bullet’ and we don’t expect it to remove every last plant.”

Mr McKenzie says there are currently a number of biocontrol agents at work in Northland including insects, fungi and rusts which attack a variety of pest plants and insects including tropical grass webworm, mistflower, gorse and ragwort.