Bugs make short work of buddleia

Lynne Douglas' buddleia bush is being eaten to ribbons by introduced weevils. Wanganui Chronicle Photograph by Lewis Gardner

The new bugs Whanganui resident Lynne Douglas has found in her garden will eventually keep a pest plant to low levels, Horizons Regonal Council's Craig Davey says.

The tiny bugs and their slug-like larvae were new to Mrs Douglas, a keen photographer and observer of nature. They were eating the bushes (Buddleia davidii) she had planted in her garden to attract butterflies.

Her bushes may not last long, because they are a species unwanted in New Zealand and a biological control insect species has been released to control them.

Buddleia was introduced from China in 1946, and became a pest plant, especially affecting new forestry plantations.

It can tolerate a wide range of conditions and spreads fast through seeds and suckering, Mr Davey said. The bushes form a canopy 3-4m high, with smaller seedlings underneath, and they smother other plants.

Buddleia is not listed in Horizons' pest plant strategy, and it is so widespread that attempting to control it by spraying is impossible.

Crown forestry research institute Scion found a bug to control it, and Horizons released the bug here too. It's a little weevil, Cleopus japonica, and is also from China. It was first released in the central North Island in 2006 and has been moved throughout New Zealand.

"The bug itself looks really cool, through a microscope. They're like little elephants with big long noses," Mr Davey said.

In the Horizons Region the bugs "sat around for a while" but are now making an impact.

They are right across the region, from the Ruahines to Whanganui, and starting to move into areas where buddleia is entrenched.

The weevils have a boom and bust pattern - eating a plant to nothing, then flying to another plant. They re-appear when there is another flush of leaves.

"Some of these trees are actually just dying. They don't have enough reserves from their short period of life to carry on," Mr Davey said.

The weevils are likely to keep buddleia numbers in balance with other plants, and the change will happen gradually.

Mrs Douglas isn't best pleased, and members of the Monarch Butterfly Trust are worried there will be less nectar available for butterflies. Propagation of the plant isn't banned, but it may become harder to source and not last long for those who have it.