Biocontrol for pest plant broom

Posted on 15 December 2008

Extra pressure is being brought to bear on the pest plant broom with the introduction of the broom shoot moth in the Wellington region.

The broom shoot moth (Agonopterix assimilella) was released on broom plants, as a biological control, at Porirua Reserve last week by Biosecurity staff from Greater Wellington Regional Council (GW).

Broom (Cytisus scoparius) is largely an agricultural weed that takes over pasture, in a very similar way to gorse. It grows to about four metres, is dark green and has yellow flowers, although it does not have prickles like gorse. It can also be a problem in newly planted forestry blocks where it out-competes young trees and can out-compete native plants in exposed areas with poor soil.

GW Senior Biosecurity Officer Richard Grimmett says the caterpillars of the moth feed on the leaves of the broom plant.

“We are hoping that once caterpillar numbers build up on a plant there is the possibility that they may strip the plants bare. But as with most biocontrols, that would take many years.”

While broom is a pest plant that spreads quickly and dominates in New Zealand conditions, it does not cause the same problems in its native habitats of Europe and northern Asia.

“This is because there are a host of native predators (insects and fungi) that stop it from becoming a pest in its native ecosystems. We are trialling introducing some of these predators as biological controls in New Zealand. We are trying to reduce the plant’s ability to reproduce and grow healthily and hope this will make it less of a pest.”

“Once the moth establishes itself at this site we can look at moving it to other sites in the region.”

Fast Facts
There are currently six bio control agents working on broom within New Zealand. Four of these are currently in the Wellington region. There are broom leaf beetle, broom seed beetle, broom twig miner and broom psyllid.

There are biological control agents working on a range of pest plants in the Wellington region including on gorse, ragwort and thistles

Biological control agents work together by attacking different parts of the plant such as the seed, leaves, roots and weakening them enough that they may become less of a threat or may even kill them.