Broom Agent Success
Hugh Gourlay examines successful gall mites. Image – Tom Fraser
Scientists are celebrating some early Christmas cheer with news that two biocontrol agents for nasty pest plants have successfully established.
Broom gall mite was released a year ago on broom plants around the Landcare Research Lincoln campus and has been found to be well established, while an investigation of another release site in North Canterbury will be undertaken soon to check on whether the mite has become well established there too.
Meanwhile, there have been confirmed outbreaks of the green thistle beetle Cassida rubiginosa, which targets Californian and other thistles, released in Otago and Southland a year ago.
Scientist Hugh Gourlay says the establishments are ‘fantastic news’. Establishing a new insect in a foreign environment is difficult, but once we succeed we know the insects will always be out there eating our weedy plants, he says.
The broom gall mite attacks Scotch broom (Cytisus scoparius), which is a highly invasive weed that forms dense impenetrable thickets throughout most of New Zealand. It is unpalatable to livestock, shades out desirable pasture species and out-competes native plants.
Mr Gourlay says the mite gets into new buds and forms a gall which deforms the new bud and stops it from growing and flowering and seeding.
‘Californian thistle spreads via its extensive root system and it’s not uncommon to have some roots spreading several metres from a single plant,’ he says.
‘The Californian green thistle beetle has the potential to have a major impact on Californian and other thistles because both stages (the adult beetle and larvae) consume so much leaf material. Previous work has shown that if you remove the foliage of the thistle then you also reduce the amount of root material growing below ground.’
Mr Gourlay will also undertake a number of releases of the broom gall mite into plantation forests around Ashley Gorge over the summer months as part of the wider programme to establish biocontrol agents that act against broom.