Radio NZ has been reporting on the problem of weeds, both in native vegetation and on farms and orchards, and on the the Department of Conservation’s War on Weeds.
One interviewee was Hugh Gourlay, from Landcare Research, who discussed (here) his work as part of a team that works to find biocontrol agents to kill or knock back the worst weeds.
The work involves visiting the country of origin of a plant and hunting for diseases that infect it. The team is also looking for insects that attack and eat any part of the weed, including the seeds, flowers, leaves or stems.
Hugh told Radio NZ it is sometimes possible to “knock a plant invasion on the head” with prompt action to hunt down and kill all the plants. Chemical control, using herbicides, is usually the next step, but at best it will “keep the problem at bay.” This leaves biocontrol as the best answer for widespread weeds.
Once potential biocontrol agents are identified they are imported into New Zealand and kept in strict quarantine, so they can be tested to see if they attack any plants other than the weed. “If they don’t attack anything else then we can look to release them.”
Describing some great biocontrol successes in New Zealand, Hugh cites St Johns Wort control in the 1960s as a “guiding light” success story. Ragwort (Jacobaea vulgaris), some thistle species and mist flower (Ageratina riparia) have had successful biocontrol agents introduced.
More recently, the broom gall mite and several species of beetle that attack Tradescantia have been showing great promise.
Darwin’s barberry is among Hugh’s recent projects.
“Darwin’s barberry continues to advance across the countryside in Otago and Southland. It takes over entire hillsides, and becomes quite a monoculture.”
Beginning in the early 2000s, the biocontrol team identified a seed weevil and a flower weevil in the plant’s native range in South America. The focus has been on the seed weevils, which have been released over two years.
Radio NZ’s “Our Changing World” recently featured a story about the wasp mite that is being investigated as a potential wasp biocontrol agent. It has also looked at the buddleia leaf weevil.