In New Zealand, most of our weeds are introduced, either accidentally or intentionally, from overseas. But they usually arrive here without the attendant insects and diseases that keep their numbers in check in their country of origin. And so they flourish here unhindered.
Manual labour and herbicides are familiar weapons in the seemingly endless war against many of these weeds. However, quietly and often un-noticed, there is an ever-swelling army of insects and diseases helping with weed control work.
Landcare Research scientists, with the assistance of central government and regional councils, have a programme of identifying and importing some of these organisms to help in the battle against some of our more serious weeds. These ‘biological control agents’ are rigorously tested before release in New Zealand to ensure that they attack the targeted weeds only.
Biological control of weeds is not new. One of the great ‘bio-control’ success stories began in 1943 with the introduction from Europe of a beetle to control St John’s wort, then considered one of our worst weeds. A wave of black beetles subsequently advanced across the countryside devouring all the St John’s wort plants in their path. The result is that this particular weed is no longer a problem.
Today there is an array of insects at work on gorse, ragwort and old man’s beard to name a few, and the search is on for agents to control many more weeds such as tradescantia (Tradescantia fluminensis, otherwise known as wandering Jew or wandering Willie) and moth plant (Araujia sericifera).
One bio-control agent to keep an eye out for at the moment is the gorse spider mite, which sucks the sap from the plant, eventually killing all or part of the bush. Gorse bushes infested with gorse spider mite are very distinctive; look for pale green branches enclosed in a fine white webbing. A closer look at this webbing will reveal masses of the tiny red spider mites, all working away to reduce the vigour of this invasive plant – a heartening sight for those involved in the fight against weeds.