It may interest you to know that this June, ERMA approved the release of a species of fungus into the New Zealand ecosystem.
The fungus is called Uromyces pencanus, and it is being released as a biological control agent for Chilean needlegrass. This means that ERMA considers Chilean needlegrass to be a pest, and they are trying to find a natural parasite to prey upon it and remove the problem.
Of course, the question one always has to ask when using biological controls is: what else is the control species going to affect? Biological control has got a bad reputation for causing huge unintended problems. A pertinent example is the introduction of cane toads into Australia to suppress the problems of cane beetles. Most of us know how that turned ou—not only did they suppress the beetles, but they were enormously successful in preying upon other native insects. They also out-competed native lizards and amphibians, resulting in a huge loss in biodiversity in Australia. Another example more close to home—stoats and ferrets were originally introduced to New Zealand as biological control agents to suppress rabbit populations. Turns out the stoats acquired a taste for native birds and eggs and have added to the problem possums began. And we are still inundated with rabbits!
Fortunately for us, it looks like the ERMA has done its homework this time around. They have reported that the fungus was tested on 65 closely related species, including species expected to grow in the same environment, and that the fungus did not take root in any of the species besides the Chilean needlegrass. What this tells us is that the risk of the fungus spreading is minimal. The biggest risk with biocontrols is for the agent to spread to other unexpected species and adapt to them. However, if the agents are unable to grow in other places (in this case, on other plants) then they have little opportunity to adapt and spread. And just to be a little safer, the Uromyces pencanus has also reported to have been tested on a number of native flora species in a separate document.
Incidentally, there has been some research into the use of biological controls for possums in New Zealand. Naturally existing possum predators and parasites would cause too many problems if introduced to our native forests (see stoats above), so more complicated methods are required. Mostly these involve genetically modifying micro-organisms, which needless to say is not a popular approach. There have been a few years of research, but nothing solid has yet been developed.
Biological controls can be a useful and cost-effective way to deal with invasive species into our ecosystem. However, due to the potential irreversibility and severity of the side effects, thorough research is a must. New Zealand is of course particularly vulnerable to new organisms due to its isolation and fragile native species, so we must be extra careful. *