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Biological control of pests

From Biosecurity 64, December 2005.

Biological control of pests is well established in New Zealand and is increasingly supported by science. It is cheap, safe, self-perpetuating and can be very effective. The history of biological control in the pastoral sector is interesting and varied and, in spite of its squeaky-clean reputation, it has not entirely been without its detractors.

For quite some time after World War Two, pest management in New Zealand pastures was readily achieved using various manifestations of DDT. DDT was cheap, very persistent and quite often mixed with superphosphate. All in all, pest problems were seen to have been defeated once and for all. Indeed, much of the pest research literature at this time is focused on ever•more imaginative ways to deliver DDT (and its near relatives, organochlorines).

Doubts about this ‘solution’ surfaced in 1962 with Rachel Carson’s book ‘Silent Spring’ when it was becoming increasingly apparent that DDT was having a negative environmental impact (such as thinning bird eggshells). DDT was banned from use on New Zealand pasture at the end of the 1960s although, because of its persistence, its ‘benefits’ continued for some time.

New Zealand’s response at the time was to try other insecticides. These were mainly in the form of organophosphates that break down quickly in the environment but tend to be very poisonous, and bird kills were not unknown. In fact, widespread use of these chemicals never really occurred and synthetic pesticide use for broadacre control of pasture pests faded. It was then that biological control made its first serious appearance.
A number of ‘hippy’ commentators (including the author) declared biocontrol to be the solution to pests; you simply went to the country of origin, found a natural enemy and released it into New Zealand to redress the balance. This approach was taken up with alacrity but it was rapidly recognised that there was not an ideal biological control solution to every problem. Even when it did work it was not always consistent, working in some places but not others, and in some seasons but not others.

Take that! A parasitic wasp lays its eggs in an unsuspecting clover root weevil.

Since then a great deal of work has gone on to understand the mechanisms that support biological control. In particular, integrated pest management was developed, in which a number of approaches are directed at a pest at the same time. For example, in the last 20 years the problems caused by Argentine stem weevil – estimated to cost the economy between $70 million and $250 million annually – have been significantly reduced through a combination of plant resistance (using endophytes) and parasites collected in South America.

Another approach has been to discover insect diseases and formulate these into biological pesticides. This has required some ground-breaking inventiveness to ensure the diseases survive long enough once released to impact on the pests. AgResearch developed the use of ‘biopolymers’ to make this possible, and this technology is now being used in all sorts of applications. Likewise, biocontrol can be used for the control of weeds in a similar way. Insects that feed specifically on weeds may be introduced. Or diseases of weeds may be reformulated and applied as bioherbicides. It is the detailed analysis of the behaviours of the pests and control agents over many years that has allowed integrated pest management to be developed to manage many pest species. Such long-term research has resulted in remarkable success in the use of natural enemies against pasture pests.

It can be speculated that many pests from overseas, like the Argentine stem weevil and the lucerne weevil, build up enormously high numbers through a lack of natural enemies and plentiful food plant species (such as pasture). It can equally be speculated that the reason appropriately chosen biological control agents have done well also has something to do with their lack of natural enemies.

Most recent work has involved the use of DNA to match biocontrol agents to pests. Some species of control agents are found all around the world and it is useful to use biotechnology to close in on which ones to use. There has been ongoing concern that biological control agents may attack species other than the pest and adversely affect native populations. In response, AgResearch scientists over the last 15 years have worked assiduously to minimise this possibility.

Currently, a major effort is under way to find a biocontrol agent that may be active against the clover root weevil [see sidebar], thought to cost the economy about half a billion dollars annually. With this, though, it is important that any agent that is brought in is tested to ensure it does not damage native insect populations. The results of a comprehensive study are now being examined through the ERMA-based process.

Dr Stephen Goldson, Chief Science Strategist, AgResearch

This article was first published in AgResearch’s Now magazine in October 2005 and is reproduced with permission.



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