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What is biological control?

Biological control is the use of one living organism to control another.  This usually involves the permanent introduction of a natural enemy of the target weed from the country of origin of the weed.  Once a population has established, the natural enemy maintains itself from year to year, spreads itself from place to place, and regulates its numbers in response to the abundance of the weed (when the weed becomes uncommon, the natural enemy will also be uncommon).

How does it work?

These plants aren’t weeds at home. For example, gorse is treasured in the nature reserves of southern England. It never grows tall, and there are so few plants you can walk amongst it. It is no more dominant than any other plant.  Plants growing in their home range are attacked by insects, mites, diseases and other organisms. In many cases these are the factors that limit the health, size, rate of spread, and density of the plant. Most plants introduced to New Zealand were imported without the natural enemies that kept them in check at home, and some have responded by becoming aggressive and weedy. Biological control tries to restore the balance by introducing some of the natural enemies that attack the plant at home, but only selecting those that are both safe and effective.

But insects are pests, aren’t they?

Well, no. The public perception of plant-feeding insects is biased by images of plague locusts and similar super-generalists.  Insects that can ‘eat anything’ are the exception, not the rule. Most insect species feed on just one plant or on a small group of closely-related plants. Most never come to our notice because they are hidden away, carrying on generation after generation on the same anonymous host plant.

We know some of these specialist insects very well. Everyone accepts that monarch butterflies only lay eggs on swan plants, and that white butterfly caterpillars only feed on cabbages, cauliflowers, and other close relatives such as turnips. It is insects like these that biological control researchers choose for evaluation as control agents. The same applies to diseases that are used for biological control of weeds.&nb

How do you go about biological control in New Zealand?

  • Find out what is attacking your weed in New Zealand. There is no point in introducing something that is already present, or introducing an agent that will compete with an insect or disease that is already doing a good job.
  • Find out what attacks the weed where it came from (Europe, Central America etc) using field surveys, the scientific literature, museum specimens etc.
  • Select from that list the species that seems to be the most damaging to populations of the weed in its home range, and at the same time can feed on only a narrow range of plants. This is not as hard as it sounds. Most plant-eating insects in the world feed on only one or two host plants.
  • Check safety by experimentation. Define exactly which plant species the natural enemy can lay eggs on and eat.
  • Seek permission to introduce the natural enemy to New Zealand.
  • Import a population into quarantine and rear it to ensure that no parasitoids or disease-causing organisms are present.
  • Release from quarantine, rear large numbers, and release into the field.
  • Monitor establishment and spread.
  • Evaluate success in limiting the health and abundance of the target weed.
  • This process may take 10-20 years for each control agent. The key point is that the agent must be safe.

What does ‘safe’ mean?

A biological control agent is safe to introduce into New Zealand if populations do not significantly harm New Zealand’s environment, economy, human or animal health or social structure. Amongst others, key rules are:

  • The agent must not cause significant damage to populations of any native plant.
  • There must be little if any chance of the agent interfering with how New Zealand ecosystems work
  • If there is any non-target damage, it must be heavily outweighed by the benefits to be gained by introducing the control agent. 

How do you prove it is safe? How can you be sure they won’t damage other species? What do the agents eat once the weed has gone?

You can’t test every native plant to see if it is safe from the control agent – there are just too many too test. But if you only test a selection of native plants, how can you assess the risk to the plants that you don’t test?

Biocontrol researchers have been safety testing biological control agents for weeds since the 1920s, and all projects are well-documented. In the early1970s, An Australian scientist used that assembled information to formulate a set of protocols for safety-testing biological control agents. Those protocols have been used ever since, and 30 years of experience suggests that they predict risk well.

Evolutionary theory tells that as plant populations evolve and change, and eventually become new species, the specialist insects and diseases that attack those plants tend to evolve as well.  This is called co-evolution. The evidence for this is all around us. For example, many of the plants in the same plant tribe as broom have weevils that feed in their pods. The plants are closely related, and the weevils are closely related, but the weevils are only ever found in the pods of their own plant - gorse seed weevil attacks gorse but not broom, broom seed weevil attacks broom but not gorse. There are many other examples. If this close tracking between herbivores and hosts is true, then specialist insects feeding on a target weed are more likely to feed on plants that are closely related to the weed than on plants that are less closely related. If you can only test a small number of plants, it is therefore more important to test the plants that are most related to the target. Once you have determined the level of specificity (for example, the agent feeds on several species within a genus, or several species within a family), you can pretty much dismiss any risk to plants that are less closely related.

Apart from being well-supported by science, this is also common sense. If your own observations tell you that white butterfly larvae feed on brassica-like species, why would you test pine trees.

Introducing a new species always upsets the ecological balance, doesn’t it?

It depends what you mean; not all change is bad.  When any new organism is introduced to an ecosystem, relationships between species in that ecosystem will change.  This is clear. The organism might simply occupy space that another species previously used, or might become prey for generalist predators such as spiders or predatory mites. What we need to know is whether the changes are ecologically important, and whether the change is positive or negative for the environment? In many cases the answer to these questions is sometimes yes and sometimes no, depending where you are. Ecosystems vary from place to place.  What we know to be true in one place may be false in another.

There is perpetual change in the New Zealand environment, especially in areas modified but humans (and what areas haven’t been). Is the change associated with biological control important when compared to other changes occurring at the same time?For example, how important is the introduction of the agent to a habitat compared to the change in forest ecosystem function as possum numbers go up and down? It is questionable whether there is such a thing as an ‘environmental balance’, and accurately assessing risk under this level of uncertainty is almost impossible. It is a matter of judgement, and a major point of discussion in all EPA applications

Aren’t plants in the same family really closely related?

Not really. Family is a relatively high level in the plant classification and can be subdivided into sub-families, then tribes, then sub-tribes then genus then species. It is usually only at the species level that plants can be regarded as very closely related. Some families don’t have all subdivisions, but all have genus and species. Here are some examples of relationships in the bean family:

  • Wattles and broom are in different subfamilies of the bean family, the Fabaceae
  • Broom and clover are in different tribes within the subfamily Faboideae of the family Fabaceae
  • Broom and gorse are in the tribe Genisteae but each in a different genus
  • White broom and scotch broom are separate species in the genus Cytisus
  • Dwarf green beans and runner beans are separate species in the genus Phaseolus but
  • Mung beans and dwarf beans are in different genera within the sub-tribe Phaseolineae and
  • Soybeans and dwarf beans are in different subtribes within the tribe Phaseolea

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