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  • Here, we describe transmissible and non-transmissible methods for delivering biocontrol agents to possums.

    Scientists in New Zealand investigated a range of biocontrol agents to reduce possum numbers, because possums are a pest. Biocontrol agents may be toxins or vaccines against specific possum proteins involved in reproduction.

    Delivering the biocontrol agent to possums is difficult because most possums live in dense bush or mountainous areas, which are not easily accessible.

    Transmissible versus non-transmissible delivery

    Transmissible delivery systems spread through the possum population, whereas non-transmissible delivery systems do not spread.

    Transmissible delivery systems include:

    • viruses
    • parasites.

    Find out more in the article, Biological control of possums.


    Transmissible delivery systems are expensive and time-consuming to develop, but ultimately they are cheaper and more cost effective to deliver, because they only have to be introduced once (or very few times) and will spread through the possum population.


    Transmissible delivery systems are not as easy to control. If adverse effects occur, for example, the organism mutates or ceases to be effective, it will be difficult to remove the transmissible agent from the environment.

    Non-transmissible delivery systems include:

    • baits, which can be dropped from the air or used in bait stations on the ground
    • aerosols, which spray the biocontrol agent into the possum’s nose or eyes
    • transgenic plantsgenetically modified plants that produce the biocontrol agent in their leaves or fruit. The plants could be fed directly to the possums or the biocontrol agent could be purified from the plant and made into a bait that is delivered to the possums. No research on transgenic plants is currently being done.


    Non-transmissible delivery systems are under our control. We can target where and when to apply the delivery. If adverse effects occur, delivery can be stopped.


    Non-transmissible delivery systems are expensive, labour-intensive and time-consuming. Repeat applications of the toxin or vaccine are needed to ensure all the possums in a population are treated.

    Genetically modified organisms and biocontrol

    Biocontrol agents that use genetically modified organisms (GMOs), like the parasitic worm, were also studied. GMOs could be delivered by transmissible or non-transmissible delivery system. However, some people oppose the use of GMOs in New Zealand.

    An update

    In a 2011 report, Dr Jan Wright, Parliamentary Commissioner for the Environment, wrote that although significant research effort and resources were put into possum contraceptive vaccines and hormone toxins, funding for the projects finished in September 2010. There was concern about the length of time required to produce results, risks associated with the biocontrol agents and potential controversy over genetic engineering.

      Published 18 December 2008, Updated 17 September 2018 Referencing Hub articles
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