Fighting aliens with enemies

Dr Candice Lyons, a biological scientist at the Agricultural Research Council, tells us how to fight alien-plant invasions with insects and pathogens. Gone are the days of uprooting and harsh chemicals.

Invasive species are detrimental to natural environments and indigenous plants, some of the most harmful impacts being their increased water usage in drought-prone areas, competition for resources with indigenous species, and the impact they have on increased fire loads and changing fire dynamics in a fire-prone habitat such as fynbos.

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Invasive plant species may also be toxic to livestock, or cause skin irritation. To manage invasive species, we make use of mechanical, chemical and biological means. Most people are familiar with the former two, but probably fewer are aware of the latter. Biological means, or biocontrol, makes use of an invasive plant’s natural enemies, be it insects or pathogens, to control the spread of this invasive plant (weed).

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People unfamiliar with the science often query the safety of such a practice. Biocontrol aims to control the weeds of concern and ensure host specificity and no spillover effects into other crop species or indigenous, non-target plant populations. To do this, researchers often spend years investigating potential agents (insects or pathogens), which includes a detailed literature review, several overseas surveying trips, and in-depth experiments to ensure the host specificity of the targeted agent, and that only the targeted plant or weed succumbs to the effect of the introduced agent.

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The role of biocontrol is not to kill the weed population but to reduce it to manageable levels whereby integrated control strategies, such as a combination of mechanical- and biocontrol, may be employed. There is little to no risk of these introduced agents shifting their feeding or breeding behaviour to non-target plant species.

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South Africa has been involved in several very successful, high-impact, biocontrol programmes for more than 100 years. The most famous of these is the control of the prickly pear (genus Opuntia). Several decades ago, prickly pear grew in such high abundance and density that livestock would become trapped in these fields and die. With the introduction of the cochineal insect and prickly pear moth, the abundance and density of prickly pear have been reduced to nothing across landscapes where it used to be the sole and prominent feature. The South African biocontrol community encompasses government, universities, and public-funded entities.

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There is a wide array of people involved in research into the biocontrol of several of the world’s most damaging invasive plant species that have become established, and often widespread, in South Africa. Some of the species of focus include famine weed (Parthenium hysterophorus), black wattle (Acacia mearnsii), water hyacinth (Eichhornia crassipes), and lantana (Lantana camara).

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The success or failure of these programmes varies and depends on several factors, including the environmental impact of fires and climate, among others. As a result, the research into biocontrol of invasive weeds is dynamic, exciting and rewarding.

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