Insects to control infestation

The end of invasive species in sight

19 November 2018 | Environment

The Namibian Chamber of Environment is proposing the biological control of alien invasive cacti in the capital.

Yolanda Nel - Large areas of the capital have already been infested with three specific cactus species that are preventing other plants from growing.

For this reason, the Namibian Chamber of Environment, together with the Botanical Society of Namibia and the Rhodes University Centre for Biological Control in South Africa, are working on introducing biological control against these plants.

A public notice that appeared in various media, stated that the project entails releasing three host-specific insect species to control infestations of these specific invasive cacti species. The problematic cacti species are the Imbricated, Pest Pear and Snake cacti – all likely to spread and invade other parts of the country if control measures are not implemented.

These species are such a problem, that “their spines can damage and kill local wildlife and make the areas impenetrable for people”.

Although the clearing of cacti can be done manually, it is expensive and time-consuming and no people can ever get ahead of new infestations that constantly appear in new locations. Control using poisons is dangerous due to the pollution risk and it also expensive. Also, these methods can only react to the presence of new plants once they are detected.

“Introducing species-specific natural enemies of each cactus species, will effectively control their abundance at very low levels,” according to the document.

The insects that can control the invasive cactus plants will be imported from South Africa, where they have been proven to be very effective against the targeted cacti. Two of the agents are cochineal insects, which appear as white waxy blotches, like cotton wool on the cactus leaves. The other is a mealy bug which forms lumps in the cactus plants and prevents it from propagating.

These species feed on the plants by sucking the sap, which kills or sterilises the plant.

Biological control

Biological control is considered to be the most effective and environmentally friendly method of controlling alien invasive cacti in Southern Africa and Australia, where very problematic cacti have been controlled for more than 100 years. All the agents are host specific, only feeding on the target weed and a few other closely related cactus species that are also alien to Africa.

No indigenous plants can be harmed by the release of these agents.

Since there are no active biological control practitioners in the country, this project will include students who will participate in the trial by mapping the invasive alien species in the country and monitoring the outcomes.

The Namibia University of Science and Technology (NUST) has committed to undertake long-term studies ­in collaboration with this programme.

“The release of these agents in Windhoek will be beneficial as it would result in the sustainable, long-term control of these damaging invasive species for the protection of ­agricultural productivity and indigenous biodiversity,” the document read.


The cactus mealy bug can damage other alien columnar cactus plants, and a small amount of spread to garden plants may occur.

However, the dispersal of the mealy bug is likely only over short distances, not more than a few hundred metres.

Since alien cacti should not be

encouraged at all, nurseries are

being asked to rather sell and promote indigenous succulents which are not invasive and do not cause problems for local wildlife.

“Any damage to local garden cacti should be considered a small price to pay for the huge

benefit that the bio-control will have on invasive cacti.”

For comments or concerns about the project, contact [email protected] or [email protected] by Monday 26 November.

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