Alien invasive plants eradicated by insects

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KwaZulu-Natal is often referred to as South Africa’s garden province because everything grows well. Unfortunately this includes invasive weeds like Mexico’s prickly pear (Opuntia monacantha), and queen of the night (Cereus jamacaru), two species threatening Bonamanzi Game Reserve’s unique vegetation.

“The queen of the night and prickly pear were introduced into South Africa as ornamental plants and barrier hedges,” says Frik-Jan de Lange, ecologist at Bonamanzi Game Reserve. The plants have now invaded the critically endangered sand forest on the reserve, where the slow-growing endangered hardwood, Lebombo wattle (Newtonia hildebrandtii), is found.

Queen of the night. Photo: Frik-Jan de Lange

Instead of using costly herbicides to eradicate these spiny invasives, de Lange resorted to biocontrol. Biological control is a way of eradicating invasive species using natural mechanisms like predation or parasitism. “The insects take years to kill the host plant, but it is a lot more cost-effective and less time-consuming,” he says.

In South America, queen of the night is one of the host plants for the harrisia cactus mealybug (Hypogeococcus pungens). This mealybug is a tiny pink insect that feeds on the growth points of most organ-pipe cactus species. In 2017, de Lange manually introduced the mealybugs to Bonamanzi. A white mass appears on the tips of the infected plant and branches become distorted. “When severely populated by biocontrol agents, the plant will stop growing and reproducing seeds or flowers,” de Lange explains.
Mealybugs do not attack native vegetation. They also cannot fly and are dispersed via wind. To help the mealybugs spread faster, they need to be manually transferred by tying a piece of vegetation containing the biocontrol agent on it to an infected plant.

Cochineal on prickly pear. Photo: Frik-Jan de Lange

De Lange also introduced wild cochineal (Dactylopius ceylonicus), a miniscule scale insect, to control the prickly pear invaders. It is nearly impossible to fully exterminate prickly pears, but the wild cochineal can bring the population under control. “The cochineal is completely dependent on the prickly pear and cannot survive on any other Opuntia species,” de Lange says.

Prickly pear. Photo: Frik-Jan de Lange

Unlike the mealybugs, the cochineal attack mature plant tissue and young leaves, and collect in the shady areas of the cactus pads where they feed. Both the harrisia cactus mealybug and the wild cochineal are similar in appearance and produce a white wax-like substance to protect them from the elements.

Written by Georgina Lockwood

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