The Case of the Prickly Pear Cactus

A few weeks ago, in Population and Community Ecology class (BIOL 3170), in a context of the effects of predation on populations, we learned about a particularly wide-spreading species: the prickly pear cactus. This common name refers to some species belonging to the genus Opontia in the Cactaceae family. It made me remember an ecology class I took last semester in my second year in University in France, so I decided to talk about it here (after all, it is also the story of a butterfly).

The prickly pear cactus is an edible plant originated from the American continent, where we can find different species almost everywhere, especially near Mexico and the south of the United States, and is particularly cold-tolerant, spreading until cooler regions like southern Canada. They have been introduced in the Mediterranean region, South Africa and Australia, the latter being the most interesting one here.

The cactus moth, Cactoblastis cactorum, whose larvae feed on cactus, is originated from South America and has been spread in other locations such as Africa, Australia and the Caribbean or North America. In Australia its introduction has been a blessing.

Indeed, in the 18th century the prickly pear cactus was intentionally introduced into Australia for decorative purposes and as natural fence. However, it quickly became impossible to control its spread,  with up to 260,000 km2 of agricultural land transformed into a dense jungle of cacti that could reach 6.1 m in height. It became imperative to do something to control the spread of this invasive weed, so authorities employed mechanical and chemical methods to eradicate it but in vain.

The key was biological control: Cactoblastis cactorum was introduced in 1925 and quickly the cacti-invaded areas got cleared. It is considered as the most successful case of biological control on an invasive species. Upon hearing this success story, other countries like South Africa or Caribean islands imported the cactus moth to get rid of invasive Opuntia sp.

However, this moth spread from the Caribbean to North America, being first detected in Florida in 1989. Here its presence is less than welcome, because Cactoblastis cactorum is targetting, among others, the endangered semaphore cactus Opuntia corallicola, and causing concern for the ornemental cactus industry in the southern United States.

This is an example of some negative effects of biological control: what do we do if the non-native species introduced to eradicate an invasive species turns to be invasive as well? Introducing another species seems a little bit risky… For now, tentatives have failed.

To learn more about these species:

The University of Florida’s page about the cactus moth

Desert USA – Prickly Pear Cactus

Thank you for taking some time reading this post,

Claire Hoarau

 

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