A successful example of biological control and its explanation

Cactoblastis cactorum caterpillars feeding on prickly pear.

Opuntia stricta (prickly pear) was introduced into Australia around 1840, but became a pest of epidemic proportions in the first two decades of the 20th century. It eventually infested 30 million hectares in 1930 in a wide area around Brisbane. After a systematic survey of insects that eat prickly pear from around the world, the moth Cactoblastis cactorum was introduced into Australia to infest the prickly pear. Within less than a decade an estimated 1.5 billion tonnes of prickly pear was consumed by trillions of larvae. This is one of the successful examples of the introduction of a new organism into Australia without generating secondary damage. This raised two important questions: First, why could Opuntia stricta spread so successfully after introduction into Australia, and second why did Cactoblastis cactorum specifically lay its eggs only onto the prickly pear?

Barry Osmond and Gert Stange from RSBS solved this puzzle. Gas exchange measurements showed that the prickly pear carries out a specific form of photosynthetic carbon fixation.  During the day when it is hot the plant shuts all its 'windows' (stomata) to restrict evaporation of water from the plant. During the night it opens its 'windows' to bring in carbon dioxide. The carbon dioxide is stored in organic acids overnight and used during the next day to convert carbon dioxide into sugar using energy derived from photosynthesis. As a result the prickly pear could spread easily in a hot and dry climate. Remarkably, Cactoblastis senses the depletion of carbon dioxide near the surface of the plant through a specific sensor, and lays its eggs only on plants where this is the case.

Reference: 1: Osmond B, Neales T, Stange G. Curiosity and context revisited: crassulacean acid metabolism in the Anthropocene. J Exp Bot. 2008;59(7):1489-502. doi: 10.1093/jxb/ern052.

In 2017, we celebrate 50 years of Biology at ANU. This article is one of a set featuring the achievements and memorable occasions of ANU biologists those first 50 years.
Read more at Biology at ANU – the first 50 years.

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