Prickly Pear Indeed

By Joe Merkelbach (May 2008)

Last month, we mentioned the establishment of prickly pear cacti worldwide in an effort to break the Spanish and Portuguese control of the cochineal red dye trade. Prickly pears were moved to other areas of the planet with similar soils and climates to serve as the necessary substrate for the cochineal insect.

One place where the cacti moved in almost as soon as Europeans settled was Australia, and very soon, the people in eastern Australia regretted that effort. The first settlers landed at Botany Bay had some cochineal-infected prickly pears that were gathered in Brazil on the voyage out. These plants – Opuntia vulgaris, aka Opuntia humifusa – have persisted on parts of the coast, but did not invade the interior. The second use for the opuntias was as stock fodder. The plants thrived in areas of poor rainfall and soils.

Soon enough, the second imported species, O. stricta, grew from food plants in stock paddocks to become a surging vegetative invader. Between 1850 and 1924, it spread to the outback and smothered the native plant species. Australians realized even before 1900 that they had a brutal and rapidly expanding invader threatening their landscapes. They tried various varieties of poisons, arsenic fumes as an example, but the cacti were still clearly winning.

In 1926, larvae of the Cactoblastis moth from South America were released in Australia, where they rapidly and effectively obliterated huge stands of the invading cacti. These moths were carefully researched before they were turned loose in the wild, and the impact on native species has been negligible.

The moths are considered a standard for biological control of pestiferous invading species. They cleared up to 16 million acres for return to agricultural use. Cochineal insects are also now used as a secondary biological control for prickly pears.

The moths have been released in South Africa, Hawaii and the islands of the Caribbean Sea with effective eradication of pest plants but also undesirable side effects. The spineless opuntia pads used for cattle feed in South Africa were severely attacked, and this agricultural use was damaged.

The moths from the islands of the Caribbean have now spread to the Florida mainland and pose a threat to rare, less numerous Opuntia species. There is also concern that the moths will move westward to Texas and Mexico, where they would have the potential to seriously affect opuntias used in agriculture there.

So, be careful what you wish for, and keep in mind that constant vigilance is indeed required.