The soils of the San Joaquin Valley in California have a dirty secret. They contain deposits of the element selenium left by the evaporated ancient sea that once covered the now semi-arid landscape. As humans put the land into cultivation, irrigation can wash that selenium into waterways were it can be toxic to wildlife.
The prickly pear cactus can provide a natural remedy to this poisonous problem, however using non-native plants to solve problems can be a double-edged sword.
The U.S. Department of Agriculture found that prickly pears pull selenium from the soil and then volatilize the element, thus evaporating it into the air where it harmlessly floats away. The USDA's Agricultural Research Service evaluated varieties of prickly pear from Mexico, Brazil, and Chile for their tolerance to the salt and boron also found in the San Joaquin Valley's soils, according to a press release. The Chilean cacti proved to be the best as squelching selenium and withstanding the Valley's soils. They even produced a tasty crop of sweet prickly pears, or nopals.
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The problem is the cacti were from Chile, as opposed to the numerous native varieties that grow coast-to-coast across the United States. Prickly pear have proven to be a very thorny problem when introduced into non-native areas.
Some of the first European settlers in Australia brought prickly pear cactus (probably Opuntia monacantha) with them and set the plants loose down under. The cacti were infested with the cochineal insect, a bug used to produce a vibrant red dye. The goal was to break the empires of the Iberian Peninsula’s monopoly on the dye.
The mission was a great success and the dye gave the infamous red coats of the English army their bull's eye hue. However, there were unintended and costly consequences to looking fashionable on the battlefield.
The original prickly pear were joined by later introductions of other varieties, particularly Opuntia stricta. The plants rapidly became a biological invasion. The cacti formed needle studded stands that choked the land and overwhelmed native plants. The prickly pears resisted all forms of chemical and physical control.
Finally, in the 1920s, the cactoblastis caterpillar (Cactoblastis cactorum) was identified as a natural control for the cacti. The Aussies had learned their lesson and first tested the insect in the lab to be sure it wouldn't spread to other plants.
IMAGES: Prickly pear at Minnesota Landscape Arboretum (SEWilco, Wikimedia Commons)