Prickly Pear Cacti in Hawaii? Ouch!

14 Nov

I spent most of my career as an earth scientist in the deserts of the American West, where there are tree-sized cactus species everywhere. So it’s natural that I came to think of “big cacti” as growing ONLY in places like the arid sand deserts of mainland North America. Imagine my surprise and confusion when I encountered hundreds of thickets of a really large species of cactus, the Prickly Pear, on Maui, a tropical island in the middle of the Pacific Ocean that receives more than 300 inches (760 centimeters) of annual rainfall.

cactus 1

Mature Prickly Pear Cactus growing on cliff face near Ho’okipa Beach, east Maui.
[ Click on image to view larger version. ]

The Prickly Pear Cactus, Opuntia ficus-indica (“Panini” in Hawaiian*) was introduced (ostensibly as cattle fodder) to the Hawaiian Islands early in the nineteenth century but is now naturalized on all the main islands. It prefers sunny open land on gentle slopes, although it is not unusual to see large mature plants clinging to nearly vertical cliff faces, particularly in drier leeward coastal areas. It prefers rocky well-drained soils with a pH range of 7 to 8.5, soil conditions that increase root growth and branching in this species of Opuntia. It also tolerates moderately saline soils and air temperatures ranging from minus-6º C to a high of 65º C.

cactus 2

A very mature tree-like Prickly Pear Cactus growing in a cattle pasture on Pulehu Road, south Maui. Measured more than 10 meters horizontally!
[ Click on image to view a larger version. ]

The Prickly Pear is a massive trunk-forming segmented cactus that typically reaches heights of 3 to 5 meters with a lateral span of two to three times its height at maturity (usually at 20 years of age). The individual leaves (“pads” or “nopales”) of this species are edible, but are covered with widely spaced aureoles that include one to as many as six white or yellowish spines 1-3 cm long. In addition to these formidable spines, aureoles include numerous glochids, tiny hair-like spines that are smaller than typical spines but no less formidable. Incautious handling of pads and fruit of the Prickly Pear can cause scores of these tiny spines to lodge in the skin and soft tissues of the handler where they can remain for days or weeks, often becoming infected. Old pads form the substantial woody stem of the tree.

cactus 3

A very mature tree-like Prickly Pear Cactus growing in a cattle pasture on Pulehu Road, south Maui. Measured more than 10 meters horizontally!
[ Click on image to view a larger version. ]

The fruit (“tunas” or “pears”) of this species is typically greenish white to yellow, yellowish brown, or reddish-purple (depending on the strain). Individual fruits are fleshy and barrel-shaped, and at maturity are about 10 cm long and half that wide. Like the pads, the tasty nopales possess glochids in their aureoles and must be carefully handled and properly prepared prior to consumption.

cactus 4

A “nopale” (leaf) with several fruits (“tunas”) of a Prickly Pear Cactus growing in a cattle pasture on Pulehu Road, south Maui.
[ Click on image to view a larger version. ]

The flowers of the Prickly Pear are pollinated by insects. They are cup-shaped, bright yellow or orange, and 5-7 cm in diameter. They form on the perimeters of individual pads and bloom in spring or early summer.

prickly pear flower

Flower of a Prickly Pear Cactus, Pulehu Road, south Maui.
[ Click on image to view a larger version. ]

Seeds are dispersed by birds, feral pigs, and lizards that feed on the fruits. The seeds from intact fruits and animal droppings germinate after long rainy spells followed by months of hot weather. However, seeds in the soil can remain viable for several years. Vegetative reproduction is accomplished by cladodes (flattened succulent segments and stems) falling close to parental plants and quickly taking root. The latter form of reproduction often results in dense stands of many large plants.

new "nopale"

Healthy nopale of a Prickly Pear Cactus, Pulehu Road, south Maui.
[ Click on image to view a larger version. ]

Opuntia ficus-indica was probably introduced into Hawaii from Mexico by Don Francisco de Paulo Marin in about 1809. Originally intended to serve as a barrier in cattle pastures (panini means “fence wall” in Hawaiian*), cattle fodder, and a human food source, its innate shortcomings prevented it from becoming popular with ranchers. Cultivated in the wrong place, this species can develop into a destructive weed: it can overrun cattle grazing lands, eventually ruining them with its invasive overgrowth. It also may out-compete certain native plants and upset local ecosystems. By 1910, several thousand acres of pastureland were under cultivation with Prickly Pear Cactus. Although only the spineless varieties were initially established, the species gradually reverted back to the spiny wild genotype by means of genetic recombination and selective grazing pressure. Over a period of nearly 200 years, there were serious invasions of Prickly Pear in many countries (including the Hawaiian Islands).

cactus bud

New bud growing on the tip of a “nopale” of a Prickly Pear Cactus.
[ Click on image to view a larger version. ]

Biological Control — At one time, Prickly Pear Cactus nearly overran Maui and the neighbor islands. The plant is considered a pest species due to its ability to spread rapidly beyond the areas in which it was originally cultivated. The environmental threat posed by invasions of the cactus became so grave that large-scale biological control programs had to be initiated. It was discovered that the most effective biological control agent was the tunneling larval caterpillar of the Cactus Moth (Cactoblastis cactorum), a species native to Argentina. Releases of the caterpillar in Australia (where Opuntia ficus-indica infestations most seriously affected the environment) in the early twentieth century led to almost complete control of the species. On the strength of the successes in Australia, Cactoblastis was introduced into Hawaii in 1950.

cactoblastis larva

A larval caterpillar of the Cactus Moth (Cactoblastis cactorum) thinking about its next meal. Kula, south Maui.
[ Click on image to view a larger version. ]

The larvae of Cactoblastis cactorum feed on the entire cactus plant, burrowing into the fruits, pads, stems, and root system where they live communally, devouring the entire biomass from the inside out, leaving nothing above ground. Even if the cactus plant is only subjected to limited predation by the caterpillar, the resulting tissue damage facilitates the growth of bacteria and fungi, hastening the destruction of the plant. The Cactus Moth larvae are capable of destroying entire stands of cacti. In 1985 they were introduced into an invasive stand of Prickly Pear Cactus in Volcano National Park on the island of Hawai’i, successfully eliminating the threat of a damaging invasion. Today there are many isolated stands of Opuntia ficus-indica in the upland mesic forests of the western slopes of Maui’s Haleakala volcano, especially in the areas surrounding the towns of Pukalani, Makawao, Kula, and Ulupalakua. The cacti are relatively abundant here, but most individuals are in poor condition due to predation by the moth larvae.

Medicinal and Pharmacological Uses — Despite its well-deserved reputation as a noxious weed and invasive species, the Prickly Pear Cactus does possess some redeeming characteristics: recent laboratory studies have shown that extracts of various parts of the plant have significant medicinal benefits and are potential sources of raw material for the pharmaceuticals industry.

Prickly Pear fruit possesses antioxidants – When a cell’s capacity to protect itself fails, oxidative stress occurs. Oxidative stress results from an oxidant/antioxidant imbalance, an excess of oxidants, and/or a depletion of antioxidants. A considerable body of recent evidence suggests that oxidative stress plays a major role in several aspects of acute and chronic cellular and tissue inflammation. U.S. Department of Agriculture studies have shown that fruits like blueberries, cranberries, raspberries, strawberries, apples, and the Prickly Pear contain high concentrations of inflammation-fighting antioxidants. Researchers reported that betalains (a class of red and yellow indole-derived pigments found in plants like the common beet) contribute to the free radical-scavenging and antioxidant activities of Prickly Pear fruits. The Prickly Pear Cactus is the only plant species that contains all 24 of the known betalains, which are also a class of rare and potent healing antioxidants. These antioxidants are used to treat many serious inflammatory diseases in humans. They may also prove useful in promoting healing of organs and tissues, as well as mitigating the more unpleasant symptoms accompanying an alcoholic hangover. Not bad for a noxious weed, eh?

A Postscript for DIYers
If you plan to try your hand at preparing Prickly Pear tunas and nopales for your dinner table, be sure to first visit the website “How to Eat Prickly Pear Cactus”. It contains very detailed instructions for removing the nasty spines and double-nasty glochids. Please, BE CAREFUL if you eat fruits you’ve prepared yourself! My advice: try them at a genuine Mexican restaurant where they know how to prepare these bad boys!

Malama pono!

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