Opuntia cacti are a successful and widespread family in the group Cactaceae, but they are not all that well understood. My first introduction to the group was as a child, when I blundered into one of them on one of those weekend fishing trips in Colorado that we took quite frequently in my salad days. I’m sure you can guess the results: bloody socks, screams of bloody murder, and plenty of tears. And I’ll bet that, at that tender age, I would have had no compunction about unleashing any sort of revenge on these plants, up to and including biological weapons.
Monday’s trip to Yamato Scrub, and my subsequent research online, gave me a new insight into these little spiny succulents. I’d always known that Florida’s “native cactus” was the prickly pear, but I really had no idea how many different species of Opuntia there are (up to 200, depending on how you draw the taxonomic lines)…
My ignorance would come as no surprise to Jon Rebman and Donald Pinkava, two cactus specialists from the Southwest: according to their survey of the genus, “Of the groups within the cactus family, the opuntias are one of the most successful and widespread, but they exhibit many taxonomic difficulties and are, therefore, not well understood” (474).
If you have the time to learn more about cacti in general, and opuntias in particular, I strongly recommend that you read Rebman and Pinkava’s overview of the genus in the December 2001 issue of Florida Entomologist. One thing I learned from their very first paragraph was that, despite their wide geographic distribution (“from just south of the Arctic circle in Canada to the tip of Patagonia in South America”), “native species of cacti are restricted to the New World, except for one species. . .which prehistorically migrated to Africa, Madagascar, and Ceylon,” presumably spread by migrating birds.
The opuntias (from Opus, an ancient city in Greece, not Berkeley Breathed’s penguin of the same name) are one of five genera in the subfamily Opuntioidea found in the United States, two of which (Nopalea and Consolea) have US distributions limited to Florida. According to Stiling and Moon, Florida has six native species of Opuntia [the Atlas of Florida Vascular Plants lists some additional, introduced opuntia species (cochenillifera, ficus–indica, leucotricha, and monocanthos)]:
- O. stricta (Haw.) Haworth, common prickly pear
- O. humifusa (Raf.) Rafinesque
- O. pusilla (Haw) Nutt.
- O. corallicola Small
- O. triacantha (Willdenow) Sweet
- O. cubensis Britton & Rose
The last three (corallicola, triacantha, and cubensis) are restricted to the Florida Keys, so there are only three opuntias on the peninsula to deal with: stricta, humifusa, and pusilla.
Opuntia cacti have a distinctive growth pattern that results from their adaptations to the wet and dry seasons of the climate in which they evolved. They are segmented into “pads” whose growth ends with the end of the rainy season; the next rainy season sees a new pad growing above the old one. This differentiates them from most other forms of cactus (think the giant colums of the saguaro, for instance), which have unsegmented, columnar growth.
The limited worldwide spread of cacti in prehistoric times was a result of natural barriers that no longer apply in this era of global commerce. Cacti are now “among the most cosmopolitan and destructive of invasive, alien plants” (Zimmerman et al., 543). The astonishing geographic distribution of the plants in the New World is adequate testimony to the ability of the cactus to thrive and spread; once it began to be aided by quick and easy travel via the horticultural trade, the invasive nature of many cacti became apparent. And the prickly pear in particular have made themselves a nuisance.
Australia had significant problems with Opuntia species in the 1860s and 70s, much as my feet and ankles had on that weekend in the 1970s. But in the 1920s, the Aussies decided to fight back, rather than just cry in their beer (six-year-olds don’t have the option of crying in their beer, so I just cried). They brought in Cactoblastis cactorum, the Cactus Moth, as a biological control agent, and it was extremely successful. “In Australasia and in the Old World, where there are no native Opuntia species…the release of C. cactorum was rational and safe” (Zimmerman et al., 545). In Australia, where the infestation covered nearly 24 million hectares in the 1930s, it took only two years for ninety percent of the introduced cacti to be cleared by the introduced moth.
The cactus moth, as Eric and I witnessed just Monday, simply loves to feed on prickly pear cacti. Plants that have been fed upon display a tell-tale mark, ” ‘green slime’ exuding from fed-upon pads or, in the later stage of attack, a characteristic skeletal appearance of the cactus with all the tissue between the epidermal layers eaten” (508).
The moth can also apparently distinguish between healthy, fast-growing plants, and less robust, slow growers. Maybe they just taste better, or smell better, or something. An experiment in the Florida Keys demonstrated that those cacti grown in the shade were strongly preferred by the moth. (Yes, it’s true–although they are widely thought of as sun plants, cacti almost always require shade for successful germination and rapid growth.) Plants in the shade grew nearly twice as quickly as those in direct sunlight. Unfortunately, attack rates by the larvae of the cactus moth were also significantly higher: these caterpillars like healthy, fast-growing plants!
My next mission is to expand on this photo gallery of prickly pear cacti seen in the various scrub sites in south Florida:
References
Mahr, Daniel L. Cactoblastis cactorum (Lepidoptera: Pyralidae) in North America: A workshop of assessment and planning. Florida Entomologist 84(4), 465-473.
Pemberton, Robert W. and Hugo A. Cordo. 2001. Potential and risks of biological control of Cactoblastis cactorum (Lepidoptera: pyralidae) in North America. Florida Entomologist 84(4), 513-526.
Rebman, Jon P. and Donald J. Pinkava. 2001. Opuntia cacti of North America–An overview. Florida Entomologist 84(4), 474-483.
Stiling, Peter and Daniel C. Moon. 2001. Protecting rare Florida cacti from attack by the exotic cactus moth, Cactoblastis cactorum (Lepidoptera: Pyralidae). Florida Entomologist 84(4), 506-509.
Zimmerman, H.G., V.C. Moran, and J. H. Hoffmann. 2001. The renowned cactus moth, Cactoblastis cactorum (Lepidoptera: Pyralidae): Its natural history and threat to native opuntia floras in Mexico and the United States of America. Florida Entomologist 84(4), 543-551.