A prickly situation

Prickly pear cactus has its detractors.  Long hated for its long spines with a bite, and its clusters of barbed spines (glochids) that are heck to remove, it has been cursed, hacked, burned and sprayed. But prickly pear (Opuntia spp.) is also used by a variety of  wildlife and cattle, and is prized as a part of the Mexican-American diet.  There is even a small industry devoted to rearing insects, called cochineal scale, that feed exclusively on prickly pear (these scales produce a vivid red dye, called cochineal or carmine, sometimes used as a natural coloring agent in cosmetics and beverages–including some Starbucks frappucchinos).

rancher burning prickly pear in south Texas

A rancher in South Texas burns the spines off of prickly pear cactus to feed his cattle during a drought. Image courtesy Texas A&M AgriLife Extension. Photo by Omar Montemayor

There are over 100 species of Opuntia native to the Americas, and most are not considered pests. Though ranchers may curse prickly pears as “weeds”, they also rely on them to provide emergency food for cattle during times of drought. In addition, many insect and vertebrate species rely on different kinds of prickly pears for food and shelter. Despite our sometimes love/hate relationship, most Texans view the various prickly pear species as valuable native plants.

Unfortunately, a small moth called Cactoblastis cactorum poses a new threat to the ecological stability of Opuntia species in Texas. Cactoblastis is a predator of prickly pear in its native home of Argentina in South America. It was distributed by humans into the Caribbean in 1959. Since then it has expanded its territory slowly through Cuba and Florida, and most recently Louisiana.  The bad news is that Cactoblastis has now become established and is spreading in Texas, according to a recent post on the Facebook page of University of Texas biologist Larry Gilbert.

Cactoblastis damage Texas 2019

Initial point of entry by Cactoblastis, and hollowed-out pad, is evident in this backlighted photo of an Opuntia cactus. Photo courtesy Larry Gilbert.

According to reports, the moth appears to have leapfrogged over the Houston area into Brazoria County and is now established as far south as Mad Island, east of Victoria, TX.  According to Robert Vines’ book, Trees, Shrubs and Woody Vines of the Southwest, over 50 native species of Opuntia can be found in Texas and surrounding states.  It is not certain how many of these species will ultimately be affected by Cactoblastis.

The problem with invasive species is that natural control agents are often left behind in their country of origin.  When this occurs, the invading species is free of ecological restraints to reproduction.  This seems to be the case with Cactoblastis. Its impact on Opuntia is much worse here than in its native home.

Entomologists hope that an Argentinian wasp, Apanteles opuntiarum, might be enlisted in the struggle to preserve native OpuntiaResearch is being conducted to learn how to rear this tiny parasite wasp and learn whether it might be safe to release into Texas.

Ultimately, if Cactoblastis continues to spread, it could have an effect on ornamental cacti grown by Texas gardeners.  Of course as gardeners we have a variety of insecticides that can be sprayed on cacti–but who wants to have to do that?  Let’s hope that the Argentinian wasp can come to rescue, and tip the scales in the favor of the cactus.

Be sure to see Larry Gilbert’s post and excellent images showing Cactoblastis damage.




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