Moth Threatens Prickly Pear Cactus

Despite its iconic association with the Southwest, many people may not love Prickly Pear Cactus (Opuntia). However, various species of Opuntia are extremely important plants to most ecosystems in Texas and Mexico. They produce a huge quantity of fruits that are a critical resource for many species. Other species survive on the pads, especially in droughts, including a wide variety of insects that in turn are part of the cactus food-web. Many more species of animals (especially small mammals, birds, and reptiles) use clumps of cactus for cover and protection, and they also serve to protect other plants from over-grazing. In addition, Opuntia is an important food and cash crop for people in Mexico. But the invasion of a moth whose larvae eat Prickly Pear threatens to change these iconic ecosystems in Texas and Mexico forever.

Why? The South American cactus moth, Cactoblastis cactorum, is now spreading through Texas. But how did it get here?

For the answers, we must travel back in time to Australia. The North American Prickly Pear became a major invasive weed in Australia by the early 1900s, densely covering over 240,000 square kilometers. There were many failed biocontrol attempts, but in 1925 after the introduction of the cactus moth, they were finally able to bring the invasive cactus under control.

The process is like this: a female moth lays “egg sticks” on the cladode (pad) of the cactus. These “sticks” have 30-50 eggs in them that hatch into larvae who then bore into the cactus pad. The damage shows itself via hollowed out, yellow pads dripping with insect frass (waste). Eventually, the larvae leave nothing behind but the surface cuticle and some thin fibrous tissue. With the loss of enough pads, the plants die.

The cactus moth got the spread of the cactus under control in Australia, and all seemed well. However, the moth was then introduced to other parts of the world for the same biological control purposes. The seemingly beneficial effect of this was all to change when the moth reached Florida via the Caribbean Islands less than three decades ago. The moths have had major impacts on the wild cacti of Florida and are considered a serious threat to the 79 species of cacti in Mexico and US.

Here in the Lone Star State, the moth was first found in Brazoria County in 2017. In December 2019, Dr. Larry Gilbert with Colin Morrison of the Brackenridge Field Lab's Invasive Species project, found evidence of the moth's spread and extensive impacts at Texas Parks and Wildlife Department Mad Island WMA. Since then it has been found further south along the coast, and inland at Columbus. It may well reach Mexico in a few short years.

How does the moth remain under control in its native Argentina? Here, Cactoblastis cactorum has many natural predators such as ants that feed on egg sticks and wasp parasitoids that feed on the larvae.

Currently, Brackenridge Field Lab’s Invasive Species team is collaborating with the USDA and Florida Dept of Agriculture to hasten parasitoid wasps into production for biological control of the moth. The wasps lay their eggs in cactus moth larvae, and when the eggs hatch, they eat the moth larvae from the inside out.

Dr. Gilbert estimates if introduced early enough, the Argentinean wasp specialized on this moth will spread with the moth and slow its expansion as it moves south and west into the heart of Texas and Northern Mexico, where the moths will devastate this keystone species for biodiversity in Texas, as well as severely impact agriculture in Mexico.

As Dr. Gilbert says about the iconic Prickly Pear, “There is time but not much. We won't know how much we miss it until it's gone.”

Thanks to Dr. David Hillis, Dr. Rob Plowes, and Dr. Larry Gilbert for their input.

Austin American Statesman has also published a piece on the efforts the Invasive Species team is making towards this moth's spread. Check it out here.