FRIEND OR FOE?
Cactus moth: damaging
prickly pear pest


June 17, 2011

by Lupe Cook , Victoria County Master Gardener

edited by Charla Borchers Leon,
Victoria County Master Gardener
Gardeners' Dirt
PHOTOS BY LYLE BUSS/UNIVERSITY OF FLORIDA
Mature larvae, in white spun cocoons in leaf litter or tree crevices, hatch into adult moths in four to five weeks.  Thus continues the cycle of the cactus moth, the population of which multiplies quickly.
The cactus moth egg stick consists of 60 to 100 eggs in the form of a chain and can usually be found on the underside of the cactus pad.
Once hatched, cactus moth larvae burrow into the cactus pad and feed together as a group.  Infestation cannot be detected without splitting the pad open.  Indication of harm to the pad includes oozing sap and insect droppings from openings in the pad.
Two years ago this July, a two-part series on cactus gardening was published in this column. I wrote the material based on personal experience of researching more than 100 species of cacti and planting a cactus garden in my own landscape. (Go to vcmga.org.; click on "Gardeners' Dirt 2009" then July 23 and 30.) I have continued to learn about these plants - and how their natural state can so easily be altered by the introduction of a pest.

GOOD OR BAD?

The cactus moth (Cactoblastis cactorum) is one of the most successful biological control agents in history. It has been used around the world for the control of prickly pear cactus (Opuntia spp.) to destroy the cactus for clearing acres of land for agriculture use. It was transported to Australia and South Africa in the early 1900s from its native land of South America to be used as a controlling agent.

Unfortunately, it was accidentally introduced into Florida becoming invasive and threatening the native and endemic prickly pear population. The moth first appeared in the Florida Keys in 1989. In 2002, the moth had spread from both coastlines of the Florida Keys to the Florida Panhandle and South Carolina. It has invaded Georgia, Mississippi and presently it is in the barrier islands of southern Louisiana.

It can travel 100 miles per year depending on the abundance of Opuntia species it has in its path. With Louisiana being our border state, it wouldn't take long for the moth to migrate into Texas.

BIOLOGY OF CACTUS MOTH

The cactus moth is gray with a zigzag line across the outer fourth of the forewing. The female is larger than the male, and the span of both wings slightly less than an inch to an inch and a third.

The female cactus moth, after maturing and mating, lays 60-100 eggs in the form of a chain. The eggs are stacked on top of each other to form an egg-stick that resembles, of all things, a cactus spine. Within a few days, it can lay 200-300 eggs. These egg-sticks are usually on the underside of the pad or other protected parts of the plant pads. The egg stage lasts about three to four weeks.

After hatching, the larvae burrow into the cactus pad and begin feeding together as a group. You can not detect any infestation of the feeding without splitting the pad open.

As the larvae mature inside the pad, oozing sap and insect droppings will be pushed out of openings in the pad. The pad will become transparent and hollow and die. Numerous pad infestations can kill the entire plant. The mature larvae usually spin white cocoons in leaf litter or crevices in nearby trees. Larvae mature in four to five weeks. The adult moth later emerges, and the cycle is repeated.

REAL THREAT TO PRICKLY PEAR

The cactus moth can do severe damage to the population of native prickly pear cactus in the southwestern United States and Mexico. It could also create a threat to the cactus industry and the ecosystems of the Southwest and Mexico. There are 31 species of native prickly pear cacti and numerous other native species associated with cacti in the United States that are threatened by this moth. Mexico has 53 species of prickly pear including 38 endemic species, which could result in a very serious situation to its cactus industry.

So who cares? I know numerous landowners who would love to get rid of prickly pear. But in times of drought, the prickly pear has been the savior of the cattle industry supplying food and water for livestock. Numerous wildlife also use prickly pear for food, water and habitat. Additionally, we import 1.5 million pounds of prickly pear annually for human food.

We need to be watchful of this potential new pest, and several local Master Gardeners and Master Naturalists are assisting with pheromone traps to monitor the movement of this pest into Texas. I personally have two pheromone traps that I am monitoring, looking to see if the cactus moth is in our area. So far, none has been found, but I'll keep a watchful eye for this potential pest.

If you see any signs of this pest on cactus in your area, contact your County Extension Agent, so it can officially be documented, or leave your name if you are also interested in monitoring for this pest. For more information on the cactus moth, visit gri.msstate.edu/research/cmdmn.
THREE PHOTOS DIRECTLY ABOVE BY LUPE COOK/VICTORIA COUNTY MASTER GARDENER
(Please, click on photo for larger view and then use backspace to return to article.)
LEFT:  This pheromone trap as shown currently in Master Gardener Lupe Cook's cactus garden is being used to monitor the cactus moth.
UPPER RIGHT:  The cow-tongue prickly pear, Opuntia spp., is commonly used as a landscape plant.
LOWER RIGHT:  An infested pad will become transparent and hollow -- and die.  Numerous pad infestations can kill an entire plant.
CACTUS MOTH THREAT

Native of Argentina, South America
Accidentally introduced into Florida in 1989
Nationally-listed invasive insect
Potentially invading Texas; recently found in Louisiana
Threatens Texas' 31 species of native prickly pear cacti

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BENEFITS OF PRESERVING PRICKLY PEAR

Industry
Cactus pads and fruit are sold in grocery stores
Fruit is used to make jelly, syrup, drinks; is eaten raw
Wildlife
Food resource for mammals, birds and insects
Major vegetation component where goats and sheep are raised
Emergency forage for cattle in Texas during drought spells
Environment
Serves as bird habitat
Predator protection for small animals
Helps prevent soil erosion

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CACTUS MOTH GENERATIONS

*March-May:  1st flight period
*July-August:  2nd flight period
*September-October:  3rd flight period
*Larvae from last generation overwinters inside the cactus pad
The Gardeners' Dirt is written by members of the Victoria County Master Gardener Association, an educational outreach of Texas AgriLife Extension - Victoria County. Mail your questions in care of the Advocate, P.O. Box 1518, Victoria, TX 77901; or vcmga@vicad.com, or comment on this column at www.VictoriaAdvocate.com.