Cactoblastis cactorum, the cactus moth, South American cactus moth or nopal moth, is native to Argentina, Paraguay, Uruguay and southern Brazil. It is one of five species in the genus Cactoblastis that inhabit South America, where many parasitoids, predators and pathogens control the expansion of the moths' population. This species has been introduced into many areas outside its natural range, including Australia, the Caribbean, and South Africa. In some locations, it has spread uncontrollably and was consequently classified an invasive species.Zimmermann, H., Bloem, S., Klei, H., "Biology, History, Threat, Surveillance and Control of the Cactus Moth, Cactoblastis cactorum", April 10, 2004. However, in other places such as Australia, it has gained favor for its role in the biological control of cacti from the genus Opuntia, such as prickly pear.
Interactions in native habitat In South America, Cactoblastis cactorum has many natural predators, including ants and New World monkeys. Ants, the moths' main predators, consume its larvae. New World monkeys dig the larvae and pupae out from the flattened leaf-like stems, or "cladodes", of the cacti. The relationship between Cactoblastis cactorum and Opuntia cactus species is parasitic: the moth feeds on the host cactus. Recent work in South America has identified four genetically-structured ecotypes of C. cactorum that infest different hosts and possess different larval morphology. The mechanism driving this isolation and pattern of host-association in the field remains unexplored.
Anatomy Adults of Cactoblastis cactorum are nondescript brownish-gray moths with long legs and long antennae. The moth can be identified only by a microscopic examination of dissected male genitalia. They generally appear as typical Pyralidae, with the pronounced labial palps of the female, thus the name "snout moths".
The forewings show a characteristic banding pattern similar to other related moths. The hindwings are whitish and semitransparent, and the wingspan of adult moths varies by age and sex. The average wingspan is 27–40 mm for females, and 23–32 mm for males.Materson, J., "Cactoblastis cactorum. Smithsonian Marine Station at Fort Pierce", March 10, 2007.
The larvae of C. cactorum are caterpillars that start out with a pink-cream color and gradually become orange, with distinctive black spots or bands.
Reproduction and lifespan Cactoblastis cactorum mating occurs before sunrise. Mates are found by scent rather than sight.Everlyn, K. (2011). "How Does a moth reproduce? " Once a female finds a mate, she begins to release sex pheromones that signal to males her readiness. When the male responds, the mating process is initiated.
The initial process of mating begins when the female and male attach themselves at their abdomens. The male passes a sac, known as the "spermatophore", and the female stores the sac in her abdomen's reproductive center. After an incubation period, the female deposits an "egg stick" that contains 30–50 eggs. The eggs are laid on either the tip of the cactus spine, the cactus leaf, the cladode, or the cactus fruit.
Egg sticks that resemble cactus spines develop and hatch in 25–30 days. The gregarious larvae bore into the cactus pad through a single entry hole by chewing through the tough outer cuticle of the cladode. The external damage that results is characterized by yellowed plant tissue with plant fluid ooze and insect frass. The larvae feed inside the cactus and eventually hollow out the cactus pad, consuming everything but the vascular tissues.
Larvae will typically spend two months within the host cactus during the summer, and approximately four months during the winter. Mature larvae exit the cactus pad to form cocoons. They pupate under debris on the ground at the base of the plant. As soon as the moths emerge, they search for a mate, and usually reproduce three to four times within their lifetime. The average longevity is nine days for females and eleven days for males. During this time, the female moth does not eat; she uses all of her energy to travel up to in search of dense cactus patches for reproduction. The male moth devotes his energy to maximizing his mating opportunities. Males mate between two and five times, and wait two to three days on average between mating events.
Effects and status
As a biological control agent
Native to Argentina, Cactoblastis cactorum was introduced in 1925 to Australia, where it was successfully used as a biological control agent for Opuntia cacti.Capinera, John L., Encyclopedia of Entomology, Springer, April 10, 2008. Due to this success, it was subsequently introduced into other countries, including South Africa in 1933 and the Caribbean in the 1950s.
Following introduction, Cactoblastis exerted an immediate effect on the agricultural community in South Africa, where it diminished the population of the spineless Opuntia species valued as "cattle fodder". In 1956, the moth was introduced to the Caribbean island of Nevis and successfully controlled a complex of native "prickly pear" cacti. In 1960 Cactoblastis was introduced into Montserrat and Antigua as a successful biological control agent.
Spread Following its introduction into the Caribbean, Cactoblastis cactorum was able to spread across the Atlantic Ocean and throughout the Caribbean through an unknown mix of natural dispersal, intentional and unintentional human transport, and importation on infested livestock fodder. It has been spotted in Saint Kitts, the US Virgin Islands, Haiti, Cuba, the Dominican Republic, the Bahamas, the Cayman Islands, Puerto Rico and Barbados.
It eventually reached the Southeastern United States and was first detected in Florida in 1989. Cactoblastis cactorum likely entered Florida through importation of Opuntia from the Dominican Republic. It is estimated that Cactoblastis is currently expanding its range along both the Gulf and Atlantic coasts at a rate of 100 miles per year, with a constant increase in the rate of colonization along the Gulf Coast. As it spreads, it threatens the population of Opuntia cacti in Florida, in the Atlantic coast up to Charleston, South Carolina, and around the Gulf Coast up to New Orleans.
As the moth moves through the Southeastern United States, it endangers many cactus species and threatens many ecosystems. In Florida, the greatest concern is for the critically endangered semaphore cactus Consolea corallicola.Stiling, P., "A Worm That Turned", Natural History, 109(5), 40-43, March 5, 2000. Studies aim to identify the most efficient way to prevent the invasion of Cactoblastis cactorum in the semaphore cactus population. In addition to the semaphore cactus, the arrival of Cactoblastis cactorum to the United States caused concern for the ornamental cactus industry in Arizona, California, Nevada, New Mexico and Texas. Arizona has the largest economic stake in Cactoblastis; the retail value of its cactus industry is around $9.5 million.
Cactoblastis cactorum has spread across the Gulf to Mexico, where it was first discovered on Isla Mujeres, a small island off the northeast coast of the Yucatán Peninsula. It is unknown how the moth migrated to Mexico; speculated means of propagation include winds and hurricanes, unintentional transport by humans, or commercial trade. Cactoblastis cactorum began to attack Opuntia cacti on San Salvador Island in the Bahamas, where Opuntia cacti, especially the prickly pear cactus, are a major food source for the Cyclura iguanas. The decrease in Opuntia cacti population raises concern of severe damage to the iguana population.
In 2020 a study found that C. cactorum is unselective as to which Opuntia species it chooses to feed upon. This may result in even more widespread devastation than previously expected.
Projected spread Researchers are projecting a westward expansion of Cactoblastis cactorum in North America. This threatens cactus industries in the Southwestern United States and inner parts of Mexico. In the Western United States, over sixty Opuntia species are a vital part of the ecosystem. In Mexico, Opuntia is a vital plant; its fruit and pads (nopal) are a staple food, chopped cacti are used to sustain cattle in times of drought, and some Opuntia species support the cochineal dye industry.Robyn, R., W., Shaharra U., "Cactus Moth, Cactoblastis cactorum", 2011 Survey Plan for PPQ and State Cooperators, March 5, 2011. Loss of these cacti would have a major environmental and economical impact on the country. Estimated stress factors in the moths' native community indicate that it is restricted to temperate climates, due to the incubation period of its larvae, but host associations may also play an important role in determining spread. Models based on environmental tolerances predict that its eventual range limits in the United States are bound by Charleston, South Carolina, to the north and by California to the west. However, the eventual extent of the western spread remains unclear because we know little about biotic interactions that will occur in this region.
Ecological interactions and mechanisms
Host selection The ecological relationship between the Cactoblastis cactorum moth and the Opuntia cactus is a parasite–host relationship.
Laboratory feeding studies suggest that Cactoblastis cactorum is one of the least selective moths in the genus Cactoblastis when it comes to host selection, but the observed patterns of infestation in the field suggest that host identity is important in determining which sites become infested. Moths target specific plant hosts, like Opuntia, using chemoreceptors on their antennae. Compared to male moths, females respond more strongly to volatile organic compounds emitted by the cacti, allowing them to best determine where to oviposit.
Competition There have been no formal studies of competition between C. cactorum and other cactophagous species to date. This is an area of great potential interest in the Southwestern United States and Mexico because we know little about how of the diversity of cactophagous insects that are found in the region might influence the spread of C. cactorum.
Predation Another factor that allows the moth to spread so easily in the United States is lack of predation. In South America, several parasitoid species as well as many diseases help to control the spread of the moth and its larvae. The parasitoids and diseases seem to be enough to control the spread of Cactoblastis cactorum. These parasitoids and diseases are not present in the areas where the moth has become a problem. This may be allowing the moth to spread more rapidly than normal. It has yet to be determined if these organisms that limit the growth of the Cactoblastis are host-specific enough to be introduced into affected areas as a method of biological control of the moth itself.
Life history strategy
Cactoblastis cactorum shows both r-selected and k-selected life history traits. Although the moth reproduces more than once in its lifetime (a more k-selected trait), it produces a large number of eggs at one time (a more r-selected trait). A female moth can lay up to 50 eggs per generation, and produce at least three generations. Multiple generations are a sign of iteroparous reproduction. However, large amounts of offspring during a single generation time is a trait of semelparous reproduction. In this particular case, the moth also has high adult mortality rates which tend to push organisms towards semelparous reproduction. Also, these generations occur over a short period of time, considering that an adult moth only lives for about nine days.