Cactoblastis cactorum

An Opuntia infested area in Australia, before Cactoblastis was introduced

Cactoblastis cactorum was first introduced to Australia in 1925 from Argentina, where it was successfully used as a biological control agent for Opuntia cacti.[10] Due to this success, it was subsequently introduced into other countries, including South Africa in 1933 and the Caribbean in the 1950s.[5]

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".[11] 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.[10]

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.[5] 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.[10]

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.[10] 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.[5] 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.[5]

Cactoblastis distribution in the United States:

  Prickly pear habitat

  Cactus moth habitat

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.[12] 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.[10] Arizona has the largest economic stake in Cactoblastis; the retail value of its cactus industry is around $9.5 million.[10]

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.[10] It is unknown how the moth migrated to Mexico; speculated means of propagation include winds and hurricanes, unintentional transport by humans, or commercial trade.[10] 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.[12]

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.[13][14][15]

Researchers are projecting a westward expansion of Cactoblastis cactorum in North America.[12] This threatens cactus industries in the Southwestern United States and inner parts of Mexico.[12] 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.[12][16] 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,[10] but host associations may also play an important role in determining spread.[17] 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.[6] However, the eventual extent of the western spread remains unclear because we know little about biotic interactions that will occur in this region.

The ecological relationship between the Cactoblastis cactorum moth and the Opuntia cactus is a parasite–host relationship.[18]

Cactoblastis larvae feeding on Opuntia cacti

Laboratory feeding studies suggest that Cactoblastis cactorum is one of the least selective moths in the genus Cactoblastis when it comes to host selection,[1] but the observed patterns of infestation in the field suggest that host identity is important in determining which sites become infested.[17] The moth selects its host by detecting CAM production in Opuntia cacti.[18] They have a detection system that enables them to detect the carbon and nitrogen gradients in the air surrounding the host.[18] The females have a superior detection system to that of the males because they use this to determine where to implant the larvae, which then destroy the cactus.[18]

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.

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.[19] The parasitoids and diseases seem to be enough to control the spread of Cactoblastis cactorum.[19] These parasitoids and diseases are not present in the areas where the moth has become a problem.[19] 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.[19]

Adult female moth in the process of laying an egg stick on a cladode.

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).[1] A female moth can lay up to 50 eggs per generation, and produce at least three generations.[1] 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.[1] Also, these generations occur over a short period of time, considering that an adult moth only lives for about nine days.[1]

There are many different combinations and gradients between semelparity and iteroparity. However, it is clear in this case that the moth is closer to the semelparous side of the scale. Semelparity is an r-selected trait, whereas iteroparity is a k-selected trait. Other r-selected traits that the moth exhibits besides large breeds of offspring and short life span are: a small body structure, low adult investment in rearing offspring, and high dispersal ability.[1] This shows that the moth is much more r-selected than it is k-selected. Most invasive species tend to be r-selected individuals because of their high growth rate and dispersal ability.[20]

One option currently being explored is a bacterium, Bacillus thuringiensis, which would kill middle-aged larvae of Cactoblastis cactorum.[21] The bacterium was discovered by a group of scientists working in a colonization facility in Tifton, Georgia, who were attempting to rear large colonies of the moth.[21] In their efforts to produce a sterile variety of the moth to eradicate it from its introduced habitat, a unique strain of bacterium was discovered.[21] A particularly virulent variety of the bacterium was cultured that killed 100% of developing Cactoblastis cactorum larvae.[5] This bacterium, when raised with the developing moth larvae, caused 100% mortality due to a combination of excreted exotoxins.[21] The primary lethal exotoxin found in the guts of Cactoblastis cactorum after being exposed to Bacillus thuringiensis is referred to commonly as BtCc.[21] It is being explored as short-term pest suppression because the bacterium is currently unable to be transferred from generation to generation in the gut of the moth.[21] BtCc disrupts the digestive process of the larvae and causes near immediate larvae death.[21] In order for this to be a long-term solution to control the spread of Cactoblastis cactorum, the bacterium or exotoxin would have to be able to reproduce and sporulate within the moth for its effects to spread in the population.[21]

Some researchers are also looking at using a parasitic wasp to curb the spread of Cactoblastis cactorum in the United States.[12] These wasps, native to South America, lay their eggs in Cactoblastis larvae and eat the larvae from the inside out.[12] Current concerns are that the wasp itself could become an invasive species, parasitizing native caterpillars and other native insect larvae.[12]

Another possible control option being explored would utilize ants to serve in a mutually beneficial relationship with the Opuntia cacti. Many ant species in the natural world participate in mutualistic relationships with various species of cactus and it is hoped that this general trend of interaction can be exploited to protect the Opuntia cacti from the Cactoblastis moth.[22] This relationship would offer Opuntia protection from the invader, Cactoblastis, and would offer the ants a place to rear their young and receive nourishment.[23] In South Africa, a mutualism already exists between many species of cacti and ants to prevent the spread of Cactoblastis.[22] Many cactus species throughout the world excrete an extrafloral nectar (ENF) that initially attracts the ants.[23] The ants then feed on this nectar and attack anything that disturbs the cacti.[23]

Researchers at Rice University in Houston and the Florida A&M University are collaborating in their research to explore such a beneficial relationship that could be reproduced in the United States.[22] Initial lab experiments showed that the presence of ants living together with the cacti increased the mortality of Cactoblastis eggs laid on the cacti.[22]

Currently in the United States, most populations of Cactoblastis cactorum have been discovered in Florida, Georgia, and most recently, in Louisiana. Many of these states have already begun their own programs to halt the progress of the moth in conjunction with the 2009 Strategic Plan.[5][24]

The United States began a plan in 2009 through the Animal and Plant Health Inspection Service (APHIS) to quarantine afflicted Opuntia species and slow the migration of Cactoblastis cactorum across the United States. The APHIS hopes to create a permanent barrier across which colonization of the invasive moth would be impossible.[5] This barrier includes not only colonization of new areas across land, but also by sea. By sea, new regulations by APHIS require livestock fodder in transit found with evidence of Cactoblastis cactorum to be destroyed, fumigated, or returned to its country of origin.[5] By land, quarantining means creating a barrier of area over which the moth will be unable to reproduce across the gap; this has been accomplished by physically removing all cacti in swaths of area or by removing and replacing afflicted cacti.[5]

One of the primary implementations proposed would be to create a sterile version of Cactoblastis cactorum that would serve to eliminate the westernmost population of the moth and push its current territory eastward.[5] This method would create a generation of moths which is unable to reproduce and would slowly but effectively curb the spread of the species. However, the sterile version of the moth has had little success in spreading to the majority of the affected area.[1]

Other methods include identifying infected areas and then mechanically destroying all cacti in that area. Crude methods such as these are being used in Louisiana to eliminate the presence of the moth in swampy areas and generally involve the heavy use of removal by chainsaw, hacksaw, or other mechanical chopping device. Chemical pesticides in most areas have proven to be ineffective due to the large quantity and frequency of treatment of pesticide needed to limit the population and also the effective protection the moth is allotted by the cactus leaves.[5]