| Cactoblastis cactorum | |
|---|---|
| Female moth | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Insecta |
| Order: | Lepidoptera |
| Family: | Pyralidae |
| Tribe: | Phycitini |
| Genus: | Cactoblastis |
| Species: | C. cactorum |
| Binomial name | |
| Cactoblastis cactorum (Berg, 1885) |
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| Synonyms | |
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Cactoblastis cactorum, commonly known as 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 lives in South America. The native habitat of South America is home to many parasitoids and pathogens that control the growth of the moth; however, its introduction to Australia, the Caribbean, South Africa, and other areas has led to its uncontrolled spread and its classification as an invasive species.[1]
In South America, Cactoblastis cactorum has many natural predators, including ants and new world monkeys.[1] Ants, the main predators of the moth, prey upon the larvae of the moth. New world monkeys hollow out the Cactoblastis larvae and pupae from the flattened leaf-like stems, or cladodes, of the cacti.[1] The relationship between Cactoblastis cactorum and Opuntia cactus species is a parasitic one, in which the moth is the parasite and feeds upon the host cactus. In South America, the cactus moth has been found to feed on all Opuntia species with only a few exceptions[1] (e.g. Opuntia sulphurea, Opuntia pampeana, Opuntia quimilo, and a few others). Cactoblastis cactorum, unlike other Cactoblastis species, is not host-specific.[1] Outside of these specific interactions, much of the moth’s relationship within its South American habitat is unexplored.
The adults of Cactoblastis cactorum are non-descript brownish-gray moths with long legs and long antennae. The moth can only be definitively identified by a microscopic examination of dissected male genitalia. They generally appear as typical Pyralide moths with pronounced labial palps of the female, thus the name “snout moths”. The fore-wings show a characteristic banding pattern; however other related moths have similar banding.[2] The hind wings of the Cactoblastis cactorum are whitish and semitransparent. The wingspan between adult moths varies between genders. The average size wingspan is 27-40mm for females and 23-32mm for males.[3] The larvae of Cactoblastis cactorum are caterpillars that start out with a pink-cream color and with age become orange with very distinctive black spots or bands.[3]
Mating in Cactoblastis cactorum occurs before sunrise. Moths find their mates by a sense of smell, not by sight.[4] Once a female has found her mate, she begins to release sex pheromones, which signal to males that she is ready to mate. When the males respond, the mating is initiated. The initial process of mating begins when the female and male attach themselves at their abdomens.[4] The male passes a sac, known as the “spermatophore” and the female will store the sac in her abdomen’s reproductive center.[4] After an incubation period, the female deposits an egg stick ranging from 30-50 eggs.[5] The eggs are laid on the tip of the cactus spine, directly onto the cactus leaf, the cladode, or cactus fruit.[5] Egg sticks, which 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.[5] The external damage is characterized by yellowing of plant tissue, with oozing of plant fluids and insect frass. The larvae feed inside the cactus, and eventually hollow out the cactus pad. The larvae eventually consume all but the vascular tissues. Larvae will typically spend two months within the host cactus during the summer months and during the winter months will spend approximately four months within their host.[3] 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.[5] Most moths reproduce three to four times within their lifetime.[5] The average longevity for female adults is nine days and male adults live around eleven days.[2][6] During this time the female moth does not eat, but spends all of her energy traveling up to ten kilometers looking for dense cactus patches for reproduction. The male moth, however, devotes his time maximizing mating.[6] Males mate between two to five times and wait, on average, two to three days between mating events.[6]
Cactoblastis cactorum was first introduced in Australia in 1925 from Argentina to be used as a biological control agent for Opuntia cacti.[7] It was greatly successful at controlling the population of Opuntia and continues to play a role in regulation of the cactus in Australia to date.[7] Because of the success of Cactoblastis as a biological control agent in Australia, it continued to be introduced in many other places around the world including South Africa in 1933 and the Caribbean in the 1950s.[2] Cactoblastis had an immediate effect on the agricultural community in South Africa: the spineless Opuntia species was valued as a cattle fodder before the onset of Cactoblastis diminished the cactus population .[8] Cactoblastis was introduced to the Caribbean island of Nevis in the 1956, to control a complex of native prickly pear cacti. Following Nevis island, Cactoblastis was also successfully introduced in Montserrat and Antigua in 1960 as a biological control agent.[7]
Following this 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 human transport, unintentional human transport or importation on infected livestock fodder.[2] It has been reported in St. Kitts, the US Virgin Islands, Haiti, Cuba, the Dominican Republic, the Bahamas, the Cayman Islands, Puerto Rico and Barbados .[7] Eventually Cactoblastis was able to travel to the Southeastern United States where it was first detected in Florida in 1989. Cactoblastis cactorum likely entered Florida through importations of Opuntia from the Dominican Republic to Miami.[7] Cactoblastis is currently moving along both the Gulf and Atlantic Coasts at a rate of 100 miles per year and the rate of colonization along the gulf coast is increasing every year.[2] As it spreads, it is threatening the life of Opuntia cacti in Florida, up the Atlantic coast to Charleston, South Carolina and around the Gulf coast to New Orleans.[2]
As the moth moves through the southeastern United States, it is endangering many cactus species and threatening many ecosystems. In Florida concern is greatest for the endangered semaphore cactus, Opuntia corallicola .[9] Current studies are being done 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 cactus industry.[7] Cacti are grown for ornamental sale in Arizona, California, Nevada, New Mexico and Texas. Arizona has the largest economic stake in Cactoblastis, with the retail value of its cactus industry placed at around $9.5 million .[7] 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 Yucatan peninsula .[7] It is unknown how the moth was able to migrate to Mexico. However it is speculated that it was enabled by either winds and hurricanes or by unintentional transport from humans or commercial trade .[7] More recently, Cactoblastis cactorum began to attack Opuntia cacti on San Salvador Island in the Bahamas. Here the Opuntia cacti, especially the prickly pear cactus, are a major food source for the Cyclura, a genus of iguana that lives on rocks. As the population of Opuntia cacti decrease from the infestation of Cactoblastis cactorum, fears that the iguana population is going to be severely hurt are growing .[9]
In the future, researchers are projecting a westward expansion of Cactoblastis cactorum in North America.[9] This westward expansion threatens cactus industries in the southwestern U.S. and in inner parts of Mexico.[9] In the west, there are over sixty Opuntia species that are a vital part of the ecosystem, and Opuntia cacti are producers of the edible prickly pear fruit and serve as a staple part of the human population’s diet, mostly in Mexico. Additionally in Mexico, chopped cactus plants serve as an alternative food for cattle in times of drought and many of the different species of Opuntia support the cochineal dye industry.[9][10] Loss of these cacti would have a major impact on Mexico, environmentally and economically. From estimated biological and stress factors in the moths native community, it is predicted that the moth is bound by a temperate climate due to the incubation period of its larvae.[7] It is therefore predicted that it will be able to live as far north as Charleston, South Carolina, and as far westward as California.[3] However, since it is predicted that the moth is unable to survive in extremely arid desert environments because of the effects that limit its growth, parts of the west may be protected.[3]
The ecological relationship between the Cactoblastis cactorum moth and the Opuntia cactus is a parasite/host relationship.[11] Cactoblastis cactorum is one of the least selective moths in the Cactoblastis genus when it comes to host selection.[1] It does, however, exhibit some form of host selection. The moth selects its host by detecting CAM production in Opuntia cacti.[11] They have a detection system that enables them to detect the carbon and nitrogen gradients in the air surrounding the host.[11] The females have a more superior detection system than the males because they use this to determine where to implant the larvae, which then destroy the cactus.[11]
Many of the other moths in Cactoblastis’ native South American habitat are extremely host specific and cannot prey on many different types of cacti as the Cactoblastis does.[1] Cactoblastis cactorum is therefore able to survive in a number of habitats as long as one of the many species it uses as a host is present.[1] This in turn increases its competitive ability. An example of the moth’s superior competitive ability can be seen when the moth was introduced into Australia with fifty other species. This test was done in order to determine which would be the best biological control agent for the cactus in the area.[9] After some time, it became clear that the moth was more effective at attacking the cactus species than the effect of all of the other species combined.[9]
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.[12] The parasitoids and diseases seem to be enough to control the spread of Cactoblastis cactorum.[12] These parasitoids and diseases are not present in the areas where the moth has become a problem.[12] 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.[12]
Cactoblastis cactorum shows both r-selected and k-selected life history traits. Although the moth reproduces more than once in its life time (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.[13]
Many attempts are being made to halt the expansion of Cactoblastis cactorum in order to prevent further damage to Opuntia cacti across Central America and the Southeast United States, where the effect of the moth has been identified as the most dangerous to the native flora. Some attempts to control the population are biological in nature whereas others are more physical attempts to quarantine afflicted Opuntia.
One optioned currently being explored is a bacteria, Bacillus thuringiensis, which would kill middle-aged larvae of Cactoblastis cactorum.[14] The bacteria 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.[14] In their efforts to produce a sterile variety of the moth to eradicate it from its introduced habitat, a unique strand of bacteria was discovered.[14] A particularly virulent variety of the bacteria was cultured that killed 100% of developing Cactoblastis cactorum larvae.[2] These bacteria, when raised with the developing moth larvae, caused 100% mortality due to combination of excreted exotoxins.[14] The primary lethal exotoxin found in the guts of Cactoblastis cactorum after being exposed to Bacillus thuringiensis is referred to commonly as BtCc.[14] It is being explored as short-term pest suppression because the bacteria is currently unable to be transferred from generation to generation in the gut of the moth.[14] BtCc disrupts the digestive process of the larvae and causes near immediate larvae death.[14] In order for this to be a long-term solution to control the spread of Cactoblastis cactorum, the bacteria or exotoxin would have be able to reproduce and sporulate within the moth for its effects to spread in the population.[14]
Some researchers are also looking at using a parasitic wasp to curb the spread of Cactoblastis cactorum in the United States.[9] These wasps, native to South America, lay their eggs in Cactoblastis larvae and eat the larvae from the inside out.[9] Current concerns are that the wasp itself could become an invasive species, parasitizing native caterpillars and other native insect larvae.[9]
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.[15] This relationship would offer Opuntia protection from the invader,Cactoblastis, and would offer the ants a place to rear their young and receive nourishment.[16] In South Africa, a mutualism already exists between many species of cacti and ants to prevent the spread of Cactoblastis.[15] Many cactus species throughout the world excrete an extrafloral nectar(ENF) that initially attracts the ants.[16] The ants then feed on this nectar and attack anything that disturbs the cacti.[16]
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.[15] Initial lab experiments showed that the presence of ants living together with the cacti increased the mortality of Cactoblastis eggs laid on the cacti.[15]
Currently in the United States, 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.[2][17]
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.[2] 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.[2] 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.[2]
One of the primary implementations proposed would be to create a sterile version of Cactoblastis cactorum that would serve to eliminate the western most population of the moth and push its current territory eastward.[2] This method would create a generation of moth 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.[2]
There is still much to be learned about Cactoblastis cactorum. Its native habitat of South America remains a mostly-unexplored area of scientific research. The moth’s interactions with other species are not well understood. Although it is well-known that the moth is capable of switching hosts, the full range of host plants susceptible to the moth is unknown. How the moth will affect agriculture in North America will be seen in the next few years and the effectiveness of many of the control tactics may be seen in even less time. Lastly, a big unknown in the study of Cactoblastis cactorum is how Opuntia species may develop defenses in response to its invasion. More studies need to be done both to understand the biological mechanisms of the moth and to halt its spread as an invasive species.
Dalby in Queensland, Australia, has a monument commemorating eradication of Opuntia by the moth in a park by Myall Creek, which runs through the town.
The Boonarga Cactoblastis Hall is located 10 km east of Chinchilla, and purports to be "the only building dedicated to an insect."[18] It was erected in 1936, and is the first insect memorial ever built.[19]
|title= specified when using {{Cite web}}". Texas Invasives. Texas Invasive Plant and Pest Council. http://texasinvasives.org/. Retrieved 27 September 2011.