Cactoblastis cactorum
Cactus moth - Cactoblastis cactorum Bergroth
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Cactus moth
In 1989, the cactus moth, Cactoblastis cactorum, was collected on Big Pine Key in Florida. A biological control agent typically released for controlling Opuntia prickly pear cacti in other parts of the world, this was the first U.S. record of this moth. Since then, C. cactorum has spread through Florida, north to Bulls Island, South Carolina, and west along the coast to Dauphin Island, Alabama (J. Floyd pers. comm., 2004). If C. cactorum continues to migrate west, it is expected to reach Texas in 2007 and from there it may spread into Mexico (Solis et al., 2004). If C. cactorum reaches the southwestern U.S. and Mexico, it is expected that the impacts to cacti, wild and cultivated, will be devastating.
In 1920, C. cactorum was introduced from Argentina as a biological control agent for invasive Opuntia in Australia (Solis et al., 2004). The Opuntia controls resulting from these releases were a huge success (90% of the non-native Opuntia were destroyed ) and so distribution to other countries for Opuntia control soon followed. South Africa, St. Helena, Hawaii, New Caledonia, Pakistan (where the moth did not establish), Kenya (where the moth did not establish), and the Ascension Islands were all release sites (Stiling, 2002; Zimmerman et al., 2000). In South Africa, eleven species of non-native but established Opuntia, including 9 originally from Mexico, are attacked by C. cactorum (Perez-Sandi, C., 2001). Releases in the Caribbean, starting on the island of Nevis in 1956, proved less successful as both non-native and native cacti were attacked (Solis et al., 2004). A researcher in 1986 unsuccessfully searched for C. cactorum in central Florida, but did find the moth in 1988 in Cuba; the Cuban discovery led to the search and discovery of C. cactorum in the Florida Keys (Stiling, 2002). The C. cactorum found in the Florida Keys could have originated from the Caribbean by natural means--either by flight or wind (Solis et al., 2004; Stiling, 2002) -- or through the horticultural trade of prickly pear cacti (Pemberton, 1995). Pemberton (1995) reports that in Miami ports, Cactoblastis was detected 17 times between 1981 and 1993, all on vegetation imported for propagation. Since 1991, C. cactorum has also been found infesting commercial nurseries in Florida (Pemberton, 1995).
The native range of the moth is Argentina, Paraguay, Uruguay, and southern Brazil. In Argentina, C. cactorum is a major pest in cultivated opuntias. A single female typically lays 50-90 eggs on top of each other forming what looks like a cactus spine (the stack of eggs is referred to as an eggstick); 3-4 eggsticks are produced in a female's lifetime (Stiling, 2002). Larvae hatch from the eggs in 4-5 weeks (Carpenter et al., 2001b). The larvae are orange to red and black, and burrow into the cactus pad using a single entry hole (Stiling, 2002). Larvae feed and grow for two months during the summer months or about four months during the winter; subsequently, the larvae exit the cactus pad to pupate in the soil or leaf-litter. Adults do not have functional mouthparts, and emerge only to reproduce. While adults in the laboratory survive for as long as two weeks, adult females in outdoor cages live for only 3-5 days in the summer and a week during the winter S. Hight, pers. comm.) Two (sometimes three) generations per year are common in temperate climates, but in tropical climates there may be more generations per year (Zimmerman et al., 2000). Adults may be seen in any month in the Florida Keys (C. Bergh, pers. comm.). Carpenter et al. (2001b), observed in the lab that C. cactorum collected from Georgia completed their lifecycle (oviposition to adult) in about 90 days at 26-27 degrees C.
Cactus damage is primarily from larval feeding, but the associated damage can also provide an entry for secondary pathogens such as fungi or bacteria (Martin & Dale, 2001). Cactus pads that are fed upon by C. cactorum weep a "green slime." Pads are often skeletonized as the larvae feed inside the pad, leaving the outer epidermis intact (Stiling and Moon, 2001). The larvae from a single eggstick are estimated to eat the equivalent of approximately four cactus pads as they complete their development to the pupal stage (Carpenter et al., 2001b).
Opuntia species occur throughout North America, but particularly along the Gulf of Mexico and the western U.S. and Mexico; many species occur in great numbers (Hight et al., 2001), so if C. cactorum spreads to these areas, negative impacts are expected. The U.S.A. has 31 platyopuntiae, including 9 endemic taxa (Stiling, 2002). Of particular concern are rare species such as the Opuntia treleasei, O. corrallicola, O. triacantha, and O. cubensis (Pemberton, 1995). In Texas, Mahr (2001) reports six taxa of Opuntia with localized distributions that could be at risk.
In the U.S., Opuntia is important for the nursery industry, with Arizona and southern California having the greatest number of nurseries. Arizona's nursery industry (wholesale and retail) is valued at $14 million (Irish, 2001). Agriculturally, O. ficus-indica varieties are grown to feed livestock and fruit (Zimmerman et al., 2000). In 1998, California was the primary U.S. state with commercial production of Opuntia products for food: Monterey County generated a crop value of $2 million per year (Garrett, 2004). U.S. imports of Opuntia vegetables are worth over $27 million (Garrett, 2004).
In Mexico, there are 56 platyopuntiae, of which 38 species are endemic. Opuntia in Mexico covers 3,000,000 ha. Both the pads and fruit are cultivated for food, and the cacti pads are also used to feed cattle. Commercially, Opuntia is used in the dye and other industries such as in medicinal products and cosmetics. Opuntia products generate an income valued at over US $50 million/year (Stiling, 2002). Cactoblastis cactorum has been found on imported cactus pads in Mexico, but surveys have not found any moths there (Zimmerman et al., 2000).
Ecologically, Opuntia is an important food source for mammals and insects and a nesting place for birds and rodents. The possible impacts of declining cacti populations include increased erosion, decreased moisture in the soil, and the invasion of other plant species (Stiling, 2002). In Cuba, a population of the rare O. dillani was destroyed by C. cactorum feeding. Ninety percent of Opuntia cacti in Florida have been attacked, although mortality is highest in smaller cacti (Stiling, 2002). As a result, reduced populations will most likely come from the death of juvenile cacti (Zimmerman et al., 2000). Plant form may also play a part in the totality of impact. Small, prostrate cacti, such as O. triancantha, seem to be preferred by C. cactorum (Stiling, 2002). In O. corallicola cacti planted for restoration in Florida, greater C. cactorum feeding was seen on plants in shady areas as opposed to in sunny sites. In Florida C. cactorum has been observed feeding on O. stricta, O. pusilla, O. humifusa, O. cochenillifera, and O. ficus-indica. Rare, native Floridian cacti such as O. corallicola and O. triacantha have also been eaten (Solis et al., 2004).
Interestingly, in some areas of its native range, C. cactorum spread has been quite limited. In Brazil, C. cactorum has not spread from its native range in southern Brazil to central Brazil, even though hosts are present and the climate would not inhibit establishment. In Argentina, C. cactorum has not infested cultivated O. ficus-indica along the Andean foothills (Zimmerman et al., 2000).
In Mexico, the estimated potential distribution of C. cactorum is in the east and northeast with the possibility of distribution in the southeast and along the coast (Soberon et al., 2001). The estimated potential distribution of C. cactorum in the U.S. is north to Charleston, South Carolina and west through Texas and the lower elevations of New Mexico, Arizona and California (Pemberton, 1995).
In 2002, it was estimated that C. cactorum populations were capable of expanding their range in Florida by 50-75 km (31-47 miles) per year (Stiling, 2002). Recently, it was estimated that C. cactorum was spreading westward at 161 km (100 miles) per year (Solis et al., 2004). This is a much faster rate of spread than recorded in Australia with 16-24 km (10-15 miles) in 2.5 years or in South Africa, where moths spread 3-6 km in 2.5 years. Female moths have been recorded flying 24 km (15 miles) to lay their eggs (Zimmerman et al., 2000). Because females are attracted to lights (Zimmerman et al., 2000), dispersal by vehicles and trains may explain the fast rate of spread in some but not all instances (Stiling, 2002). A combination of multiple introductions and the movement within infected nursery stock could also explain faster spread throughout Florida (Zimmerman et al., 2000).
There are not many choices for controlling infestations or stopping the spread of these moths. Infested cacti are typically in sensitive natural areas and the use of chemical control measures is not advised (Stiling, 2002). In Florida, Papilio aristodemus ponceanus Schaus (Schaus swallowtail), Anaea floridalis Johnson and Comstock (the Florida leafwing), Strymon acis (Drury) (Bartram's scrub-hairstreak) and Gerstaeckeria fasciata Pierce (the Gerstaeckeria cactus weevil) are rare and endangered insects that could be negatively impacted by broad spectrum insecticides. The injection of a systemic insecticide into cactus pads does not provide adequate control, nor would it be practical for use in natural areas (Carpenter et al., 2001a). Similarly, the removal of egg sticks is recommended in Opuntia plantations but may not be practical in natural areas (Stiling, 2002).
Known biological control agents, both pathogens and insects, are too general and could negatively impact native Lepidoptera. Nine parasitoids attack C. cactorum in South America (Pemberton and Cordo, 2001b). The most common, a braconid, Apantales alexanderi, is a generalist and so may not be suitable for biological control (Zimmerman et al., 2000) because it could also parasitize related cactus moths (Pemberton & Cordo, 2001b). Larvae of a related cactus moth, Melitara prodenalis feed on the same hosts and in the same geographic area and are very similar to C. cactorum, making this cactus moth the most at risk.
Pathogens considered for the biological control of C. cactorum include a fungus (probably Beauveria bassiana) which has a wide host range with 200 hosts, and two protozoans (Nosema cactoblastis and N. cactorum) which have been found attacking C. cactorum in South Africa. The protozoans have been observed causing high mortality to C. cactorum, but it is unknown if they would attack other moth species (Pemberton & Cordo, 2001a; 2001b).
Currently the idea of genetic control is being researched, such as using Sterile Insect Technique (SIT), or inherited sterility (sometimes also called F1 sterility), to control populations and stop the spread of C. cactorum (Bloem et al., 2003; Carpenter et al., 2001a). With SIT, large numbers of males are raised and then sterilized, typically with irradiation. These sterile males are released, where they will mate with wild females who will lay no eggs. With inherited (F1) sterility, large numbers of males are raised and also irradiated, but with a lower dose so that when they mate with wild females, these females produce eggs, but the eggs hatch and grow to adults that are sterile, increasing the numbers of sterile moths (Stiling, 2002). In order to be successful, a genetic control program needs to be able to raise large numbers of C. cactorum that can successfully compete with wild moths for mating opportunities. Sterilization procedures would also need to be determined and a protocol to monitor the success of the releases is important.
A laboratory colony of C. cactorum has been established by researchers working for the Agricultural Research Service (ARS) in Georgia. Originally raised on Opuntia pads, the researchers are now using an artificial diet to raise large numbers moths, which seem to be competitive with wild moths (J. Floyd, pers. comm., 2004).
Although females use a pheromone to attract males for mating, a specific, synthetic pheromone has not been developed. Monitoring populations in research has typically used a sticky trap baited with virgin C. cactorum females (Hight et al., 2002), but this is not recommended in uninfested areas (Bloem et al., 2003). Bloem et al. (2003) found that irradiated (200 Gy) females are equally attractive to males as untreated females in sticky traps. The ability to trap moth populations would be important for monitoring population movement, detecting new establishments, and determining success for sterile releases.
Carpenter et al. (2001b) explored the effects of irradiation on sterility in the parental and F1 generation. The authors suggest that a dose between 100 and 200 Gy would be useful for research and genetic control strategies, but more research at these levels should be done. Inherited sterility could be useful for determining the potential distribution and host range of C. cactorum as well as for the control of newly established populations or as a barrier to prevent further spread to the western U.S. and Mexico (Carpenter et al., 2001a).
The current plan is to conduct a field trial of the F1 SIT in spring-summer 2005 through collaboration between the Animal and Plant Health Inspection Service (USDA APHIS) and ARS. Researchers will be removing eggsticks and infested pads along the leading edge of the western-most infestation (Dauphin Island, Alabama and Santa Rosa Island, Florida) before inundating the islands with sterile moths. Populations of C. cactorum will be monitored before and after sterile releases to determine efficacy (J. Floyd, pers. comm., 2004). Volunteers from The Nature Conservancy and other organizations will work with University of Mississippi researchers to assist with field monitoring of moths and eggsticks.
Assuming the field trial is successful, APHIS is working to secure funding for a full-scale sterile release program to form a barrier to stop the western movement of the moth. The permanent program is estimated to cost approximately $1.5 million per year.
Sources
Bergh, Chris. 2004. The Nature Conservancy, Florida Keys office, personal communication.
Bloem, S., J.E. Carpenter and K.A. Bloem. 2003. Performance of sterile Cactoblastis cactorum (Lepidoptera: Pyralidae) females in luring males to traps. Florida Entomologist 86(4): 395-399.
Carpenter, J.E., K.A. Bloem and S. Bloem. 2001a. Applications of F1 sterility for research and managment of Cactoblastis cactorum (Lepidoptera: Pyralidae). Florida Entomologist 84(4): 531-536.
Carpenter, J.E., K.A. Bloem and S. Bloem. 2001b. Inherited sterility in Cactoblastis cactorum (Lepidoptera: Pyralidae). Florida Entomologist 84(4): 537-542.
Floyd, Joel P. October 2004. Animal and Plant Health Inspection Service, United States Department of Agriculture, personal communication.
Garrett, L. 2004. Economic impact from spread of Cactoblastis cactorum in the United States. White paper prepared for USDA APHIS PPQ Center for Plant Health Science and Technology, Raleigh, NC.
Hight, Stephen February 2005. personal communication.
Hight, S.D., S. Bloem, K.A. Bloem and J.E. Carpenter. 2001. Cactoblastis cactorum (Lepidoptera: Pyralidae): observations of courtship and mating behaviors at two locations on the gulf coast of Florida. Florida Entomologist 86(4): 400-408.
Irish, M. 2001. The ornamental prickly pear industry in the southwestern United States. Florida Entomologist 84(4): 484-485.
Mahr, D.L. Cactoblastis cactorum (Lepidoptera: Pyralidae) in North America: A workshop of assessment and planning. Florida Entomologist 84(4): 465-473.
Martin, H.L. and M.L. Dale. 2001. Potential of Cactoblastis cactorum as a vector for fungi pathogenic to prickly pear, Opuntia inermis. Biological Control 21: 258-263.
Pemberton, R.W. 1995. Cactoblastis cactorum (Lepidoptera: Pyralidae) in the United States: an immigrant biological control agent or an introduction of the nursery industry? American Entomologist 41: 230-232.
Pemberton, R.W. and H.A. Cordo. 2001a. Nosema (Microsporidia: Nosematidae) species as potential biological control agents of Cactoblastis cactorum (Lepidoptera: Pyralidae): surveys for the microsporidia in Argentina and South Africa. Florida Entomologist 84(4): 527-530.
Pemberton, R.W. and H.A. Cordo. 2001b. Potential and risks of biological control of Cactoblastis cactorum (Lepidoptera: Pyralidae) in North America. Florida Entomologist 84(4): 513-526.
Perez-Sandi C., M. 2001. Addressing the threat of Cactoblastis cactorum (Lepidoptera: Pyralidae), to Opuntia in Mexico. Florida Entomologist 84(4): 499-502.
Soberon, J., J. Golubov and J. Sarukhan. 2001. The importance of Opuntia in Mexico and routes of invasion and impact of Cactoblastis cactorum (Lepidoptera: Pyralidae). Florida Entomologist 84(4): 486-492.
Solis, M. A., S. D. Hight and D. R. Gordon. 2004. Tracking the cactus moth, Cactoblastis cactorum Berg., as it flies and eats its way westward in the U.S. News of the Lepidopterists' Society 46(1): 3-7.
Stiling, P. 2002. Potential non-target effects of a biological control agent, prickly pear moth, Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae), in North America, and possible management actions. Biological Invasions 4: 273-281.
Stiling, P. and D. C. Moon. 2001. Protecting rare Florida cacti from attack by the exotic cactus moth, Cactoblastis cactorum (Lepidoptera: Pyralidae). Florida Entomologist 84(4): 506-509.
Zimmerman, H.G., V.C. Moran and J.H. Hoffmann. 2000. The renowned cactus moth, Cactoblastis cactorum: its natural history and threat to native Opuntia floras in Mexico and the United States of America. Diversity and Distributions 6: 259-269.
