PRICKLY PEAR CACTUS
Opuntia inermis deCandolle & Opuntia stricta Haworth -- Cactaceae
The earliest record of the biological control of a noxious plant involved the intentional introduction of the cochineal insect, Dactylopius ceylonicus (Green) to northern India from Brazil in 1795 in the false assumption that it was D. coccus Costa, a species cultured commercially as a source of carmine dye. Instead of reproducing well on the cultivated, spineless prickly pear cactus, Opuntia ficus-indica (L.) Miller, D. ceylonicus readily transferred to its natural host plant, O. vulgaris Miller, that had become widespread in India when it escaped cultivation in the absence of its South American natural enemies. Once the value of D. ceylonicus as a biological control agent was recognized, it was introduced in 1836-1838 to southern India, where it brought about the first successful, intentional use of an insect to control a noxious plant. Shortly before 1865, D. ceylonicus also was transferred from India to Sri Lanka which resulted in the successful control of O. vulgaris throughout the island (Goeden 1978, Moran & Zimmerman 1984).
An attempt was made to introduce D. ceylonicus to Australia from Ceylon and India in 1903, without success (Goeden 1978, Moran & Zimmerman 1984). Then an intensive Australian effort on the biological control of prickly pear cacti (Opuntia spp.) began in 1913-1914, when the two membered Prickly-Pear Travelling Commission surveyed the insects and pathogens associated with these plants in Java, Sri Lanka, India, East Africa, South Africa, the Canary Islands, littoral Mediterranean countries, the United States, Mexico and parts of Central America, the West Indies, South America and Hawaii (Johnston & Tryon 1914). This effort of worldwide exploration for natural enemies of a group of noxious plants remains unequalled in scope of geographic coverage.
Biological control of the prickly pear cacti, Opuntia inermis deCandolle and O. stricta Haworth in Australia ranks as one of the most successful projects in biological control of noxious plants. The project followed the initial efforts of the Prickly Pear Travelling Commission, which first recognized the potential value of what was later to become the principal natural enemy, the moth, Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae) (Goeden & Andrés 1999). The principal entomological effort in this biological control project occurred during the 1920's when North and South America, particularly the southern United States, Mexico and Argentina, were thoroughly explored for potentially useful, cactus-feeding insects. More than 150 species of cactus insects eventually were collected and studied, many of which were new to science. From 1921 to 1925, 48 species were imported into Australia, of which 19 were liberated and 11 became established (Goeden & Andrés 1999).
A single consignment of C. cactorum was imported from Argentina in 1925. Large scale mass culture and host plant specificity tests with useful and weedy plant species were undertaken for the first time in a biological control project. Cactoblastis cactorum became widely established following the distribution of more than 2.7 billion mass reared and field collected eggs between 1925 and 1933. Almost 90% of the original stands of O. inermis and O. stricta were destroyed by 1934 through larval feeding by this moth, supplemented by airborne, soft-rot bacteria for which the borers provided entrance wounds into infested plants. Virtually complete control of the cacti was achieved in Queensland and northern New South Wales involving 24 million ha of formerly infested land that was restored to agricultural use (Dodd 1940, Goeden 1978, Moran & Zimmerman 1984).
The spectacular success of Cactoblastis cactorum tended to eclipse the benefits derived from other cactus insects used in biological control, notably several species of cochineal insects (Moran & Zimmerman 1984). For example, Dactylopius ceylonicus was successfully reintroduced to Australia during 1913-1915 and virtually eliminated O. vulgaris as a rangeland weed in Queensland. Both C. cactorum and Dactylopius spp. were transferred during the mid 1920's and 1930's to countries where prickly pear cacti also were introduced pests: Indonesia, Mauritius, New Caledonia, Reunion and South Africa (Rao et al. 1971, Greathead 1971, Goeden 1978). In South Africa and Mauritius these early successful transfer projects led to the independent development of other successful research projects in the biological control of noxious plants (Greathead et al. 1971, Goeden 1978, Julien 1982, 1987).
Details of the Collections
DeBach (1974) expounding on the prickly pear cactus in Australia, stated that it was a massive, long term project comparable to the gypsy and brown tail moth project and represents the first striking success in biological control of a weed. Many entomologists and assistants were employed and a lot of foreign exploration carried out in the Americas ranging from the United States to Argentina. DeBach (1974) considered it ironic that but for an unfortunate circumstance, the natural enemy Cactoblastis cactorum berg, which caused the ultimate destruction of the prickly pear, could have expressed itself at least 10 years earlier than it did and with a minimum of cost. It would have been another simple, nearly miraculous success like the cottony-cushion scale project. Apparently Mr. Henry Tyron, one of a pair of members of a Queensland Prickly Pear Travelling Commission, was impressed by the potentialities of the Cactoblastis work he saw in Argentina and actually brought a small number of larvae back to Australia in 1914; unfortunately he was not able to rear these to maturity. No further project activity occurred until 1920 and Cactoblastis was not imported until 1925.
The enormity of the prickly pear problem in Australia is difficult to visualize. Evidently various species were brought by the early settlers as ornamentals and escaped. Two species, Opuntia inermis and Opuntia stricta, assumed major catastrophic pest proportions but several others were pests in more localized areas. These two became established in the 1800's when Australia was being opened for grazing and the human population was sparse. After 1900 the cacti spread rapidly, reaching a peak around 1925 when 60,000,000 acres were heavily infested, 30,000,000 of which were so dense that the land was completely useless. Hundreds of miles were impenetrable to humans or animals. Otherwise it was good grazing land with potential for dairying and general farming. Eighty percent of the infested land was in Queensland and 20% in New South Wales. The cost of chemical or mechanical control was more than the land was worth. During the peak years the rate of spread of the cacti was alarming; year by year more land became unoccupied and more holdings and homesteads deserted (DeBach 1974).
The project that led to ultimate and complete success began with the appointment of the Commonwealth Prickly Pear Board in 1920. Their first approach was to send entomologists to America and to establish insectaries and quarantine facilities in Australia. They decided early to concentrate on cactus-feeding insects rather than disease organisms because disease did not appear to be important in the field. Several diseases occurred accidentally in Australia already, and there was the question of diseases transferring to other plants.
All told there were 150 species of insects discovered that were restricted to feeding on cacti. About 50 species were sent to Australia, totaling more than 500,000 individuals. All imported material was screened through a central quarantine and culture laboratory at Brisbane and subjected to starvation tests on a wide variety of other plants before being sent out to local breeding or colonization stations. Final tabulation showed that 12 species of prickly pear insects had become established and were exerting some measure of control when the establishment and final success by Cactoblastis put an end to further trials.
Larvae of Cactoblastis cactorum were collected, apparently by Alan P. Dodd, at Concordia, Argentina in late January 1925, and taken to Buenos Aires. DeBach (1974) notes that they came from two different species of Opuntia than those they were destined to attack in Australia. Moths emerged in February and readily laid many eggs. Six Wardian cages were filled with Opuntia for food and 3,000 Cactoblastis eggs were placed on the prickly pear in the cages. In March the shipment went by steamer via Cape Town to Australia. Some 25 larvae were removed for examination at Cape Town so the original shipment then included 2,750, and this gave rise to all Australian stocks. The unsupervised cages arrived in Brisbane 10 weeks and 14,000 miles later in May 1925 in excellent condition. They contained primarily half grown larvae. This was possible because cactus pads may last several months. These larvae were given food and they successfully pupated in August-September 1925, producing 1,137 cocoons. There were 1,70 moths emerged which produced 100,605 eggs, a return of nearly 36X.
DeBach (1974) considers that had this shipment not succeeded, it is doubtful whether or at least when, another would have been made. Dodd returned to North America following the shipment because Argentina had a limited fauna of Opuntia insects and he had no particular reason to believe that Cactoblastis might be more successful than other Opuntia feeding insects. According to Dodd, "certainly, its remarkable achievement could not have been foretold." Therefore, if the one shipment had not resulted in establishment, further investigations in Argentina may not have been undertaken for some years.
The first colonizations of Cactoblastis consisting of 2,263,150 eggs were made in various localities in Queensland and New South Wales during February-March 1926. By March 1927, 10,196,150 eggs had been released in the field. Redistribution of eggs obtained from field material finally amounted to 389,225,520 by November 1929. Establishment and increase occurred so rapidly that by 1930-2 the general collapse and destruction of the original stands of prickly pear had occurred. Miles of dense growth collapsed in a few months under the concentrated attack of enormous numbers of larvae. According to Dodd the most optimistic scientific opinion could not have foreseen the extent and completeness of the destruction. The spectacle of miles of heavy prickly pear growth collapsing en masse and disappearing in the short time of a few years did not appear to fall within the bounds of possibility. The other cactus natural enemies were generally eliminated at the same time but this did not make a difference. Following the die back of the cactus plants, the Cactoblastis population naturally collapsed and during 1932-3 a heavy regrowth of cactus occurred from the roots. However, Cactoblastis responded to this and destroyed the regrowth during 1933-5. From then on until now the prickly pears have remained under almost complete control and the former densely infested country reclaimed and brought into production (Dodd 1940).
For further details on biological control effort and biologies of host and natural enemies, please also see the following (Tryon 1910, 1911; Hunter et al. 1912, Warren 1914, White-Haney 1914, 1915, 1916; Johnston 1921, 1924; Hamlin 1924a, 1924b, 1925, 1932; Alexander 1925, Dodd 1927a, 1927b, 1929, 1960; Kunhi-Kannan 1928, 1930; Hutson 1926, Imms 1931, Jepson 1930, Ayyar 1931, Sweetman 1935, Pettey 1943, 1946, 1948, 1950, 1953; Bailey 1949, Sellers 1952, David & Mathukrishnan 1953, Fullaway 1954, 1958; Williams 1954, Naude 1955, Pemberton 1957, Anonymous 1960, Simmonds 1960, Wilson 1960, Holloway 1964, Davis 1966, Goeden et al. 1967, 1968, Greathead 1967).
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