Cactoblastis Cactorum Research Articles

Nosema (Microsporida: Nosematidae) Species as Potential Biological Control Agents of Cactoblastis cactorum (Lepidoptera: Pyralidae): Surveys for the Microsporidia in Argentina and South Africa

Cactoblastis cactorum Berg is an invasive moth in North America where it damages and threatens many native Opuntia cacti. Nosema species of C. cactorum may have potential as biological control agents of the moth. Surveys for Nosema species were made in South Africa, where two of these Microsporidia were described from the moth and in Argentina where these pathogens may have originated. No Nosema were found in the C. cactorum larvae from South Africa and low levels of infection (0-6%) were found in the South American larvae. The low abundance of C. cactorum and the time of collection (austral summer) may be the reasons for the absence of or rarity of Nosema in these surveys. Winter collections of the larvae are suggested to obtain more abundant Nosema for evaluation as potential biocontrols of C. cactorum.

Darwin's Naturalization Hypothesis Revisited

In The Origin of Species, Darwin (1859) drew attention to observations by Alphonse de Candolle (1855) that floras gain by naturalization far more species belonging to new genera than species belonging to native genera. Darwin (1859, p. 86) goes on to give a specific example: “In the last edition of Dr. Asa Gray’s ‘Manual of the Flora of the United States’ ... out of the 162 naturalised genera, no less than 100 genera are not there indigenous.” Darwin used these data to support his theory of intense competition between congeners, described only a few pages earlier: “As the species of the same genus usually have, though by no means invariably, much similarity in habits and constitution, and always in structure, the struggle will generally be more severe between them” (1859, p. 60). Darwin’s intriguing observations have recently attracted renewed interest, as comprehensive lists of naturalized plants have become available for various regions of the world. Two studies (Mack 1996; Rejmanek 1996, 1998) have concluded that naturalized floras provide some support for Darwin’s hypothesis, but only one of these studies used statistical tests. Analyses of additional floras are needed to test the generality of Darwin’s naturalization hypothesis. Mack (1996) tabulated data from six regional floras within the United States and noted that naturalized species more often belong to alien genera than native genera, with the curious exception of one region (New York). In addition to the possibility of strong competition between native and introduced congeners, Mack (1996) proposed that specialist native herbivores, or pathogens, may be

Potential of Cactoblastis cactorum as a Vector for Fungi Pathogenic to Pricklypear, Opuntia inermis

In Australia, fungi associated with larvae of the biological control agent Cactoblastis cactorum may contribute to the control of the exotic weed pricklypear (Opuntia inermis). C. cactorum larvae were assessed for their ability to vector pathogenic fungi into O. inermis by the infestation of larvae with fungal suspensions. Six fungal isolates caused disease after being carried into the host on external surfaces of larvae, and propagules of one isolate (UQ5109) initiated disease after being transferred from the cladode epidermis into the host by larvae feeding on the plant. Scanning electron microscopy revealed extensive hyphal growth on the external surfaces of larvae infested with several of the isolates. Fungi isolated from field-grown O. inermis cladodes were tested for pathogenicity to this plant in an in vivo plant assay. In total, 152 isolates were screened, 22 of which infected the host in pathogenicity tests. Only 1 (UQ5115) infected undamaged host tissue, whereas the remainder required the host to be wounded before infection could proceed. The majority of isolates were only weakly pathogenic, even when inoculated via wounds, suggesting that most were either saprophytes or weak parasites. This study demonstrates that it is possible for larvae of C. cactorum to transmit fungal pathogens into O. inermis tissue and it has provided a sound basis for future field work to determine the contribution that fungi make to the control of O. inermis.

The renowned cactus moth, Cactoblastis cactorum: its natural history and threat to native Opuntia floras in Mexico and the United States of America

Abstract. The cactus moth, Cactoblastis cactorum (Berg) (Phycitidae) is native to South America. It was released as a biological control agent against alien Opuntia‐cacti in Australia in the 1920s, then in southern Africa, and latterly on several islands, including those in the Caribbean. In 1989, the cactus moth was discovered in Florida, in the United States of America, where it is now threatening the survival of indigenous Opuntia species. In this paper we identify some of the attributes that have contributed to the success of C. cactorum as a weed biological control agent. Many of these same qualities account for the problems that C. cactorum has caused in Florida and predispose it as a major threat to the speciose, native Opuntia‐floras of Central and North America. An estimated 79 platyopuntia (prickly pear) species are at risk: 51 species endemic to Mexico; nine species endemic to the United States; and 19 species common to both countries. Many cultivated and wild Opuntia species, that are used in various ways, are also vulnerable to attack by C. cactorum, including at least 25 species in Mexico and three species in the United States, particularly the widely exploited and culturally important cultivars of O. ficus‐indica. Some control strategies are suggested that may minimize the risk and consequences of invasion by the cactus moth. The wider implications of this threat to the practice of weed biological control and to conservation are discussed.

The difficulties of single factor thinking in restoration: replanting a rare cactus in the Florida Keys

The Florida Semaphore cactus, Opuntia corallicola, is a rare species with a distribution currently limited to one island in the Florida Keys. The only 12 mature, flowering individuals known to exist in 1989 were threatened by the arrival of an exotic cactus feeding moth, Cactoblastis cactorum. Cages were erected in 1990 around these cacti to protect them from Cactoblastis. From 1990 to 1998 the O. corallicola population was monitored and number of new recruits were measured. In May 1996, we made an outplanting of 96 small, new cacti derived from fallen pads, in an experimental regime designed to bolster the number of extant cacti and to test for the effectiveness of cages as a protection against Cactoblastis. Both the natural and the outplanted population declined by over 30% during our monitoring periods. In both the natural population and the experimental outplanting most individuals died because of what appeared to be stem browning, probably by a plant pathogen. The next largest cause of mortality in the outplanting was by trampling, probably by Key deer. Cactoblastis destroyed one mature cactus but killed no individuals in the outplanting. A hurricane in 1998 was a minor cause of death for outplanted individuals but killed two mature cacti.

Ribosomal DNA, species structure, and biogeography of the cactophilic yeast Clavispora opuntiae.

The ribosomal DNA of the cactophilic yeast species Clavispora opuntiae was studied in order to clarify the global distribution of the yeast. Over 500 strains, including isolates from several new localities worldwide, were characterized by rDNA restriction mapping. An unusual restriction pattern previously encountered only in one strain, from Conception Island in the Bahamas, was found in several Brazilian isolates. Sequences of the D1/D2 and D7/D8 divergent domains of the large subunit (LSU) and of the intergenic spacers (IGS) confirmed that these strains represent a genetically distinct variety of Clavispora opuntiae. This divergence had previously been hypothesized on the basis of reduced genetic recombination in inter-varietal crosses and the presence of a polymorphic ApaI restriction site located in the LSU. The exact position of the ApaI site in the D8 divergent domain and the nature of the variation that it reveals were determined. The complete sequences of 12 intergenic spacers clarified the significance of the species-wide variation uncovered by restriction mapping. Most of the polymorphic sites occur in the IGS1 and IGS2 regions, on either side of the 5S gene, and the variation is largely due to differences in the numbers and the sequences of internal repeats. Two other polymorphic sites are located in the external transcribed spacer (ETS) region. The reliability of various sites as indicators of overall spacer sequence divergence differed from one case to another. Variety-specific probes were devised and used to screen 120 strains for the presence of recombinant rDNA spacers. Three strains gave ambiguous results, but these did not constitute evidence that inter-varietal recombination has taken place in nature. The hypothesis that the global movement of Clavispora opuntiae has been influenced by the worldwide biological control of prickly pear with Cactoblastis cactorum, a moth of Argentinian origin, has received additional support from the demonstration that Argentinian strains have rDNAs similar to those found where the moth has been introduced. A dramatic founder effect was identified in a yeast population collected in cacti (Maui, Hawaii) in a site where the moth had been recently introduced.

Purification, structural characterization, cloning and immunocytochemical localization of chemoreception proteins from Schistocerca gregaria.

Soluble low-molecular-mass protein isoforms were purified from chemosensory organs (antennae, tarsi and labrum) of the desert locust Schistocerca gregaria. Five genes encoding proteins of this group were amplified by PCR from cDNAs of tarsi and sequenced. Their expression products are polypeptide chains of 109 amino acids showing 40-50% sequence identity with putative olfactory proteins from Drosophila melanogaster and Cactoblastis cactorum. Direct structural investigation on isoforms purified from chemosensory organs revealed the presence in the expression products of two of the genes cloned. Two additional protein isoforms were detected and their molecular structure exhaustively characterized. MS analysis of all isoforms demonstrated that the four cysteine residues conserved in the polypeptide chain were involved in disulfide bridges (Cys29-Cys38 and Cys57-Cys60) and indicated the absence of any additional post-translational modifications. Immunocytochemistry experiments, performed with rabbit antiserum raised against the protein isoform mixture, showed selective labelling of the outer lymph in contact sensilla of tarsi, maxillary palps and antennae. Other types of sensilla were not labelled, nor were the cuticle and dendrites of the sensory cells. No binding of radioactively labelled glucose or bicarbonate was detected, in disagreement with the hypothesis that this class of proteins is involved in the CO2-sensing cascade. Our experimental data suggest that the proteins described here could be involved in contact chemoreception in Orthoptera.

Host‐plant affinities of two biotypes of Dactylopius opuntiae (Homoptera: Dactylopiidae): enhanced prospects for biological control of Opuntia stricta (Cactaceae) in South Africa

1. Until recently, neither the phycitid moth Cactoblastis cactorum nor the cochineal insect Dactylopius opuntiae have been satisfactory biological control agents of Opuntia stricta in South Africa. 2. In marked contrast, both of these agents have kept O. stricta under biological control for many decades in Australia. 3. In an attempt to improve the situation in South Africa, a stock of D. opuntiae was obtained from O. stricta in Australia during 1996. 4. Host‐specificity tests confirmed that the newly imported D. opuntiae from Australia is a different biotype to the one already established in South Africa. 5. The Australian (‘stricta’) biotype thrives on O. stricta but is unable to develop satisfactorily on O. ficus‐indica, while the converse is true for the South African (‘ficus’) biotype, which thrives on O. ficus‐indica but fares poorly on O. stricta. 6. The integrity of the host‐plant specificity of the two biotypes of D. opuntiae has important implications for biological control of Cactaceae in South Africa, and has greatly enhanced prospects that O. stricta can be brought under biological control successfully.

On the nature of keystone species

There is an unfortunate tendency to nominate large and conspicuous creatures as likely keystone species playing roles in ecosystems. Particular favorites in the tropics include fig trees (Ficus spp.), large apes, and colorful birds, but such claims are rarely supported by empirical evidence. Khanina (1998) follows this trend, suggesting that only trees can be considered as keystone species of forest communities (detritus ecosystems). I am sceptical; I suspect that inconspicuous organisms may be the ultimate arbiters of ecosystem function and appearance. Mycorrhizae play a critical, possibly pivotal, role in many forests, and they and other fungi may be more realistic candidates for the title of keystone within forest communities. Similarly, experience in Australia suggests that insects such as the Cactoblastis moth (Cactoblastis cactorum) and insect vectors of Myxomatosis have a greater influence on pasture dynamics than do the more conspicuous herbivores. I suspect that the roles of most organisms in ecosystems may be matters of degrees rather than absolutes such as pivotal (and conversely, redundant). I advocate caution in promoting these concepts without further evidence to support such claims.

Molecular cloning of cDNA for p10, a novel protein that increases in the regenerating legs of Periplaneta americana (American cockroach)

Previously, we purified a protein with a molecular mass of 10 kDa (p10) that increases transiently during the regeneration of legs in the nymphal American cockroach Periplaneta americana, and showed that it is localized exclusively in the cytosol and on the external side of the newly formed epidermis of the regenerating legs [Nomura, A. et al. (1992) Int. J. Dev. Biol. 36, 391–389]. We isolated p10 cDNA and analyzed the expression of the p10 gene. The results indicated that p10 is synthesized as a precursor protein with a putative prosegment including a signal sequence at its N-terminal. The deduced amino acid sequence of p10 showed 53% and 47% identities with those of A10 (a Drosophila antennal protein) and CLP-1 (a moth, Cactoblastis cactorum labial palp protein), respectively. Expression of the p10 gene was shown to be significantly enhanced in regenerating Periplaneta legs. Immunoblotting analysis revealed that p10 was expressed not only in the regenerating legs, but also in the antennae and heads of nymphal and adult cockroaches.

Long‐term population studies and the development of an integrated management programme for control of Opuntia stricta in Kruger National Park, South Africa

1. A cactus, Opuntia stricta, has invaded almost 16 000 ha of conserved, natural habitatand has become a major weed problem in Kruger National Park (KNP), South Africa. 2. The main objectives in the control of O. stricta are to reduce the density of the weed and to curb long‐range dispersal of seeds by preventing young plants from reaching the size (28 cladodes) at which they start to produce fruits. 3. Herbicides have failed to provide satisfactory control of O. stricta because the weed infestations are replenished from seeds in the soil and from small plants that are overlooked during spraying. 4. A phycitid moth, Cactoblastis cactorum, was released in KNP during 1988 in an attempt to control O. stricta biologically. 5. Population counts of the biological control agent and of the weed over a 5‐year period showed that, even though C. cactorum has not provided complete control of O. stricta in KNP, the moderate levels of larval damage have stunted the growth of O. stricta and have considerably extended the time that the young plants take to reach sexual maturity. 6. Comparisons of modelled (i.e. with no C. cactorum) and actual populations of O. stricta showed that C. cactorum is making a substantial contribution to the control of O. stricta in residual infestations of the weed that have been treated with herbicides. 7. The need for long‐term evaluation studies in biological weed control is demon strated by the development of an integrated management programme for effective control of O. stricta.

An integrated management plan for the control of <i>Opuntia stricta</i> (Cactaceae) in the Kruger National Park, South Africa

Opuntia stricta is the most problematic alien plant species in the Kruger National Park (KNP) where 30000 ha around Skukuza have been invaded by the weed. Control of 0. stricta is expensive and time consuming and there are insufficient resources available for the task at present. Biological control using the moth Cactoblastis cactorum has only been partially successful and herbicidal control remains the main method for tackling the problem. In order to optimise the control operation against 0. stricta, a management plan has been developed for the KNP. The infested region of the park has been divided into 18 management units, each of which will be treated in turn. During the first three years the control operations will clear 0. stricta from the peripheral units, to prevent the spread of the weed into other areas of the KNP, after which the central units will be tackled. The objective is to destroy all mature fruiting plants and those nearing maturity to curb long-range (seed) dispersal of the weed. The small juvenile plants, which are difficult to detect, will be suppressed by C. cactorum. After five years the problem should be contained to the extent that an ongoing 'holding' programme will be sufficient to keep the weed in check. The success of the plan will depend on increased resources being committed to alien plant control in the Kruger National Park.

Molecular cloning, by a novel approach, of a cDNA encoding a putative olfactory protein in the labial palps of the moth Cactoblastis cactorum

We have used the CapFinder technology, without the library construction step, to amplify and clone full-length cDNAs expressed in the labial palps (CO 2-sensing organs) of the moth Cactoblastis cactorum. The validity of our approach is exemplified by the sequence analysis of a 597-bp cDNA clone, designated CLP-1, that contains a 390-bp open reading frame (ORF) flanked by motifs characteristic to a full-length cDNA. The ORF in CLP-1 encodes a predicted polypeptide that is 47% identical to a novel protein, OS-D, found exclusively in the olfactory antennal segment of Drosophila melanogaster. Both CLP-1 and OS-D have primary structures that do not bear sequence similarity to any previously characterised proteins including odorant-binding proteins (OBPs) in vertebrates and pheromone-binding proteins (PBPs) in moths. Although they share features common to OBPs and PBPs, such as the presence of signal peptides and cysteine motifs, they clearly belong to a distinct class of olfactory proteins that appear to be unique to insects. The relative abundance of the CLP-1 message in the labial palps of females leads to the suggestion that this protein is involved in the CO 2-sensing cascade. Our results suggest that the experimental procedure can be used as an alternative, rapid method to identify genes expressed in a particular organ, or tissue, especially in situations when the amount of available tissue is a limiting factor.

Effects of changes in atmospheric carbon dioxide on the location of hosts by the moth, Cactoblastis cactorum.

Sensory organs that detect CO2 are common in herbivorous moths and butterflies, but their function has been unclear until now. As the CO2 gradients in the vicinity of a host plant depend on its physiological condition, CO2 could provide a sensory cue for the suitability of the plant as a larval food source. This study investigated whether changing the atmospheric CO2 concentration affected oviposition by Cactoblastis cactorum on its host, the cactus Opuntia stricta. On host plants exposed to rapid fluctuations in CO2 concentration, the frequency of oviposition was reduced by a factor of 3.2 compared to the control. As the fluctuations mask the much smaller CO2 signals generated by the plants, this suggests that those signals constitute an important component of the host identification process. On host plants exposed to a constant background of doubled CO2, oviposition was also reduced, by a factor of 1.8. An increased background reduces host signal detectability, partially as a consequence of a general principle of sensory physiology (Weber-Fechner's law), and partially due to other factors specific to CO2-receptor neurons.

Host Specificity of Cactoblastis cactorum (Lepidoptera: Pyralidae), an Exotic Opuntia -feeding Moth, in Florida

The recent arrival of the cactus moth, Cactoblastis cactorum (Berg) in Florida in 1989 has raised concern for the native Opuntia cacti. At particular risk is Opuntia spinosissima , whose entire U.S. population consists of 12 plants in the Florida Keys. We examined host choice of both C. cactorum larvae and ovipositing females to determine if any species of native Opuntia are preferred by C. cactorum . In addition, larval performance among Opuntia hosts was examined. Ovipositing moths showed no host preference in interspecific host choice experiments. Although larvae significantly preferred O. spinosissima over the other Opuntia species, larval survivorship on O. spinosissima was less than on O. stricta and O. humifusa . We also examined intraspecific host choice between pads of O. spinosissima to see if any of the 12 remaining plants is more resistant to attack than others. Such information is valuable for future restoration efforts to protect O. spinosissima . No adult or larval host choice was demonstrated in the intraspecific experiments.

The CO2 sense of the moth Cactoblastis cactorum and its probable role in the biological control of the CAM plant Opuntia stricta

The interaction between the moth, Cactoblastis cactorum, and the cactus, Opuntia stricta, is used as a model to examine the question of whether the CO2 sense of a herbivorous insect can detect the CO2 gradients associated with a plant's metabolic activity. Both the anatomical and the electrophysiological characteristics of CO2-sensitive receptor neurons in C. cactorum indicate an adaptation to the detection of small fluctuations around the atmospheric background. Evidence is provided that further rises in background will impair the function of the sensory organ. In the habitat of the plant, during the diurnal window of the moth's activity, two types of CO2 gradients occur that are detectable by the moth's sensors. The first gradient, associated with soil respiration, is vertical and extends from the soil surface to an altitude of approximately 1 m. Its magnitude is well above the detectability limit of the sensors. The notion that this gradient provides, to a flying insect, a cue for the maintenance of a flight altitude favourable for host detection is supported by field observations of behaviour. The second gradient, associated with CO2 fixation by the plant, extends from the surfaces of photosynthetic organs (cladodes) over a boundary layer distance of approximately 5 mm. Again, its magnitude is well above the detectability limit. The notion that this gradient provides, to a walking insect, a cue to the physiological condition of the plant is supported by the observation that females of C. cactorum, prior to oviposition, actively probe the plant surface with their CO2 sensors. In a simulation of probing, pronounced responses of the sensors to the CO2-fixing capacity of O. stricta are observed. We propose that by probing the boundary layer, females of C. cactorum can detect the healthiest, most active O. stricta cladodes, accounting for earlier observations that the most vigorous plants attract the greatest density of egg sticks.

Indirect versus direct effects of grasses on growth of a cactus (Opuntia fragilis): insect herbivory versus competition.

Variation in plant performance between microhabitats is usually attributed to direct mechanisms, such as plant physiological tolerances or competitive interactions. However, indirect mechanisms, such as differences in herbivore pressure mediated by microhabitat differences, could create the same pattern of variation. In this study, we investigated the effect of insect herbivore pressure on the growth of the grassland cactus Opuntia fragilis under different regimes of grassland canopy cover. Our purpose was to establish the extent to which canopy cover plays a direct, competitive role versus an indirect, mediatory role in cactus growth. We manipulated aboveground microhabitat, specifically the cover of adjacent grasses. The three treatments were: (1) open canopy, with grass pinned down away from the cactus; (2) shaded canopy, with a partial mesh cage staked over the cactus; and (3) ambient grass canopy. We measured seasonal plant growth and recorded changes in insect herbivore occurrence and damage in relation to cover. Cactus growth, defined as the change in number of live cladodes, was higher in the open than under either treatment where the plant was more shaded (P<0.05). However, allocation to new growth, measured as the proportion of new segments (cladodes) in a patch, did not differ among cover treatments. Thus, the hypothesis that physiological constraints, or competition for light, limited cactus performance in grass is rejected. Instead, we found that both cladode mortality, caused by the larvae of a cactus moth borer (Melitara dentata), and occurrence of the moth were lower in the open microhabitat than in either shaded microhabitat. Thus, higher net growth in the open, unshaded treatment, rather than representing a release from competition for light with grasses, was better explained as an indirect effect of grass cover on the activity and impact of the cactus moth. These results show that indirect effects can lead to a misinterpretation of experimental data on direct effects. These data also contribute to an improved understanding of mixed results in the biological control of weedy cacti. Clearly, future evaluations of the relative importance of physiology, competition, and insect herbivory in plant performance must be environmentally explicit.

Biological control of Opuntia stricta (Haw.) Haw. var. stricta using Dactylopius opuntiae (Cockerell) in an area of New South Wales, Australia, where Cactoblastis cactorum (Berg) is not a successful biological control agent

Dactylopius opuntiae reduced an Opuntia stricta var. stricta population in the Central Tablelands of New South Wales, an area where Cactoblastis cactorum has little impact. Cactoblastis cactorum was bivoltine at the experimental site. The proportion of C. cactorum eggs hatching and larvae penetrating segments was much greater for the generation commencing between November and January compared with that commencing between February and June. Failure of first instar C. cactorum to establish during the latter generation appeared to prevent C. cactorum from having a significant impact on the cactus population. Dactylopius opuntiae released at the site decreased new growth of O. stricta var. stricta and caused direct damage to segments. This damage led to a decline in the cactus population in the area. The use of D. opuntiae for biological control of O. stricta var. stricta in similar areas is discussed.

Prickly Pear Menace in Eastern Australia 1880-1940

Prickly Pear Menace in Eastern Australia 1880-1940

Biological control of prickly pear, Opuntia ficusindica (Cactaceae), in South Africa

The outstanding success of a cochineal insect, Dactylopius opuntiae (Cockerell), and, to a far lesser extent, a phycitid moth, Cactoblastis cactorum (Bergroth), in biocontrol of the prickly pear, Opuntia ficus-indica (L.) Miller, in South Africa was fully reviewed in 1978. The present account updates this information. Today, the weed infests less than 100 000 ha (of the 900 000 ha originally colonized) in the cooler, higher rainfall areas where D. opuntiae is relatively ineffective. Cactoblastis cactorum is able to kill only the small prickly pear plants and suffers lossess because of ant predation, climatic extremes and, partly, to host plant incompatibility. Integrated chemical and biological control methods are described which allow O. ficus-indica to be managed as necessary and also to be exploited as a drought fodder for stock, as a fruit and vegetable source for humans, and as a host plant for the commercial production of dye stuff from the cochineal, Dactylopius coccus O. Costa. As a result of successful biological control over the past 50 years, the public has forgotten the former severity of the prickly pear weed problem in South Africa. The residual O. ficus-indica populations are now highly valued and the biological control agents themselves are regarded as pests. The consequences of these changed perceptions are discussed.