Description and comparison of the pupae of a further two Culicoides (Avaritia) species from the dung of large herbivores in South Africa (Diptera:Ceratopogonidae).

In 2007 Nevill, Venter, Meiswinkel & Nevill demonstrated that the pupae of five Culicoides species belonging to the Imicola complex of the subgenus Avaritia could readily be differentiated from one another using various morphological characters. Three of the described species, Culicoides bolitinos Meiswinkel 1989; Culicoides loxodontis Meiswinkel 1992 and Culicoides sp. # 107 (= C. kwagga, Meiswinkel, unpublished thesis 1995), were reared from the dung of large herbivores, which included buffaloes, elephants, white and black rhinoceroses and zebras. However, during that study a further two Avaritia species, neither of which belonged to the Imicola complex, were reared from dung and these are the subject of the present study. For the past 20 years the adults of these two new closely related species have been known as Culicoides sp. # 54 pale form (p.f.) Meiswinkel and Culicoides sp. # 54 dark form (d.f.) Meiswinkel. The taxonomic description and formal naming of the adults of these two species has yet to be done. The present description and comparison of their pupae show that they are two clearly distinct species; that there is no group of morphological characters that can be used to differentiate these two species from the previously described five species of the Imicola complex; and finally that there was no difference between the pupae of C. sp. # 54 d.f. nor C. sp. # 54 p.f. reared from the dung of different host animals.

Comparative descriptions of the pupae of five species of the Culicoides imicola complex (Diptera, Ceratopogonidae) from South Africa.

The viruses causing the economically important livestock diseases of African horse sickness (AHS) and bluetongue (BT) are transmitted by biting midges of the genus Culicoides (Diptera, Ceratopogonidae). In the Old World the most important vectors of these diseases are Culicoides imicola Kieffer, 1913, Culicoides brevitarsis Kieffer, 1917 and Culicoides bolitinos Meiswinkel, 1989. All three of these vectors belong to the Imicola complex of the subgenus Avaritia Fox, 1955. This species complex now comprises 12 sibling species; ten occur in sub-Saharan Africa and are difficult to identify (based mostly on subtle variations in the wing latterns) and so additional methods of reliable identification are needed. The pupal exuviae of the five commonest sibling species (C. imicola, C. bolitinos, Culicoides loxodontis Meiswinkel, 1992, Culicoides tuttifrutti Meiswinkel, Cornet & Dyce, 2003 and Culicoides sp. # 107) harvested from a variety of large herbivore dung types and from decaying fruits, are described and illustrated in detail. It is shown that they can be differentiated clearly on a number of morphological characters and, furthermore, are separable into two distinct groups based (principally) on the shape of the respiratory organ. A key for identifying and differentiating these five pupae is provided. Also, the pupa of the Oriental-Australasian C. brevitarsis was compared with its allopatric sister taxon, C. bolitinos. Because they share a common larval habitat (cattle and buffalo dung) and are almost inseparable in the adult phenotype, the question of their possible synonymy is raised. However, their respective pupae could not be differentiated on gross morphology and so it is argued that this unresolved problem requires a molecular solution.

Selective use of odour-baited, insecticide-treated targets to control tsetse flies Glossina austeni and G. brevipalpis in South Africa.

The effectiveness of odour-baited targets treated with 0.8% deltamethrin in controlling Glossina austeni Newstead and G. brevipalpis Newstead (Diptera: Glossinidae) was evaluated in Zululand, South Africa. Targets were initially deployed in the three habitat types (grassland, woodland and forest) of two adjacent areas at a density of four targets per km(2). One area functioned as the treatment block (c. 35 km(2)) and included the focus of the target deployment, and the second area functioned as a barrier block (c. 40 km(2)) against tsetse fly re-invasion from the untreated area to the south. After 8 months, targets were removed from open grassland in both areas and target density in wooded habitats and sand forest was increased to eight per km(2). Twelve months later, all targets were removed from the barrier block and used to increase target density in the wooded and sand forest habitats of the treatment block to 12 per km(2). This target density was maintained for 14 months. In the treatment area, a 99% reduction in G. austeni females occurred after 13 months at a target density of eight per km(2) in wooded habitat; this was maintained for 22 months. Reduction in G. brevipalpis was less marked. The relatively poor reduction in G. brevipalpis is attributed to the high mobility of this species and its distribution throughout less wooded and more open habitats.

Review of blackfly (Diptera: Simuliidae) control in South Africa.

The medical, veterinary and economic importance of blackflies in South Africa, and the historical development of blackfly control programmes in various South African rivers, are reviewed in this paper. In 1996 it was estimated that blackflies can cause more than R 88 million damages per annum along the middle and lower Orange River where Simulium chutteri is considered the main pest species. A clear link between the construction of dams and the spread of the blackfly problem was shown. Four phases characterize the development of blackfly control in South Africa: (1) during the 1960s blackflies in the Vaal River were controlled with DDT; (2), during the 1970s and into the 1980s blackflies were controlled using water-flow manipulation; (3) when used at strategic times, water-flow manipulation could be used to enhance the effect of natural predator populations; and (4) during the 1990s the organophosphate temephos and toxins produced by the bacterium Bacillus thuringiensis var. israelensis were tested for their efficacy against blackflies. The larvicides temephos and B. thuringiensis proved to be effective and are still used in several control programmes. The latest research focuses on the factors that influence adult blackfly survival and annoyance, as well as the development of methods that can be used to protect sheep from blackfly attacks.

Evaluation of coloured targets for the attraction of Glossina brevipalpis and Glossina austeni (Diptera: Glossinidae) in South Africa.

Studies on the attractiveness of various coloured targets for Glossina brevipalpis and G. austeni in South Africa showed black and pthalogen blue (p.blue) combinations to be the most effective for both species. A 2 m wide (all targets 1 m high) black/p.blue/black (colour ratio 1:2:1) conformation caught nearly three times more G. brevipalpis and nearly five times more G. austeni than a 1.5 m wide black standard control target. For G. brevipalpis the black/p.blue/black (1:2:1) target should be at least 2 m wide in order to increase catches significantly while a 1.5-2.0 m wide target is optimal for G. austeni. The p.blue section of a 2 m black/p.blue/black target should not make up less than 20% of the total target width for either species. The most effective combination of practical target sizes and colour ratios were a 1.75 m wide black/p.blue/black (1:1.5:1) or 2 m wide target (1.5:1:1.5). Between 61-95% of G. brevipalpis and 34-90% of G. austeni that were attracted, settled first on the black section of black/p.blue targets (> 1 m wide). Further studies revealed that for G. brevipalpis only the black parts of the 2 m wide target need to be treated with insecticide, while the entire 1.75 m wide target should be treated. For G. austeni the total width of either target should be treated with insecticide since this species readily settles on both blue and black.

Evaluation of conventional odour attractants for Glossina brevipalpis and Glossina austeni (Diptera: Glossinidae) in South Africa.

The components of the synthetic ox-odour used in Zimbabwe against Glossina pallidipes and G. m. morsitans were evaluated for the attraction of G. brevipalpis and G. austeni in South Africa. The Zim babwe mixture (Zim-mix), which consisted or acetone and a 1:4:8 mixture of 3-n-propyl phenol, 4-methyl phenol and 1-octen-3-ol, increased the catches of G. brevipalpis by c. 2.1-4.4 times compared to when no odours were used. One of the odour components, namely 3-n-propyl phenol, did not significantly increase the size of the catches. Acetone was an essential component for G. brevipalpis, especially during the warm and wet season when it acted synergistically with high doses of 1-octen-3-ol and 4-methyl phenol. The most attractive odour combination for G. brevipalpis was 1-octen-3-ol released at 2.3-9.1 mg/h with 4-methyl phenol at c. 15.5 mg/h and acetone at c. 350 mg/h. This combination increased the catches by another 2.3-2.8 times when compared to the Zim-mix and 10.1-12.3 times compared to 'no odour'. None of the odour components was attractive for G. austeni. None of the components was repellent for either species.

Evaluation of a proposed odour-baited target to control the tsetse flies Glossina brevipalpis and Glossina austeni (Diptera: Glossinidae) in South Africa.

The most effective odour attractant for G. brevipalpis Newstead, namely a combination of octenol released at c. 9.1 mg/h, 4-methyl phenol released at c. 15.5 mg/h and acetone released at c. 350 mg/h, when used together with the smallest recommended colour target (as determined in previous studies), namely a 1.75 m wide x 1 m high black/pthalogen-blue/black target, was evaluated for the control of G. brevipalpis and G. austeni Newstead. This combination increased the catches of G. brevipalpis by 3.5 fold when compared to the number of those caught on a 1.5 m wide x 1 m high black target baited with a synthetic ox-odour as was used in a trial to control this species in the Hluhluwe-Umfolozi Game Reserve in 1992. There was an indication that odour (olfaction) plays a far more important role in attracting G. brevipalpis than does colour (vision). For G. austeni visual attraction appears to play the major role as the odours used were relatively unattractive to them. The odour-baited target should, however, attract G. austeni in sufficient numbers (visually) to achieve control to the fly.

Review of tsetse flies and trypanosomosis in South Africa.

The history of tsetse flies and nagana (trypanosomosis) in South Africa, and especially in Zululand, is reviewed. Four valid tsetse fly species have been recorded from South Africa. Glossina morsitans morsitans disappeared from the most northerly parts of South Africa during the rinderpest epizootic between 1896-1897. Of the three remaining species that occurred in Zululand, now part of KwaZuluNatal Province, G. pallidipes was the most common vector of nagana in cattle, but was eradicated from this area in 1954. G. brevipalpis and G. austeni remained but were responsible for only a few sporadic cases of nagana up until 1990. A widespread outbreak occurred in 1990 where cattle served by 61 diptanks were found infected with Trypanosoma congolense and T. vivax. Dipping of cattle in a pyrethroid plus the therapeutic treatment of infected animals brought the disease under control. The outbreak also led to a trial to control G. brevipalpis from the most northerly parts of the Hluhluwe/Umfolozi Game Reserve making use of target technology as for savannah species. The results were not satisfactory and the trial was discontinued until further research could provide a more appropriate system for the control of this species. A Tsetse Research Station was established at Hellsgate near St. Lucia Lake where research on G. brevipalpis and G. austeni is conducted into ways and means of monitoring and controlling these species.

Seasonal abundance and parity of stock-associated Culicoides species (Diptera: Ceratopogonidae) in different climatic regions in southern Africa in relation to their viral vector potential.

Seasonal abundance and parity in Culicoides populations, in the vicinity of livestock, were determined at seven sites in five different climatic regions with 220 V down-draught blacklight-traps. In 418 collections made between October 1983 and December 1986, a total of 2,134,171 Culicoides, of which 342,571 were identified to species level and sexed, were collected; 267 of these collections (182,321 Culicoides) were graded for parity. In the frost-free summer rainfall area, Culicoides were collected in large numbers in light-traps throughout the year; this implies breeding and possible virus transmission throughout the winter in certain parts of South Africa. However, where frost occurred, Culicoides numbers usually peaked in late summer and dropped sharply after the first frost. In the latter areas, small Culicoides collections during winter may be due to low winter temperatures and rainfall; low temperatures negatively affect adult activity and reduce the rate of development of larvae and pupae; low rainfall would lead to a reduction of available larval habitats. Relatively large numbers of Culicoides were collected in winter in the temperature frost-free winter rainfall area. In each of the four summer rainfall areas, one Culicoides species remained dominant throughout the year: at two of these areas this species was C. imicola. Other abundant species in some of these summer rainfall areas were C. schultzei s.l. and C. zuluensis. In the winter rainfall area, C. zuluensis, C. magnus, C. gulbenkiani and C. imicola shared abundance. It was established that abdominal pigmentation is an indicator of parity in C. imicola in South Africa. With the increase in Culicoides numbers towards the end of summer, there was also a rise in the proportion of parous (pigmented) females in most Culicoides species, which signifies a higher vector potential for African horsesickness and bluetongue towards the end of summer. This coincides with the seasonal occurrence of viral diseases transmitted by Culicoides species. Nulliparous (unpigmented) females of all Culicoides species were present throughout the year at all sites where Culicoides were continuously collected, confirming uninterrupted breeding in these areas.

Control of pest blackflies (Diptera:Simuliidae) along the Orange River, South Africa: 1990-1995.

The efficacy of Bacillus thuringiensis var. israelensis (B.t.i.) and temephos in controlling the pest blackfly Simulium chutteri Lewis along the middle Orange River between 1990 and 1995, was assessed. Larvicides were applied by helicopter to rapids and riffles between Hopetown and Onseepkans, a river distance of 807 km. Larvicidal efficacy was based on the change in larval abundance at selected sites before and after each treatment. The success of the control programme was assessed independently by local farmers, who ranked adult blackfly annoyance on a 4-point scale. Before treatment, blackfly annoyance showed consistent peaks in spring, and sometimes in autumn, and levels were unacceptably high for between 17 and 36 weeks of the year. After treatment started, blackfly annoyance levels were reduced significantly. The number of annual treatments necessary to reduce blackfly annoyance to acceptable levels was highly variable (3-13), and depended on river conditions, as well as the efficacy and timing of each treatment. During low-flow conditions (< 50 m3/s), applications became increasingly difficult in braided sections of the river, and dosage calculations were inaccurate because of local abstraction and return flows. Both larvicides worked well in winter (water temperature 11-13 degrees C). Control of the spring outbreak can be planned well in advance, with the first treatment starting in mid July. A flexible protocol is required to control outbreaks at other times of the year. We recommended the use of B.t.i. for most applications, with increased dosages during algal blooms (> 1500 cells/ml). The use of temphos in the Orange River should be considered only during algal blooms or when flows exceed 300 m3/s. We conclude that helicopter application of larvicides is an effective method or controlling blackflies, along the middle Orange River.

Culicoides (Diptera:Ceratopogonidae) associated with livestock in the Onderstepoort area, Gauteng, South Africa as determined by light-trap collections.

In 54 light-trap collections made at 28 sites in the Onderstepoort area a total of 178,941 Culicoides midges of 35 species was collected in March 1988; the survey was repeated at 26 sites in September and yielded 19,518 Culicoides of 24 species. The number of Culicoides species collected totalled 38. C,imicola was the most abundant species at 27 of the 28 sites sampled, and accounted for 88% and 67% of all midges collected in the two months respectively. This study not only confirms that C. imicola is widespread and abundant in the greater Onderstepoort area, but also that its numbers correlate positively with the historical prevalence of African horse sickness (AHS) and bluetongue (BT) locally. The high numbers of C. imicola make Onderstepoort the ideal site for the study of its laboratory vector capacity. The relatively low numbers of Culicoides spp. other than C. imicola in the Onderstepoort area, will severely limit studies on their roles in the transmission of arboviruses. The origin of the blood-meals of 1338 engorged Culicoides belonging to 13 species was determined by means of cross-over electrophoresis precipitin test; C. imicola fed on cattle, horses, sheep of pigs, Four other Culicoides species showed a similarly wide host range.

Geographical distribution and relative abundance of stock-associated Culicoides species (Diptera: Ceratopogonidae) in southern Africa in relation to their potential as viral vectors.

To determine the geographical distribution and relative abundance of Culicoides species associated with livestock, 220-V down-draught light-traps equipped with 8-W blacklight tubes were operated at 34 sites in different climatic regions in South Africa and Lesotho. From January 1984 to September 1986, 3 041 631 Culicoides, belonging to at least 50 species, were collected in a total of 959 collections. Of these, 572 412 individuals were identified and sexed. Culicoides species were found to be widespread in South Africa and were collected in varying numbers at all the sites sampled. The average catch size, however, was larger in frost-free areas than in areas with extreme winters. The more abundant and widespread species, which have the potential to be vectors of stock-associated viruses such as bluetongue and African horsesickness, were C. imicola, C. leucostictus, C. schultzei s.l., C. pycnostictus, C. nivosus, C. similis, C. zuluensis, C. magnus, C. bedfordi, C. neavei, C. brucei, C. tropicalis, C. exspectator, C. gulbenkiani, C. bolitinos, C. ravus, C. coarctatus and C. onderstepoortensis. Of these, C. imicola was the most abundant species, being dominant at 17 of the 34 sites sampled and accounting for 71.4% of the specimens collected. As C. imicola is relatively uncommon in hot and dry as well as cool and wet areas, this species cannot be regarded as the only vector of stock-associated viruses in southern Africa. Further laboratory vector-competence studies, i.e. determination of viral-infection and -transmission rates, should first concentrate on the above-mentioned Culicoides species, especially those known to feed on livestock.

A survey of the Culicoides (Diptera: Ceratopogonidae) of the Umlalazi Nature Reserve in Zululand, South Africa, with notes on two species biting man.

Culicoides biting midges were intermittently collected between July 1988 and December 1992 in the Umlalazi Nature. Reserve on the subtropical eastern coastal margin of South Africa. Altogether 34 species were collected in a diversity of habitats that included a mangrove community, dune forest and mixed thornveld. Most Culicoides were collected with the aid of light traps and whilst biting man. The pupae of ten species were collected from substrata in an open salt marsh, as well as from fresh and stagnant groundwater situations. It was the first time that the pupae of six of these ten species were collected. Of the species collected in light traps, the two most abundant species, C. leucostictus Kieffer (49.1% of 16,563 identified) and C. rhizophorensis Khamala & Kettle (22.3%), were also the two species found biting man. The larval habitat of C. leucostictus was widespread except in the more saline, tidal areas, but that of C. rhizophorensis appeared to be restricted to the tidal salt-march area. Two of the species collected, C. fulvithorax Austen and C. moreli Clastrier, are new records for South Africa.

The use of a membrane feeding technique to determine the infection rate of Culicoides imicola (Diptera, Ceratopogonidae) for 2 bluetongue virus serotypes in South Africa.

Culicoides spp. in the Lowveld of the northern Transvaal, Republic of South Africa, were fed bluetongue virus serotypes 3 and 6 and African horsesickness virus serotype 1 through latex and chicken skin membranes. After an incubation period of 10 days at 25-27 degrees C, the infection rate of C. imicola for bluetongue virus serotypes 3 and 6 was established at 31% and 24% respectively. No African horsesickness virus could be recovered. The membrane feeding technique and handling procedures proved to be suitable for field studies.

Culicoides species associated with livestock in the Stellenbosch area of the Western Cape Province, Republic of South Africa (Diptera: Ceratopogonidae).

A total of 33,564 Culicoides midges was collected in 44 light trap collections made at 22 sites in the Stellenbosch area during November 1986. Of the 23 species present in these collections 8 were frequently encountered namely, C. magnus, C. imicola, Culicoides sp. 49, C. zuluensis, C. gulbenkiani, C. pycnostictus, C. distinctipennis and C. nivosus. Although C. magnus was abundant at all trap sites, the prevalence of the other species appeared to be affected by the proximity of the light trap to different host animals and/or larval habitats. Plain-wing species and members of the C. schultzei group were rarely collected. The larval habitats of most of the above species were located by the use of tent-type emergence traps. All these habitats were found on irrigated pastures or where drainage water had accumulated. The difference in the requirements of the various species was associated with certain factors, such as degree of moisture, the type and amount of organic matter present and the particle size of the underlying soil. The identity of the blood-meals of 69 individual Culicoides belonging to 7 species was determined. The 5 commonest species had all fed on cattle and 4 of these on sheep. Two species, C. pycnostictus and C. distinctipennis were positive for bird blood.

The control of Parafilaria bovicola transmission in South Africa.

Ivermectin treatment of all cattle on a badly infected farm failed to interrupt the transmission of P. bovicola, even though ovipositional blood spots were drastically reduced in numbers for an entire summer season following treatment. Regular weekly to fortnightly dipping of all cattle in 50 ppm deltamethrin immediately reduced vector fly numbers to less than 1 fly per cow face. Sustained dipping for 9 months effectively reduced P. bovicola transmission from approximately 50% to less than 2%. However, cessation of fly control led to a return to predipping P. bovicola infection levels. Ovipositional blood spot counts and the ELISA technique for evaluating P. bovicola infection in a herd were compared and were both effective methods. Best results for the blood spot method, however, are obtained in spring at the peak of the bleeding season whereas the ELISA method does not have this limitation.

The colonization and life-cycles of Musca lusoria, Musca xanthomelas and Musca nevilli, vectors of Parafilaria bovicola in South Africa.

Thriving, permanent colonies of Musca xanthomelas and later of Musca nevilli were successfully established. However, because of the low reproduction potential of Musca lusoria a small colony only was kept for a limited period until life-cycle studies were completed. Larvae were reared on fresh dung from cattle fed lucerne, while in general adults were fed 0.3% citrated ox-blood, whole milk powder, sugar crystals, fresh dung and water. M. nevilli could be colonized only when ox-liver was substituted for ox-blood. A comparison of the life-cycles of M. lusoria and M. xanthomelas under laboratory conditions at a constant temperature of approximately 27 degrees C, 60% R.H. and 24 h illumination revealed major differences between these 2 vector species. M. lusoria deposits single larvae at intervals of approximately 2 days and a female can produce up to 27 in her life-time. An M. xanthomelas female can lay up to 4 batches of eggs, with as many as 33 eggs per batch, at intervals of approximately 5 days. A single female can produce a maximum of 94 eggs. M. lusoria, however, showed survival advantages over M. xanthomelas in that its larvae reached the pupal stage at least a day sooner and its adults survived more than twice as long. The life-cycles of M. xanthomelas and M. nevilli were similar in the laboratory, except for adult dietary requirements. The mean number of mature oocytes in the ovaries of M. nevilli, however, was only 15.7 compared with 26.1 in M. xanthomelas.

The effect of arsenical dips on Parafilaria bovicola in artificially infected cattle in South Africa.

The possible adverse effect of arsenical tick control dips on Parafilaria bovicola infections was investigated in 48 artificially infected cattle. A treatment group of 24 cattle was dipped in a plunge dip containing 1600 ppm arsenic trioxide. A control group of the same size was dipped in an organophosphate dip containing a mixture of chlorfenvinphos and dioxathion. Regular weekly to 3-weekly dipping had no effect initially on the prevalence of ovipositional blood spots of P. bovicola in either group. However, from 4 months after bleeding commenced there was a significant reduction in blood spots in the arsenic-dipped cattle and, on slaughter at 12-14 months after infection, the arsenic group had significantly fewer live worms and fewer carcass lesions. Arsenic residues in muscle samples of treated cattle were 11.6 times higher than in the controls. It is proposed that arsenic residues in the sub-cutaneous muscle layers increase with repeated dipping until a level toxic to P. bovicola is finally reached. Older cattle would therefore be refractory to infection and their carcasses at slaughter would not be affected.

The epidemiology of Parafilaria bovicola in the Transvaal Bushveld of South Africa.

A total of 20 375 flies collected off cattle on 12 farms over 36 months were identified and examined for 3rd stage P. bovicola. The 3 vector species accounted for 64.1% of the flies collected and were the only fly species found to be infected. Musca lusoria was clearly the dominant vector fly, although large numbers of Musca sp. A appeared regularly between February and April each year. This phenomenon, coupled with high numbers of M. lusoria throughout most of the year, led to an increase in the numbers of vector flies from their lowest level in June to a peak in February-April. Of the 13 070 vector flies examined for 3rd stage larvae only 64 (0.52%) were positive; of these 41 were M. lusoria and 17 Musca sp. A. No positive male flies were found. Incubation of wild-caught flies for up to 13 days at 27 degrees C noticeably increased the larval recovery rate. Flies were found to be infected mainly from August-March. Infected M. lusoria were recorded from July-March and infected Musca sp. A from January-May. Only 6 infected M. xanthomelas were collected and this was during the period August-December, when most ovipositional blood spots occur on cattle. It is concluded that P. bovicola transmission in the Bushveld is not correlated with peak periods of bleeding but rather with high numbers of vector flies, the various species augmenting each other so that transmission may take place almost throughout the year.

WHO/FAO Working Team Report: entomology.

A summary of the more important concerns of the Working Team with particular reference to bluetongue (BT) virus (BTV), is as follows: With the exception of Australia, the US, and possibly parts of Africa, there are almost no concrete data that could be used to direct control efforts against the responsible vector(s) of BTV in a specific geographic area. In Australia, a broad plan of attack yielded data that showed that members of the subgenus Avaritia are the primary vector species. It has been clearly shown that Culicoides variipennis (Coq.) is the dominant vector species for most of the US. In the Middle East and certain areas of Africa, isolations from possible vector species indicated that C. imicola K., again probably a species complex and belonging to the subgenus Avaritia, is one of the likely vectors. Except for some subgenera, the taxonomy for much of the Culicoides remains in disarray. The subgenus Oecacta for example still remains a catchall for many species. Although a number of proposals have been made for delineating the species of the C. (Monoculicoides) variipennis species complex, none have been initiated because of the lack of funds. The Working Team strongly feels that the lack of definition of important species complexes is the result of short-term planning that does not consider the fact that efficiency can be increased and a great deal of monies saved by directing control toward the specific species or populations thereof that are responsible for a problem.(ABSTRACT TRUNCATED AT 250 WORDS)