Hexapoda: Insecta: Muscidae
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An investigation into the biological-integrated control of pestiferous flies in Kwajalein Atoll, Marshall Islands revealed 6 principal species in the families Calliphoridae, Sarcophagidae and Muscidae, among which some were successfully reduced to non-annoyance population levels in 18 months. Musca vetustissima Walker, of the Musca sorbens complex, was satisfactorily reduced on some islands through habitat reduction, the introduction of natural enemies and baiting after 21 months. The history, biology and control of M. vetustissima is reviewed and future efforts outlined for the importation of natural enemies. Possibilities for resistance to insecticides are indicated and precautions recommended for the effective long term use of poisoned baits. The potential invasion of M. sorbens in North America is discussed.
Pestiferous flies in the Marshall Islands provide a classic example of the adaptation of invading noxious insects to a salubrious tropical climatic area. With nearly perfect temperature-humidity conditions for their development, an abundance of carbohydrate and protein-rich wastes in the form of garbage and excreta provided by man and his animals, and a general absence of effective natural enemies, several species were able to reach maximum numbers.
There are principally 4 types of pestiferous flies in Kwajalein atoll of the Marshall Islands, with the Australian bush fly, Musca vetustissima Walker of the Musca sorbens complex, being by far the most pestiferous species (FIG. I). The common house fly, Musca demestica L., of lesser importance, frequents houses and is attracted to food in recreation areas. The remaining 2 types are the Calliphoridae [Chrysomya megacephala (Fab.), and Phaenicia cuprina (Wiedemann)) , and the Sarcophagidae [ Parasarcophaga misera (Walker), and Phytosarcophaga gressitti Hall and Bohart). These latter species are abundant around garbage disposal sites and wherever rotting meat and I decaying fish are available. The Calliphoridae are copper to bluish-green, sometimes blue, on the thorax making them easily distinguishable from the large striped gray Sarcophagidae. Both of these flies differ from the common house fly and the bush fly in being more sluggish and noisy and by their general avoidance of humans. Because residents do not a distinguish different kinds of flies, these latter 2 types are often blamed as nuisances when in fact they may be considered to fulfill a useful role in the biodegradation of garbage and rotting a meat.
Studies were begun in April 1971 at the request of the United States Army and the United States Trust Territory of the Pacific, to investigate fly abatement possibilities in Kwajalein Atoll, Marshall Islands. The atoll, the largest in the world, is about 120 Km long by an average 23 Km wide and contains 93 islets grouped roughly between 166-167° E. longitude and 8-9° N. latitude in an irregular ring around a central lagoon (Bryan 1972 and FIG. 2). Kwajalein, the largest island, lies at the extreme south of the atoll (FIG. 2 , 3 & 5).
An initial assessment of the problem using standard sampling methods to locate fly breeding sources and identify associated natural enemy fauna, resulted in the expedient implementation of breeding source reduction to reduce the house fly, Musca domestica L., and both the Calliphoridae and Sarcophagidae to inconspicuous levels, which largely involved slight modifications of refuse disposal sites to disfavor fly breeding. By November 1972, these simple measures resulted in an estimated 1/3rd reduction of total flies concentrating around beaches and other recreational areas (see Table 1 for proportion of fly species trapped). Because the house fly especially enters dwellings, its reduction was desirable for the general health of the community, and fly annoyances indoors diminished. Thorough surveys of breeding sites and natural enemy complexes revealed that Musca vetustissima reduction would not be quickly forthcoming, however. A schedule of importation for natural enemies was begun in 1971 and other integrated control approaches were investigated: eg. baiting and breeding habitat manipulation. The following account of this, the principal problem fly species in the atoll, reviews bush fly history and biology, at the same time pointing out the magnitude of the biological, technological and political difficulties involved in its permanent reduction.
RESULTS AND DISCUSSION
Bush Fly. Musca vetustissima Walker of the Musca sorbens 1830 complex Origin and Habits. -- This group of flies is known as the bazaar fly in North Africa, a housefly in India, and the bush fly in Australia (Yu 1971). Musca sorbens was first described from Sierra Leone in West Africa in 1830 (Wiedemann 1830) where it is a notorious nuisance to man and animals. The flies are attracted to wounds, sores, and skin lesions, searching for any possible food sources such as blood and other exudations. Although not a biting species, its habits of being able to transmit eye diseases, entheric infections, pathogenic bacteria and helminth eggs make it one of our most important and dangerous public health insects (Bell 1969; Buston and Hopkins 1927; Gaud and Faure 1951; Gaud et al. 1954; Greenberg 1971; Hafez and Attia 1958; Lamborn 1937; McGuire and Durant 1957; Patton 1920, '22, '23, '26, '32, '33; Thomson and Lamborn 1934).
The bush fly has spread through a major portion of the Old World, Africa and parts of Asia (Awati 1920; Gaud and Faure 1951; Hafez and Attia 1958; Hughes 1968; Jack 1931; Lewis 1954; Meng and Winfield 1944; Norris 1966; Patton 1920, '23, '26; Roy and Siddoils 1940; Van Emden 1965). In Oceania its distribution is in AustraIia (Norris 1966; Paterson and Norris 1970); New Guinea (Paterson and Norris 1970); Samoa (Huxton and Hopkins 1927); Guam (Harris and Down 1946, Hohart and Gressitt 1951); and the Marshall Islands (Hohart and Gressitt 1951 )0 In Hawaii it was first reported by Joyce in 1950. Since then Hardy (1952) listed it in the Catalog of Hawaiian Diptera, and Wilton (1963) reported its predilection for dog excrement. Annoyance by the bush fly increased in the 1960's when it was incriminated as a potential vector of Beta-haemolytic streptococci in an eipdemic of acute glomerulonephritis (Bell 1969).
Taxonomy. -- Paterson and Norris (1970) identified 3 biological forms in the M. sorbens complex. These are distinguished by morphometric differences and reproductive isolation in the laboratory, and consist of the African male broad-frons and narrow-frons forms, and the Australian narrow-frons form.
Examination of morphometric measurements of 220 randomly selected adult males collected on Kwajalein Island during the period April 1971 -November 1972 showed an average frons ratio (6) of 0.0590 (range 0.0341 -0.1052; SD = 0.0182), which was close to that of both the African and Australian narrow-frons forms. However, the frequency distribution showed 2 distinctive peaks representing the narrow- and broad-frons in the ratio of about 4: I (FIG 4). Also, a count of the parafrontal hairs of 20 random females (Paterson and Norris 1970) gave similar results. Thus, 2 forms may be sympatric in Kwajalein, which has been shown possible from other areas (Paterson and Norris 1970).
Breeding Habits. -- The principal breeding sites of the bush fly vary according to different geographical localities. However, human excrement is regarded by many authorities as a preferred site when this is available (Bohar,t and Gressitt 1954; Gaud et al. 1954; Hafez and Attia 1958; Harris and Down 1946; Norris 1966; Patton 1933; Peffly 1953a, b; Roy and Siddons 1940; Sabrosky 1952). The bush fly also breeds in fresh droppings of a number of larger animals, such as cattle (Bohart and Gressitt 1951; Hafez and Attia 1958; Hughes and Walker 1970; Jack 1931; Meng and Winfield 1944; Pat ton 1922, '36; Peffly 1953b); horses (Bohart and Gressitt 1951; Lewis 1954; Pat ton 1922, '36); dogs (Meng and Winfield 1944; Norris 1966; Pat ton 1922; Wilton 1963); and pigs (Bohart and Gressitt 1951; Meng and Winfield 1938; Smirov 1940; Hafez and Attia 1958). Sheep feces (Norris 1966), and goat and cat feces (Yu 1971) are also suitable. Poultry excrement will not support this fly's development (Yu 1971). Other breeding sources of much lesser importance but which are capable of producing small numbers of bush fly are carcasses of animals, decaying vegetable, matter and garbage (Bevan 1926, Norris 1966, Pat ton 1922, Yu 1971). Recent studies by Yu (1971) in Hawaii show that dog feces are the most important breeding medium on Oahu. He concluded that dung of cows, horses, pigs and goats are of minor importance in rural areas where dog feces are not readily available.
Development. -- The average developmental period in Hawaii from eggs to adults is 10 days with an average of 76 flies emerging per field sample of dog feces. On Kwajalein developmental time probably averages 8 days with over 100 flies emerging from one sample of dog feces. High precipitation may prevent the fly from full activity and breeding (Yu 1971).
In the Kwajalein atoll there is little reason to doubt that a substantial portion of the main density of Musca vetustissima emanates from dog and human feces. However, extensive inspection of pig droppings in the bush of 10 widely separated islets revealed high numbers of larvae (over 100 per dropping), making this dung, as in Guam (Bohart and Gressitt 1951 ), a primary breeding source in the atoll. Pigs that are corralled on soil or concrete slabs concentrate and trample their droppings making them less suitable breeding sites. In such situations flies were only able to complete their development along the periphery of corrals. Coconut husks placed under pigs in corrals results in the production of greater numbers of flies by reducing the effectiveness of trampling. These conclusions were reached after comparing quantitative samples taken over a period of 2 years from the respective breeding habitats.
Garbage was not found to breed M. vetustissima, although admittedly a very low percentage of the adult population could originate there judging from reports elsewhere. Nevertheless, this medium is certainly not responsible for producing more than 2% of the adult densities observed in the atoll.
Control Efforts and Evaluation Worldwide. -- Successful partial control of bush fly has been achieved only in Hawaii through a combination of the elimination of breeding sites, principally dog droppings, and the activities of parasitic and predatory insects introduced earlier for biological control of other fly species, e.g., Musca domestica (Legner 1978 ). The density of-bush fly varies in different climatic zones in Hawaii, but the importance of this fly is minimal compared to Kwajalein. At times hymenopterous parasites have been found to parasitize over 95% of flies sampled in the Waikiki area (H. Yu, unpublished data).
Other parts of Oceania were either not suitable for the maximum effectiveness of known parasitic species ( e.g. Australia) or the principal breeding habitats were not attractive to the natural enemies. Therefore, in Australia a concerted effort is being made to secure scavenger and predatory insects from Africa that would be effective in the principal unmanageable fly producing source, cattle and sheep dung (Bomemissza 1970; G. F. Bomemissza, personal communication). For example, insectary studies on Onthophagus gazella F. (Coleoptera: Scarabaeidae) showed 80-100% reduction of M. vetustissima emergence from cattle dung (Bomemissza 1970). Surviving fly larvae gave rise to small, stunted adults of reduced reproductive capability.
Kwajalein Atoll. -- Continuing integrated fly control begun in 1971 has now reached a level of partial success. Initial surveys for natural enemies of M. vetustissima in April-May 1971 revealed the presence of 4 scavenger and predatory insects, the histerid Carcinops troglodytes Erichson, the nitidulid Carpophilus pilosellus Motschulsky, the tenebrionid Alphitobius diaperinus (penzer), and the dermapteran Labidura riparia (pallas). Following our recommendations in May 1971, dogs were significantly reduced and all privies were reconstructed or improved on one island, Ebeye Dogs were reduced or tethered on Kwajalein Island and refuse fish, etc., disposed of thoroughly on l1leginni (an island that experienced outbreaks of Sarcophagidae in 1970 (FIG. 2), and other islands with American residents. Continuing importations of natural enemies were made throughout the atoll with establishment of some species verified in November 1972 (Table 2). The average density of M. vetustissima on Ebeye was, subsequently, considerably reduced by November 1972 from an estimated 8.5 flies attracted to the face per minute!, to less than 0.5 flies per minute, a reduction readily recognized by the inhabitants (7). The single most important cause appeared to be the partial elimination of breeding sources, with natural enemies playing a secondary role
The M. vetustissima density at Roi-Namur (estimated 2.5 flies attracted per minute), in the norther apex of the atoll, and Ennylabegan (6.0 flies per minute), in the southwest, did not substantially improve following the natural enemy introductions and recommendations for reducing breeding habitats. However, this is thought to be due to an increase in the number of human inhabitants, dogs and pigs on Ennylabegan and on Ennubira to the southeast of Roi-Namur (Fig. 2).
Flies apparently invade islands adjacent to their breeding origin at a very slow rate as evidenced by a lack of recovery on Kwajalein of dyed flies that were released on Ebeye and Ennylabegan in November, 1973. However, Australian studies support the fact that M. vetustissima possesses rapid, long-distance migrating behavior, which is aided by windstorms (Hughes 1970, Norris 1966). Even though mark and recapture experiments were negative, there is probably continuous slow invasion of M. vetustissima over a proposed route from Ennubira to Roi-Namur; and from Ennylabegan to Enubuj and Kwajalein (FIG. 2 & FIG. 3). Flies are absent on islands without native or American inhabitants even though adjacent islands may have high densities, which lends some support to the minimal invasion hypothesis. Fly persistence around human habitation is probably not to seek breeding sites but rather to attain foodstuffs, especially carbohydrates, meat scraps and sauces which are perennially available at recreation sites especially. Therefore, control efforts stress corralling pigs, reconstructing privies and reducing dog populations. All residents on all islands are urged to refrain from indiscriminate disposal of wastes from soft drink and beer cans. Importations of natural enemies are being stressed for those areas where pigs and dogs roam wild.
New Approaches to Bush Fly Reduction. -- New approaches to the solution of the bush fly problem in Kwajalein atoll involve the use of effective poisoned baits for adult flies, technological changes in garbage disposal and the importation of natural enemies from the presumed original endemic area of M. sorbens in central and eastern Africa.
Sugar bait mixtures that have been used for M domestica in years previous to 1972 were wholly ineffective for killing adult M. vetustissima due to their almost complete lack of attractiveness. Baits tested during November 1972 through April 1973 included a variety of decomposing foodstuffs, of which rotting eggs and rotting fish sauces were very highly attractive. In one experiment using a 6-day old mixture of one part fresh whole eggs to one part water (Legner 1970b, 1971), it is estimated that about 50,000 bush flies were attracted to the mixture and killed by a 0.5 ppm Dichlorvos, (8) additive. The poisoned mixture was poured in quantities of 100 mI each on damp sand at 20 sites in the shade and spaced every 10m along a public beach on Kwajalein (FIG 6 & FIG 7). Baits placed above the height of 1m or against walls in open pavilions were only weakly attractive. After 48 hours, flies were reduced to inconspicuous levels all over Kwajalein Island (Table 1 gives the approximate % of each species trapped). This condition endured for 3 consecutive days after which immigrating flies managed to build up to annoying levels starting on the 4th day when the baits ceased to be attractive. The former density of flies had not yet been reached, one week after the baiting experiment; and this population was subsequently reduced in the same manner by applying additional fresh poisoned baits. Flies breeding in dog feces were the principal source of the population recovery on Kwajalein, as shown by marking and recovery experiments in November 1973..
Baiting was continued through November 1973 and extended to other islands in the atoll with a result of sustained reduction of bush fly to below general annoyance levels (less than 0.01 attracted per minute on Kwajalein, Roi-Namur, Illeginni and Meck Islands). A new attractant that augmented the rotting egg mixture consisted of beach sand soaked for one week in the decomposing body fluids of buried sharks. This new attractant was far superior to rotting eggs both in rate and time of attraction, the latter sometimes exceeding 5 days.
The baiting method of control can be used effectively if applied initially twice a week. A schedule of biweekly applications or longer may follow as determined by personnel in charge of the control program.
Resistance in the tropical environment could be expected to develop very quickly if adult flies that are exposed to the baits are able to reach breeding sites. This resistance might ultimately be expected to spread through the whole M. vetustissima population in Kwajalein atoll, similar to that observed for other species (Georghiou 1966). Obviously, if breeding sites and poisoned baits are located together within the normal flight range of flies, resistance rate is increased. However, there is probably a lower resistance possibility to electric shock devices, and not much chance for resistance against natural bait such as rotting eggs or fish. Nevertheless, with full awareness of the plasticity of M vetustissima and the general resilience of nature, it cannot be ignored that the possibility for resistance to any chemical control implemented by humans does exist.
Technological and Cultural Improvements
To further ensure against breeding of M. vetustissima around refuse disposal sites and that resistance does not develop in populations of the common house fly and blowfly which are able to successfully breed in these sites, the development of suitable concrete barriers to larval flies around such sites was proposed. Fly larvae of M. domestica and Phaenicia cuprina especially, were repeatedly observed entering the soil from cans and dumpsters containing refuse. Such refuse sites placed on adequately rimmed concrete platforms would probably prevent larvae from finding suitable pupation sites in surrounding soil. The use of petroleum oils in a long perimeter around refuse deposition sites, saturating the soil within the depicted circle to a depth of at least 5 cm, might offer partial control. However, some fly larvae are capable of penetrating this barrier so that its effectiveness is not as great as concrete.
Biological control organisms are usually effective only in one or a few breeding habitats, and their activities are limited under certain types of climatic stress. For example, when house flies breed in garbage they are less prone to attack by certain natural enemies than when breeding occurs in accumulated animal excrement (Legner and Poorbaugh 1972). The aim of biological control is to locate and establish natural enemies that will perform effectively in: all problem breeding habitats. This goal is rarely achieved, so that effective control takes on an integrated aspect where cultural and chemical means are implemented.
There are no known natural enemies that will specifically attack the bush fly nor is there any information on what kinds of parasites and predators range in the droppings of humans, dogs and pigs in eastern Africa, where bush fly may have originated. The best that can be done in biological control at this time is to import natural enemies of other fly species for their possible adaptation to M. vetustissima. (Legner 1970a; Legner and Greathead 1969 ; Legner and Olton 1968, 1970, 1971 ; Legner et al. 1967).
This is the approach that has been made in Hawaii and is partially successful. As new species of natural enemies become available, they will be introduced for biological control. There is some expectation that a concerted effort will be made to seek out specific natural enemies in eastern Africa as support for the work becomes available. A list of species that should be considered for introduction in Kwajalein atoll is shown in Table 3; however, this list is expected to grow as information about new natural enemies from Africa develops.
The biological control phase of fly reduction must be viewed as a long-range approach with ever increasing fly reduction as the new beneficial species become established. An investment of research time in the initial evaluation of a natural enemy species during the importation phase would not be practical from the standpoint that such time would, (I) detract from the amount of effort placed on introductions; (2) most species will probably not become established judging from a history of biological control efforts during the past 50 years; and (3) fly densities are expected to drop gradually with the relatively small numbers of natural enemies being released at any given time. Once an obvious drop in the adult M. vetustissima density has occurred, a thorough evaluation of the natural enemy species responsible may be carried out.
POTENTIAL INVASION OF MUSCA VETUSTISSIMA IN NORTH AME RICA
There has been some speculation about the possibility of Musca vetustissima invading portions of North America, where climatic similarities with Africa and Australia exist. Undoubtedly the more tropical portions of the continent could sustain this species, with the Florida peninsula and Mexico appearing the most vulnerable. There is greater uncertainty about its establishment in the Southwestern United States where winter cold may be an effective barrier. In Australia M . vetustissima over-winters with difficulty in the colder southern quarter of the continent (Norris 1966) and probably reinvades that region annually from the more tropical north (Hughes 1966). Similarly, in the Middle East Israel seems to be out of the general range of M. vetustissima (Legner and Olton 1968), although it is a prominent species further south along the Nile River (Hafez and Attia 1958). There being no sustained breeding sites available in the intervening desert, invasion during warm seasons is precluded. Much the same conditions exist in the American Southwest where annual invasions from Mexico, a potential breeding source, may be minimal across the arid Sonoran Desert. The threat of invasion in the southeastern United States from potential breeding sites in Florida is greater, however.
In modern pest control the importance of being conscious of what effects a control action may have on the total environment as well as its permanency are recognized. Resistance to pesticides, especially in the tropics, is a greater threat in our procedure than pesticide pollution because we employ compounds that degrade and are nonpoisonous to us. In fly control on Kwajalein, resistance to Dichlorvos and related compounds would leave no alternative stop-gap measure to use in case of an epidemic of human disease in which flies were vectors. Resistance could also produce hardier strains and theoretically even more aggressive flies (G. P. Georghiou, personal communication; Legner 1970a). Therefore, it is essential that the baiting program to combat flies is minimized and that stress be placed on the implementation of technological, cultural and biological controls that could lead to a permanent reduction of fly breeding in the atoll.
As of January 2002, the baiting procedure in Kwajalein has not continued with the sophistication initially determined necessary. The result is that flies are not adequately reduced where breeding habitat such as dog feces remain. Personnel changes that resulted in critical information not being passed on are primarily responsible. Of paramount importance is the proper brewing of the baits and the latter’s placement in wind-calm areas of the islands. Also, the baiting program should logically be extended, at least experimentally, to Musca vetustissima Walker in Australia, where towns simulate islands in the vast bush land. Temporary relief from bush fly annoyance might be expected to occur during summer when these flies descend into the higher latitudes.
The assistance and cooperation of personnel in the following organizations who made this scientific study possible is gratefully acknowledged: Global Associates, Department of the Army, University of California, United States Trust Territory of the Pacific, and the people of the Marshall Islands.
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