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FLIES BREEDING IN WASTES OF

CATTLE & OTHER LIVESTOCK

Musca autumnalis, Musca vetustissima, Haematobia spp. -- Muscidae

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[Refer also to Related Research #1, #2 ]

New Zealand Project

The exophilic flies are those that persist in nature in the absence of humans, but whose populations can increase as a result of certain human activities such as provision of greater breeding habitat. They include several species in the genera Calliphora, Hippelates, Musca, Muscina, Phaenicia, and Stomoxys. Some success has been recorded with the use of natural enemies against the calliphorid species in California and Hawaii, but attempts elsewhere have not been effective (Bay et al. 1976). The braconid parasitoid Alysia ridibunda Say, indigenous to the United States, was released into an area of Texas new to its range and successfully parasitized the blowflies Phaenicia sericata (Meigen) and a Sarcophaga species. However, the parasitoid did not maintain control and became rare within a couple of years (Lindquist 1940).

The gregarious parasitoid Tachinaephagus zealandicus may have considerable potential for biological control of exophilic flies (Olton & Legner 1974, 1975 ). The range of habitats utilized by this natural enemy is considered unparelleled by any other fly parasitoid. But this genus has not been given much attention. One species, Tachinaephagus stomoxcida Subba-Rao provides overall permanent reductions of Stomoxys in Mauritius (Greathead & Monty 1982).

The complex of problems that confront field programs in biological control of exophilic flies has clearly had a dampening effect on research in this area. The unforeseen problems associated with attempts to biological control the eye gnat, Hippelates collusor (Townsend), in California exemplify those problems. In the early 1960's a concerted effort was launched to control this eye gnat with the use of both indigenous and exotic parasitoids in orchards and date palm groves of southern California. About a dozen species and strains were evaluated for several years. Some of the exotics established, but eye gnat reductions were obvious only where cultivation practices were curtailed (Legner et al. 1966, Legner 1970b). Cultivation of the orchards buried the larvae and pupae of the eye gnat below the search zone of the parasitoids and cultivation also removed vegetation that offered the parastioids protection and possibly nutrients (Legner & Olton 1969, Legner & Bay 1970). Buried eye gnats emerged from several centimeters below the soil surface and thus continued to pose a serious problem (Bay et al. 1976).

Tabanidae or horseflies, although widespread and on occasion serious pests and vectors of disease of livestock, have not received much attention. Only one successful inundative release of the egg parasitoid Phanurus emersoni Girault has been recorded (Parman 1928). Apparently this effort was precipitated by a severe outbreak of anthrax at the time and since this disease diminished and other control tactics were available, interest in their biological control has not continued.

Flies associated with cattle droppings, symbovine flies (Povolny 1971), have received the most attention for biological control since the 1970's. The primary targets for control have been the bush fly, Musca vetustissima Walker, the hornfly, Haematobia irritans (L.), and the facefly, Musca autumnalis DeGeer (Wallace & Tyndale-Biscoe 1983, Ridsdill-Smith et al. 1986, Ridsdill-Smith & Hayles 1987).

Scarab beetles have been the principal emphasis for biological control of pasture breeding symbovine flies since Albert Koebele first imported coprophages and fly predators from Europe to Hawaii in 1909 (Anderson & Loomis 1978; Bornemissza 1976; Ferrar 1975; Waterhouse 1974; Legner 1986). The largest effort took place in Australia where pasture improvement benefits were also desired (Bornemissza 1960 1976; Ferrar 1975). However, widespread significant fly reductions have not been reported (Legner 1978a 1986; Macqueen 1975).

Field and laboratory studies have shown that the survival of symbovine flies can be experimentally reduced by dung shredding, scattering and burying activities of scarab beetles (Blume et al. 1973; Bornemissza 1970; Moon et al. 1980; Hughes et al. 1978; Ridsdill-Smith 1981; Ridsdill-Smith et al. 1977; Wallace & Tyndale-Biscoe 1983). Macqueen (1975) and Hughes et al. (1978) reviewed several cases in the field where bush fly, Musca vetustissima Walker reductions may have resulted from the activities of scarab beetles; and Ridsdill-Smith & Mathiessen (1984) gave experimental evidence for some reduction by endemic and imported scarab beetles. However, the amount of control achieved was generally low. Immigration of bush flies from outside the experimental area often confounded the results.

The only known biological control reduction of symbovine flies of noticeable magnitude was reported from Fiji involving a single predator, Hister chinensis Quensel, that originally had been intended for other dipterous species (Bornemissza 1968). A minor success apparently occurred in Hawaii, which involved both dung-burying scarab and predatory beetles (Legner 1978b).

Scarab beetle field population densities are often high enough to cause significant dung removal and pasture improvement (Fincher 1981; Fincher et al. 1981; Kessler 1983; Waterhouse 1974). However, whether significant symbovine fly reductions are also achieved is not certain (Legner 1978a 1986; Macqueen 1975).

Experimental Observations

Haematobia irritans (L.) breeding in flood irrigated pastures of the lower Colorado Desert of southeastern California continues to remain unacceptably high during warm seasons (>1,000 adult flies per bovine head) despite the presence of moderately abundant populations of Onthophagus gazella F. This study suggests that densities of > 40-70 adult beetles per dung pad and giving pronounced dung shredding activity, caused fly mortality of 38-56%. The continued high abundance of adult horn flies on cattle suggests that at >50% mortality, the pasture environment still produces sufficient flies to saturate cattle, although emigration might be reduced. Additional species of scarabs may be necessary to increase fly mortality. However, the dung drying activity of existing O. gazella significantly could interfere with resident staphylinid beetle breeding, which was significantly lower in pastures where O. gazella reached densities of 40 per dung pad. Scarab beetle activity might also impede the introduction of superior predatory species for biological control.

Reasons for the above conclusions stemmed from observations in the Coachella Valley of southeastern California, where established populations of Onthophagus gazella F. seem generally ineffective in reducing adult populations of H. irritans in irrigated pastures. In the 1970's the scarab was imported from Hawaii, and establishment quickly followed (Legner 1986). The species remains firmly established throughout the Coachella Valley. and dung scattering and burying by adult beetles in autumn usually begins within an hour of deposition when pastures are under regular 21-day irrigation. Scarab beetles that remained dormant in the sandy loam soils, in some cases for six months during irrigation-free periods in this largely rainfall-free area, become highly active within a week to 18 days, following renewed irrigation and cattle stocking.

Cattle on these pastures are often stocked at densities exceeding 25 per ha. and left to graze for 12-14 days. The amount of dung that is shredded, scattered and buried daily by the 1-cm long beetles is enormous. By early autumn, beetle density generally exceeds 40 per fresh dropping, a density in the range where fly control can be expected in another species, Musca vetustissima Walker (Wallace & Tyndale-Biscoe 1983).

Ranchers generally have been pleased with the manner in which the cattle dung is incorporated into the soil, even though hornfly control seems lacking. During warm seasons the cattle sustain continuously high densities of this fly, usually exceeding 1,000 per head in autumn. These densities do not appear very different from those attained in pastures where scarab beetles are low or absent. The apparent lack of adult horn fly control is not understood, especially as beetle densities are sufficient for fly larval control to begin to take effect.

Cattle grazing in this area was gradually replaced by horse breeding in connection with the equestrian sport polo, so that by 1988 only about 10% of the former irrigated pastures were devoted to cattle grazing. The overall abundance of O. gazella declined proportionally, and the beetle population survived in a few isolated pastures. A unique opportunity to quantify differences between pastures that sustained scarab beetles and those in which they were absent or greatly reduced by delayed recolonization, presented itself in 1989 when some previously abandoned fields were reconstituted and stocked with cattle.

Experiments to quantify field breeding densities were made in autumn 1989, a time of year for maximum horn fly and O. gazella abundance in the Lower Colorado Desert area of southeastern California. Random samples were taken of dung pads shredded by established O. gazella populations where >40 adult beetles attacked a single pad. These were compared to unshredded samples from control pastures in which O. gazella populations had been reduced to <2 per pad through several continuous years of fallowing. Studies in previous years had shown that the controls were suitable for maintaining large populations of O. gazella (Legner 1986 Legner & Warkentin, unpub. data).

There were four Bermuda grass pastures with established O. gazella adults and larvae and four control pastures in the lower Coachella Valley near the towns of Coachella and Mecca. These pastures, with a sandy loam soil, ranged from 3-8 ha. in size, and control pastures were separated from those with long-established O. gazella pastures by >10 km. Herds were of mixed breeds, and were stocked at densities of 25-45 per ha., with supplemental feeding of concentrates.

Samples were taken over a period of four days from each of four pastures of both types beginning on September 28, October 26, and November 23, 1989. Twenty fresh cow pads of ca. 1,495 cc (SD 374 cc), were first marked and then collected after 120 h of exposure to the pasture. Blume et al. (1970) have shown that predators and competitors reach a pad within 48 h, and cause most horn fly mortality within five days by desiccation of the dung.

The sampling method was that of Roth et al. (1983), consisting of shovel collections both of the manure and the topsoil 5 to 6 cm from the edge of the pad and 30 cm deep. The manure and topsoil were sealed in plastic bags and brought to the laboratory. Half the number of samples (10) were manually broken to separate adult staphylinid predators and O. gazella and to obtain an estimate of their numbers. The other half were placed intact into emergence sleeve cages in a greenhouse, incubated at 26-29EC, 55-60% RH and 14:10-h L:D photoperiod for the emergence of adult horn flies. Beetles in the second incubated set were removed as they left the dung within 6 h of being caged.

Adult horn fly densities on cattle were assessed with 6X binoculars.

The average number and oven-dry weights of horn fly adults emerging per pad from dung collected in both kinds of pastures was calculated. The number of adult predatory staphylinids were counted in each pad.

Statistical Analyses--Data were transformed to sqr-rt(X + 0.5) and analyzed for significant differences using an ANOVA F-test (Steel & Torrie, 1980). Significant differences were tested at P<0.05.

Horn fly adults were produced from all pastures but significantly lower numbers were from fields where O. gazella beetles were active (Legner & Warkentin 1991). Estimated reductions ranged from 38-56%. There was also a trend for smaller flies to be collected from pads producing the highest horn fly numbers, based on oven-dry weight data, bivariate correlation analysis giving a highly significant coefficient of -0.669, 22 df, P<0.05. Although O. gazella was either absent or at densities averaging <1 per pad at the beginning of the sample period, there was a trend toward higher beetle densities on succeeding sample dates, none of which exceeded 5 per pad (Legner & Warkentin 1991). Colonization of the control fields continued in the spring and summer of 1990 so that by July 12th, 1990 the scarab population approximated that observed in the long-established fields of autumn 1989. This provides evidence for the suitability of control pastures to sustain equal scarab densities.

Adult hornfly populations on cattle during the three months sample period remained high (>1,000/head by 4 PM) in both kinds of pastures. It is unlikely that the large number of horn flies on the cattle in pastures containing high population densities of O. gazella was due to the immigration of flies from neighboring ranches, because the control pastures under study were isolated (>10 km. separation), with primarily agricultural crops (citrus and dates) in the areas between.

Two principal predators present were Philonthus discoideus Gravenhorst and Philonthus longicornis Stephens. Their abundance in control pastures was significantly greater on all collection dates than where O. gazella populations were firmly established. Another staphylinid, Platystethus spiculus Erichson, was present under both situations, but this species is probably not an obligate predator, preferring to feed on manure (Legner & Moore, 1977).

Populations of ants and predacious mites were also present, but not monitored. Other predatory species in the Histeridae, Carabidae and Cincindelidae only infrequently were found at very low densities.

Reasons For Continued Adult Fly Abundance.--The continued abundance of adult horn flies on cattle suggests that a predicted 38-56% reduction of hornflies in O. gazella pastures was insufficient to noticeably reduce the adult fly density congregating on single animals. However, emigration of excess flies from these pastures could have been reduced so that an area wide reduction of hornflies might have occurred. Nevertheless, the reduction from a supersaturated to a saturated environment did not obviously give a noticeable level of control on cattle, as judged with binocular observations at 10-11 AM 4-5 PM. A similar situation might prevail in Australia where imported scarab activity seems sufficient to cause significant reductions in bush fly, M. vetustissima, breeding but which paradoxically is not accompanied by drops in the annoyance thresholds.

Explanation For Staphylinid Reduction.--The lower numbers of Philonthus spp. in irrigated pastures where O. gazella were highly active may be found in the dynamics of scarab beetles with horn flies and their natural enemies in the dung habitat. Natural enemy habitats are undoubtedly altered or destroyed by the dung shredding process. The shredding activity of O. gazella reduces habitat configuration and moisture content to a level that may be unsuited for staphylinid oviposition and larval development. In some respects this is similar to the effects of cultivation on the natural breeding habitat of Hippelates eye gnats, which causes a marked reduction in the effectiveness of natural enemies (Legner & Olton, 1969). Other evidence that scarabs in America may disturb Philonthus species was given by Roth et al. (1983) who associated declines in these predators' abundance with rising scarab population densities.

Although the introduction of additional scarab beetle species may afford a positive means for lowering exophilic fly densities, it is important to consider whether introduced scarabs might, through habitat disruption, preclude the introduction of effective predatory species. Because there are no practical nonbiological control methods to reduce fly numbers in exophilic habitats, and the addition of more scarabs may actually exacerbate the problem, the most logical direction for research is to intensify worldwide searches for more effective natural enemies, especially predators and pathogens.

REFERENCES: [Additional references may be found at: MELVYL Library ]

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