Related, Current and Previous Work

The stable fly has long been recognized as an important pest of confined cattle during the summer months (see Foil & Hogsette 1994). This blood-feeding fly has a painful bite, and cattle attacked by stable flies often respond by bunching together in dense aggregations. In addition, animals stamp their front legs, switch their tails and drop their heads in an effort to dislodge feeding flies. Bunching increases heat stress, and in turn reduces feed intake. In beef cattle, heat stress increases time and feed required to reach slaughter weight, and consequently, increases the cost of finishing the animals (Catangui et al. 1997). Similar effects on lactation rate of dairy cattle are presumed, but have not been demonstrated experimentally. A recent three-year study under S-274 (Campbell et al., 2001) has indicated that stable flies are also an economic pest of grazing cattle. Stable flies reduced the weight gains of unprotected steers by 0.2 kg/day in 84-day trials on pasture. This estimate of reduced growth rate was probably low, however, because topical permethrin, the best available insecticide, was inadequate, and fly movement from unprotected to protected herds was probably substantial. Much other research has been accomplished under preceding multistate research projects NC-154 and S-274, and this work has led to and clarified the objectives in the present proposal.

Larval development habitats and overwintering

In the feedlot environment, stable fly larvae develop in fermenting, urine-soaked mixtures of manure and feed at drylot perimeters, in moist feed spills, in silage residues, and in large rolled hay bales (see Foil & Hogsette 1994 for review). On dairies, stored manure spilled feeds and soiled bedding under young stock and in calf hutches are probably the most important sources of stable flies (Schmidtmann 1988). Stable fly immatures can overwinter in non-frozen portions of manure mounds and silage piles in the Northern Plains (Berkebile et al. 1994). Larval development sites other than those of confinement facilities have not been investigated, and it is not known whether larvae and pupae overwinter in shallower, surface habitats that freeze in winter.

Dispersal by adult stable flies on local and regional scales

Stable flies have been found to disperse 8 km in 2 hrs (Eddy et al. 1962) and 29 km in 24 hrs (Bailey et al. 1973), but the record is 225 km in an unknown amount of time (Hogsette & Ruff 1985). Average dispersal rates are likely to be slower (Hogsette et al 1989) and affected by presence of host cattle, local habitats and terrain. These data suggest two sources of stable flies in pasture and range are possible: they may originate locally in pasture and range, or they may be immigrants from neighboring or distant confinement facilities. Previous work has not simultaneously examined both possibilities.

Extent of movement on regional scales and corresponding breeding structure of stable fly populations in North America are unclear. Wind from local fronts may be responsible for long range transport of stable fly populations (Broce 1993). Using allozyme markers, Jones et al. (1987) observed genetic differentiation among populations in the panhandle of Florida, but more recent work failed to detect differentiation in populations from Canada to Texas (Krafsur 1993, Szalanski et al. 1996). Newer genetic markers such as microsatellite polymorphisms and amplified fragment length polymorphisms (AFLPs) may provide better markers of movement and genetic variation within and among different regions of the U.S.

A method for estimating adult age from pterin content of their heads and field temperatures antecedent to field collection was developed (Lysyk & Krafsur 1993) and used to assess age and survival of adult stable flies in Iowa. Longevities of males and females were equivalent and exponentially distributed (Krafsur et al., 1995); average lifespan was about 8 days during summer. Age grading is an established tool that can be used to draw inferences about the origins of flies in a given area.

Control tactics and population management

Source reduction: This tactic, aimed at minimizing larval habitat has long been recommended to control stable flies where cattle are confined. Thomas et al. (1996) showed that sanitation alone could reduce the number of adults in cattle feedlots by a season-long average of 33%. Sanitation might be a useful tactic in pastures and range if stable flies were shown to originate in focal substrates in those environments.

Insecticides: In general, producers are most willing to use insecticides at beef and dairy confinement facilities, because they produce quick and visible results. Materials applied either directly to cattle or onto adjacent substrates include pyrethroids and organophosphates. These compounds are short lived and are generally effective if applied on a regular basis (Mock & Greene 1989). Use of the same compounds and formulations for pastured cattle is impractical. Insecticidal ear tags are less effective against stable flies than against non-resistant horn fly populations.

Traps: Much research has been done to develop traps for sampling and control of stable flies (ess Gibson & Torr 1999). An effective sticky trap is constructed of Alsynite^® fiberglass--the Williams (1973) trap is two rectangles fitted together, whereas Broce's (1988) modification is a cylinder of the same material. Rugg (1982) attempted to trap out a stable fly population, but concluded that the time required for recoating of sticky traps prohibited their use in most field situations. To overcome required maintenance, subsequent workers replaced sticky sleeves with an insecticide. Alsynite® panels treated with permethrin removed more than 30% of a stable fly population when deployed at a rate of one trap per five head of cattle in Florida (Meifert et al 1978). Similarly, efficacy of permethrin impregnated Orlon yarn on fiberglass panels was modest and limited to 6-8 weeks under simulated field conditions (Hogsette & Ruff, 1996). Torr et al. (1992) showed that pyrethroid impregnated fabric targets were effective for killing tsetse for approximately one year under field conditions, which suggests longevity of permethrin varies with substrate to which it is applied.

Recent research suggests stable fly traps could be made more attractive. Vale (1974) developed electric grid technology to study the trap approaching behaviors of tsetse identified odors and colors used by tsetse in host location and feeding behavior. Fabric targets, impregnated with insecticides and baited with synthetic host odors, were then developed (Vale, 1993). Preliminary studies of trap seeking stable flies have been conducted in Louisiana using the same grid technology. Alsynite^® cylinders were compared with the NZI trap (Mihok et al. 1995), a pyramidal device of blue and black fabric. The Alsynite^® trap captured 183 flies per hour, while the NZI captured 278 flies. These results suggest that a more attractive target could be developed, one which could be treated with permethrin (or alternative) and be deployed in pasture situations to protect grazing cattle.

Classical and augmentative biological control: Wasps in the genera Muscidifurax and Spalangia (Pteromalidae) are pupal parasitoids that are most promising (see Petersen 1989). Muscidifurax includes five species; M. raptor is endemic throughout the temperate and semitropical regions of the world, whereas another four are limited to the Western Hemisphere (Gibson 2000). Taylor et al. (1997) found mtDNA nucleotide substitution rates of 14-19% among M. raptor, M. raptorellus, and M. zaraptor. These levels of differentiation indicated divergence millions of years ago. Spalangia are more speciose in the Old World, and the subset of species now extant in the New World appear to have been introduced. Biological properties of Old World and New World forms of the same species in these two genera have yet to be compared.

Attractive features of pteromalid wasps is that they occur naturally in the environment, they can produce high levels of host mortality, and they can be mass-reared for field release. Parasitization of house fly pupae was increased in Florida following mass releases of S. endius (Morgan & Patterson 1977), and of M. raptor in dairies (Geden et al. 1992). However, attempts to control flies on larger, midwestern cattle confinements (see Greene 1990) and California dairies (Meyer et al 1990) were less successful. Natural prevalence and performance of pteromalid parasites released in pastoral settings and other habitats outside of cattle confinement facilities have not been evaluated.

Wolbachia: These alpha-Proteobacteria are obligate, intracellular symbionts that manipulate their host's reproduction in different ways (Werren 1997). Of particular interest is cytoplasmic incompatibility (CI) in embryos that results from matings of males and females that lack the same strain of Wolbachia. In early experiments (reviewed in Sinkins et al. 1997), cytoplasmically incompatible Culex quinquefasciatus males were released into a Burmese village, resulting in a temporary elimination of the resident population. Dobson and colleagues (unpubl.) recently developed a mathematical model of Wolbachia infections and their effects on host population size. This model demonstrated that releases of Wolbachia-infected hosts that are bidirectionally incompatible with the target population can reduce or even eliminate the target population. Simulations predict that this strategy will be appropriate for controlling stable fly, because it has a suitably low reproductive rate (R₀) of 1.1-3.2. Examples of insects known to have multiple, bidirectionally incompatible Wolbachia include Drosophila simulans and Cx. pipiens. A key first step will be to survey stable fly populations in North America to determine which, if any strains of Wolbachia are now present, using diagnostic methods developed initially for Wolbachia in Aedes mosquitoes (Dobson et al. 2001).

Modeling: Simulation models are tools for understanding how complex systems function, and for evaluating and optimizing current and proposed integrated pest management (IPM) strategies and tactics (Focks & McLaughlin 1988; Wilhoit et al. 1991a). Models also aid in technology transfer, illustrating effects of management options to producers. Most of the basic relations needed to develop a stable fly model are in hand. Effects of temperature on stage-specific development time, survival and reproduction of stable flies were examined, reviewed and modeled by Lysyk (1998). Additional egg-larval mortality occurs in the field, and varies according to abundance of generalist predators (mainly staphylinid beetles and macrochelid mites), and pupal mortality varies with abundance of parasitoids (Lysyk 1995). The life cycles of the principal parasitoids have been described, and their functional and numerical responses have been studied in laboratory settings (Wilhoit et al. 1991a, Lysyk 2000). Efforts are underway (Lysyk, pers. com.) to code the component mathematical relationships into a process based simulation model driven by daily min-max air temperatures as available from NOAA, and inferred for corresponding larval-pupal substrates (Wilhoit et al., 1991b). Time series descriptions of stable fly abundance in Alberta and 7 states from NY to FL (S-274, unpubl.) are available to compare predicted (model) and observed (field) patterns in stable fly abundance. Features remaining to be understood--namely, nature and supply of larval substrates, overwintering, and adult dispersal--are subjects of objectives 1 and 2 in the present proposal.

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