Mechanical Control of Greenhead Flies (Diptera: Tabanidae) in a Marsh Environment M. C. AILES, 1 L. J. BROWN,2 C. CHURCH,* D. P. FRENCH, 4 AND W. Naval Surface Warfare Center, Wallops Island, Virginia 23337
J. Med. Entomol. 29(2): 160-164 (1992) ABSTRACT The effectiveness of New Jersey box traps for the control of adult greenhead flies, Tabanus nigrovittatus, Macquart, T. conterminus Walker, was assessed at Wallops Island, Va. Trap shape and placement were tested. Shallow (0.45 m high) traps caught significantly fewer flies than cubic (0.60 m high) traps. Traps located in the inner two of four layers of traps caught fewer flies than traps in the outer layers. KEY WORDS Insecta, Tabanus spp., control, trapping
1 Public Works Office, Aegis Combat Systems Center, Wallops Island, Va. 23337. 2 Box 578, Dahlgren, Va. 22448. 3 Box 1-87 Agriculture Building, Department of Entomology, University of Missouri, Columbia, Mo. 65211. 4 Department of Natural Sciences, University of Maryland Eastern Shore, Princess Anne, Md. 21853. 5 Disease Vector Ecology and Control Center, Naval Air Station, Box 43, Jacksonville, Fla. 32212-0043.
Materials and Methods Study Area. Our study was conducted on Wallops Island, off the Eastern Shore of Virginia, at the Wallops Flight Facility, a branch of the National Aeronautics and Space Administration's Goddard Space Flight Center. Wallops Island is a sandy barrier island off the coast of Virginia's Delmarva Peninsula consisting of sandy beach, a narrow central strip of developed area in the south and forest in the north, and vast tracts of tidal marsh to the west. The tidal marsh provides excellent breeding habitat for tabanid flies. The study site is located at the interface of the northern, forested area and the southern, developed area. Although it is mostly developed, there is a finger of forest reaching to the site from the north and a barrier of shrubs both north and south of the site. The forest occurs on the highest areas and consists mostly of loblolly pine (Finns taeda L.). In slightly lower areas, there are extensive growths of bayberry {Myrica cerifera L.) and sea myrtle (Baccharis halimifolia L.). In freshwater areas, cattails (Typha sp.) and reed grass (Phragmites australis (Cav.) Trin. ex. Steud.) are encountered. A further drop in elevation results in the vegetation of the open marsh, predominated by the salt marsh grasses (Spartina alterniflora Loisel and S. patens (Aiton) Muhl. Except on the open marsh, poison ivy (Rhus radicans L.) is common in association with various other herbs. All of these communities occur in close association with the study site. Trap Description. The traps were wooden boxes raised on legs to an elevation of 0.6 m above the ground (Fig. 1). The four sides were made of plywood and were painted black on the outside but left unpainted on the inside. The top of the trap was made of 0.16-cm-mesh wire screen. The bottom of the traps consisted of two
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is not known to be an important disease vector, people working outdoors near salt marshes can be subjected seasonally to sufficient greenhead attacks which may reduce comfort and work efficiency. The work areas on Wallops Island, Va., are surrounded by saltmarsh, where greenhead flies breed in abundance. However, because of the local seafood industry's dependence on the marsh (e.g., crabs, oysters, and fish), acceptable control measures are limited by the controlling agency (National Aeronautics and Space Administration), by environmental protection laws, or by poor cost/benefit ratios. Because chemical larviciding for control of greenhead flies is impossible on Wallops Island, an environmentally safe and effective control method for adults is needed. Box traps (Hansens et al. 1971; Spencer 1971, 1972; Morgan & Lee 1977; Wall & Doane 1980) have been used for research and control purposes. On the Eastern Shore of Virginia and Maryland, box traps are often used around swimming pools and picnic areas to reduce the nuisance level of the flies. The shallow version of the box trap is the most common style used by local motels and swimming pools. There is no consistent pattern in the placement of the boxes, nor does their efficacy appear to be consistent. The purpose of our study was to determine the best shape and placement of box traps to reduce the annoyance from tabanids at a work site on Wallops Island.
ALTHOUGH THE GREENHEAD FLY
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AlLES ET AL.: CONTROL OF TABANIDAE IN MARSHES
LAYER
161
1
LAYER
CUBIC TRAP
SHALLOW TRAP
LAYER
3
LAYER
4
Fig. 1. Greenhead fly traps as used at Wallops Island, Va.
Fig. 2. Cubic box placement at Aegis study site, Wallops, Va., 1986-1987.
of live and dead greenhead flies were recorded. Dead flies were removed through the panels, preventing the escape of live flies. We tested the effectiveness of trap shape using analysis of variance (ANOVA). All counts were square root transformed to reduce heteroscedasticity (Winer 1971, Zar 1984). We performed two repeated-measure ANOVAs with shape, week, and year as within-subjects factors. Calculations were performed using BMDP program BMDP2V (BMDP Dixon 1985). The first analysis compared the two traps at each site during the 1985 season and tested whether the pairs of traps at each site were similar. The second ANOVA compared the catches during the 1986 and 1987 seasons when cubic and shallow traps were in place. Unless identified by specific date, all reported means are backtransformed. Experiment II: Effect of Trap Placement. The work area to be protected by the traps was an Aegis (Navy) training and engineering facility located on a small rise between the beach and the open marsh (Fig. 2). Dense vegetation on the sides of the site restricted fly access from the sides (Morgan & Lee 1977). Two layers of traps were placed on the marsh and two layers at the site itself. Five groups of cubic box traps were used: (1) four traps averaging 27 m apart in the first layer in the marsh west of the site, (2) four traps averaging 24 m apart in the second layer in the marsh and 20 m east of the first layer, (3) six traps in a semicircular layer around the site on the marsh side of the building averaging 45 m apart, (4) six traps in a semicircular layer around the site on the ocean side of the building aver-
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wire screens of 0.16-cm mesh, placed in an inverted "V," with a 1-cm gap at the apex. Flies enter from the bottom of the box, then move upward toward the light. The mesh "V" directs them toward the 1-cm gap at the apex, where they are funneled into the trap. For the purposes of this study, we added a hinged panel along the bottom of two opposing sides. The panels formed a floor between the bottom of the inverted "V" and the sides of the trap. The panels could be opened to clean the trap and remove dead flies. To monitor fly abundance, five cubic boxes were placed at random along the island. The flies caught in these boxes were counted weekly from 1984 through 1989, on the same day as the other boxes. Experiment I: Effect of Trap Shape. Two trap shapes were compared. The "cubic" traps had all sides measuring 0.6 m and a volume of 0.2 m 3 (Spencer 1971, Wall & Doane 1980). The "shallow" traps measured 0.9 m by 0.6 m on the two sides and 0.45 m by 0.6 m on the two ends, with a volume of 0.36 m 3 (Olkowski 1985). Both traps were raised above the ground on 0.6-m legs. In 1985, 20 cubic traps were set out in 10 pairs; 6 traps within pairs were separated by an average of 49 m (range, 17-150 m), and pairs separated by an average distance of 433 m (range, 18-2,700 m). Each pair was located in an area of low vegetation either on or adjacent to the Spartina patens marsh. One member of each pair was chosen randomly to be the experimental site. The number of flies collected at each experimental site was compared with the number collected from its mate (control) to determine whether the members of each pair of traps caught comparable numbers of flies. The following summer (1986), one cubic trap was left in place at each control site, and a shallow trap replaced the cubic trap at each experimental site. All traps were checked weekly from June to September and the numbers
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Results In Virginia, the greenhead fly complex is composed of Tabanus nigrovittatus Macquart and Tabanus conterminus Walker. The two species usually can be separated by body size, but precise differentiation requires electrophoretic analysis (Sofield et al. 1985). Most of the flies we collected fell within the 10.47 ± body length (10.47 ± 0.71 mm) reported for T. nigrovittatus (Sofield et al. 1985). There were a few larger flies observed in August about the time of a secondary population peak (Fig. 3); this may indicate the presence of a small population of T. conterminus emerging late in the summer. Because there is no known difference in the nuisance levels or trap attraction by these two species, flies were not separated to species in this study. Only adult females were found in the traps; however, hostseeking females were the portion of the population of interest to us because other portions do not bite man.
1400
1 2 3 JUNE
4
1
2 3 4 1 2 3 4 JULY AUGUST DATES (WEEKS)
Fig. 3. Adult female greenhead fly populations at Wallops Island, Va., averaged over the years 19841989.
The fly population exhibited a sharp increase in late June (Fig. 3). The weekly count at each trap increased rapidly; e.g., from 12 (6 June 1987) to 3,980 flies (29 June 1987) at the southernmost control site. On a specific date, however, catch size varied markedly among sites; e.g., catches on 29 June 1987, ranged from 15 flies (southern trap, Layer 2) to 3,980 flies (southernmost control trap). Catches at a given site remained consistent in relation to other traps; i.e., sites experiencing a low catch in June always had among the lowest catches, whereas the traps with a high catch in June continued to have the highest counts within and among years. Experiment I: Effect of Trap Shape. Before shallow traps were introduced, there was no significant difference in the mean number of flies caught at the control (x = 42.38) or the experimental (x = 34.83) sites (F = 0.65; df = 1,9; P > 0.05). The number of flies caught varied weekly, whether shallow traps were present (range x = 4.45-331.07; F = 11.79; df = 12,108; P < 0.0001) or absent (range x = 1.05-155.67; F = 33.36; df = 16,144; P < 0.0001). When the shallow traps were introduced, they caught significantly fewer flies (x = 25.44) than did the cubic traps (x = 58.61) (F = 11,22; df = 1,9; P < 0.01). When shallow traps were present, differences among years were not significant (3c1986 = 44.70, x1987 = 37.16; F = 3.37; df = 1,9; P > 0.05), but the year by trap type (F = 9.91; df = 1,9; P < 0.02), week by trap type (F = 8.17; df = 16,144; P < 0.0001), and year by week by trap type (F = 3.03; df = 16,144; P < 0.0005) interactions were significant. Experiment II: Effect of Trap Placement. The mean number of flies caught per trap in each of the layers is presented in Table 1. Analysis of variance indicated a seasonal effect (F = 83.38; df = 16,336; P < 0.0005) and yearly (F = 6.90, df = 1,21; P < 0.025) differences. There was also a
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aging 52 m apart, and (5) six controls (not shown in Fig. 2) located at distances varying from 0.25 to 2.0 km from the Aegis site (three traps located south of the test site and three north), in locations as similar to the test site as possible. Data were recorded and transformed as described above. The effect of trap placement on fly capture was tested using a repeated-measures ANOVA with trap location as the between-subjects factor and week and year as within-subject factors. Calculations were performed using SPSS procedure MANOVA (SPSSX [SPSS, Inc. 1988]). Two additional shallow traps were originally included in each of layers 1 and 2, but we excluded these during analysis because of the results of Experiment I. We predicted several possible patterns of differences among the layers of traps based on the hypothesized patterns of fly movements and trap arrangements. If flies move around the marsh, then fly out in a seaward direction toward the building sites, then traps located in or near the marsh should catch more flies than those located near buildings on the side nearest the marsh or the sea. Under this hypothesized arrangement, traps should capture linearly decreasing numbers of flies when Layers 1-4 are compared. Alternatively, traps in Layers 1-4 could have functioned as inner (Layers 2 and 3) and outer (Layers 1 and 4) layers. In this case, the inner layers would be expected to capture fewer flies than either of the outer layers. In either case, control traps should have caught more flies because of the greater spacing among them and because they were located in the marsh. We tested these possibilities using three orthogonal planned comparisons (Winer 1971). Unless identified by specific date, all reported means are backtransformed.
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Table 1. Adult female greenhead flies captured in the trap layers on Wallops Island, Va. Averages are backtransformed means for data used in the ANOVA.
Location Marsh, outer Marsh, inner Site, inner Site, outer Controls No. weeks
Designation Layer Layer Layer Layer Layer
1 2 3 4 5
—
Avg. no. flies/trap (June-Sept.) 1987 1986 94.3 21.2 43.4 81.2 319.0
62.7 13.9 41.2 70.9 269.0
18
17
Discussion Our results support the conclusion that cubic box traps catch more greenhead flies than shallow box traps and thus should be more effective in reducing annoyance from tabanids. The cubic box is deeper than the shallow box, and the cubic box lid is smaller than the shallow box lid. The lids are constructed of transparent wire mesh. As a result, the interior of the cubic box appears darker when viewed from below than does the interior of the shallow box. It may be that the insects are more attracted to the darker box. We found a seasonal difference in the relative effectiveness of the two trap shapes. The significant interactions among factors indicated that the difference in trap efficacy is at times less marked. When fly populations are at their peak (see Fig. 3), there is a large difference in the number of flies caught by the two trap types. When flies are less abundant, there is less difference in the number of flies caught by the different trap shapes. This effect may be an artifact of the changing size of the population. Although the two trap types may retain the same proportional efficacy, when the population drops, the catch sizes begin to approach similar absolute quantities. Trap placement also appears to be a factor influencing the capture of flies by box traps. We tested three hypotheses concerning fly capture: (1) whether traps around the site of interest would capture different numbers of flies than more widely spaced traps placed in the marsh, (2) whether traps closer to the marsh would catch more flies than those farther from the marsh, and
(3) whether traps in the outer layers caught more flies than those in the inner layers. The traps surrounding the work site caught fewer flies than controls placed in the marsh. It is impossible to separate the influence of intertrap distances from that of location within the marsh. Perhaps traps in close proximity to one another act competitively in collecting flies. On the open marsh it would be possible for flies to see more than one trap. Flies attracted to the traps then would have to choose among those visible, thus reducing the probability of capture in any individual trap. Because the controls were located on the marsh where the insects breed and hunt, success in this group of traps would be expected to be high. It had been suggested that the traps at the work site might reduce the populations of flies throughout the island. Although the controls can not indicate changes in absolute population sizes, it can safely be stated that the traps at the site did not eliminate flies from other areas of Wallops Island. Our results failed to support the hypothesis that the flies emerge from the marsh and travel seaward unidirectionally. There was not a decreasing linear trend in catch size from marsh to sea. As they were arranged, the traps appeared to function as inner and outer layers. Because there were significantly fewer flies in the traps in the inner layer, it would appear that the outer layers, both on the marsh side of the site and on the ocean side of the site, provided protection to the inner layers of traps. It thus would appear that a barrier of traps between the marsh and the work site will not provide sufficient protection. A site must be entirely surrounded by traps or dense vegetation to achieve maximum protection from the flies. Box traps are being used with increasing frequency in the resort areas near Wallops Island (personal observation). They do not pollute the environment and appear to reduce adult fly populations in protected areas. Any improvement in the efficiency of the traps will result in better greenhead fly control. Increased use of mechanical controls such as the box traps will result in the reduced use of potentially harmful chemicals. Our results indicated that future traps should be built in the cubic rather than in the shallow configuration. The traps should be placed in a layer around the entire perimeter of the site to be protected, except where dense vegetation protects some portion of the perimeter. Acknowledgment The authors express their gratitude to the National Aeronautic and Space Administration, Goddard Space Flight Center's Wallops Flight Facility, for permission to conduct the study on lands under their jurisdiction, and for the generous help and support offered by NASA
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significant difference among the layers of traps (F = 14.70; df = 4,21; P < 0.005). The results of the orthogonal comparisons indicated that control traps caught more than those in the layers (F = 52.38; df = 1,21; P < 0.0005). There was no decreasing linear trend for Layers 1—4 (F = 0.11; df = 1,21; P > 0.05). Traps in Layers 2 and 3 caught fewer flies than traps in Layers 1 and 4 (F = 6.27; df = 1,21; P < 0.025) and appeared to function as inner and outer layers.
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personnel. This study was supported by the Naval Surface Warfare Center, Dahlgren, Va.
References Cited
nus in New Jersey. Proc. Entomol. Soc. Wash. 87: 572-577. Spencer, R. W. 1971. A mechanical approach to the abatement of the greenhead fly. Proc. N.J. Mosq. Exterm. Assoc. 58: 71-77. 1972. A mechanical approach toward control of the greenhead fly. Public Works 103: 90-92. SPSS. 1988. Statistical package for the social sciences SPSSX version 3 user's guide. McGraw-Hill, New York. Wall, W. J. & O. W. Doane, Jr. 1980. Large scale use of box traps to study and control salt marsh greenhead flies (Diptera: Tabanidae) on Cape Cod, Massachusetts. Environ. Entomol. 9: 371-375. Winer, B. J. 1971. Statistical principles in experimental design. McGraw-Hill, New York. Zar, J. H. 1984. Biostatistical analysis. PrenticeHall, Englewood Cliffs, N.J. Received for publication 14 August 1989; accepted 16 September 1991.
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Dixon, W. J. [ed]. 1985. BMDP Statistical Software Version 6. Univ. Calif. Press, Berkeley, Ca. Hansens, E. J., E. M. Bosler & J. W. Robinson. 1971. Use of traps for study and control of salt marsh greenhead flies. J. Econ. Entomol. 64: 1481-1486. Morgan, N. O. & R. P. Lee. 1977. Vegetative barriers influence flight direction of salt marsh greenheads. Mosq. News 37: 263-267. Olkowski, W. 1985. A better horsefly trap. The IPM Practitioner 7: 10-11. Pechuman, L. L. 1973. Horse flies and deer flies of Virginia (Diptera: Tabanidae). The Insects of Virginia No. 6. Res. Div. Bull. 81, Virginia Polytechnic Institute and State University, Blacksburg. Sofield, R. K., E. J. Hansens & R. C. Vrijenhoek. 1985. The size and distribution of the sibling species Tabanus nigrovittatus and Tabanus contermi-
Vol. 29, no. 2
J. Med. Entomol. 29(2): 160-164 (1992) ABSTRACT The effectiveness of New Jersey box traps for the control of adult greenhead flies, Tabanus nigrovittatus, Macquart, T. conterminus Walker, was assessed at Wallops Island, Va. Trap shape and placement were tested. Shallow (0.45 m high) traps caught significantly fewer flies than cubic (0.60 m high) traps. Traps located in the inner two of four layers of traps caught fewer flies than traps in the outer layers. KEY WORDS Insecta, Tabanus spp., control, trapping
1 Public Works Office, Aegis Combat Systems Center, Wallops Island, Va. 23337. 2 Box 578, Dahlgren, Va. 22448. 3 Box 1-87 Agriculture Building, Department of Entomology, University of Missouri, Columbia, Mo. 65211. 4 Department of Natural Sciences, University of Maryland Eastern Shore, Princess Anne, Md. 21853. 5 Disease Vector Ecology and Control Center, Naval Air Station, Box 43, Jacksonville, Fla. 32212-0043.
Materials and Methods Study Area. Our study was conducted on Wallops Island, off the Eastern Shore of Virginia, at the Wallops Flight Facility, a branch of the National Aeronautics and Space Administration's Goddard Space Flight Center. Wallops Island is a sandy barrier island off the coast of Virginia's Delmarva Peninsula consisting of sandy beach, a narrow central strip of developed area in the south and forest in the north, and vast tracts of tidal marsh to the west. The tidal marsh provides excellent breeding habitat for tabanid flies. The study site is located at the interface of the northern, forested area and the southern, developed area. Although it is mostly developed, there is a finger of forest reaching to the site from the north and a barrier of shrubs both north and south of the site. The forest occurs on the highest areas and consists mostly of loblolly pine (Finns taeda L.). In slightly lower areas, there are extensive growths of bayberry {Myrica cerifera L.) and sea myrtle (Baccharis halimifolia L.). In freshwater areas, cattails (Typha sp.) and reed grass (Phragmites australis (Cav.) Trin. ex. Steud.) are encountered. A further drop in elevation results in the vegetation of the open marsh, predominated by the salt marsh grasses (Spartina alterniflora Loisel and S. patens (Aiton) Muhl. Except on the open marsh, poison ivy (Rhus radicans L.) is common in association with various other herbs. All of these communities occur in close association with the study site. Trap Description. The traps were wooden boxes raised on legs to an elevation of 0.6 m above the ground (Fig. 1). The four sides were made of plywood and were painted black on the outside but left unpainted on the inside. The top of the trap was made of 0.16-cm-mesh wire screen. The bottom of the traps consisted of two
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is not known to be an important disease vector, people working outdoors near salt marshes can be subjected seasonally to sufficient greenhead attacks which may reduce comfort and work efficiency. The work areas on Wallops Island, Va., are surrounded by saltmarsh, where greenhead flies breed in abundance. However, because of the local seafood industry's dependence on the marsh (e.g., crabs, oysters, and fish), acceptable control measures are limited by the controlling agency (National Aeronautics and Space Administration), by environmental protection laws, or by poor cost/benefit ratios. Because chemical larviciding for control of greenhead flies is impossible on Wallops Island, an environmentally safe and effective control method for adults is needed. Box traps (Hansens et al. 1971; Spencer 1971, 1972; Morgan & Lee 1977; Wall & Doane 1980) have been used for research and control purposes. On the Eastern Shore of Virginia and Maryland, box traps are often used around swimming pools and picnic areas to reduce the nuisance level of the flies. The shallow version of the box trap is the most common style used by local motels and swimming pools. There is no consistent pattern in the placement of the boxes, nor does their efficacy appear to be consistent. The purpose of our study was to determine the best shape and placement of box traps to reduce the annoyance from tabanids at a work site on Wallops Island.
ALTHOUGH THE GREENHEAD FLY
March 1992
AlLES ET AL.: CONTROL OF TABANIDAE IN MARSHES
LAYER
161
1
LAYER
CUBIC TRAP
SHALLOW TRAP
LAYER
3
LAYER
4
Fig. 1. Greenhead fly traps as used at Wallops Island, Va.
Fig. 2. Cubic box placement at Aegis study site, Wallops, Va., 1986-1987.
of live and dead greenhead flies were recorded. Dead flies were removed through the panels, preventing the escape of live flies. We tested the effectiveness of trap shape using analysis of variance (ANOVA). All counts were square root transformed to reduce heteroscedasticity (Winer 1971, Zar 1984). We performed two repeated-measure ANOVAs with shape, week, and year as within-subjects factors. Calculations were performed using BMDP program BMDP2V (BMDP Dixon 1985). The first analysis compared the two traps at each site during the 1985 season and tested whether the pairs of traps at each site were similar. The second ANOVA compared the catches during the 1986 and 1987 seasons when cubic and shallow traps were in place. Unless identified by specific date, all reported means are backtransformed. Experiment II: Effect of Trap Placement. The work area to be protected by the traps was an Aegis (Navy) training and engineering facility located on a small rise between the beach and the open marsh (Fig. 2). Dense vegetation on the sides of the site restricted fly access from the sides (Morgan & Lee 1977). Two layers of traps were placed on the marsh and two layers at the site itself. Five groups of cubic box traps were used: (1) four traps averaging 27 m apart in the first layer in the marsh west of the site, (2) four traps averaging 24 m apart in the second layer in the marsh and 20 m east of the first layer, (3) six traps in a semicircular layer around the site on the marsh side of the building averaging 45 m apart, (4) six traps in a semicircular layer around the site on the ocean side of the building aver-
Downloaded from http://jme.oxfordjournals.org/ by guest on April 10, 2016
wire screens of 0.16-cm mesh, placed in an inverted "V," with a 1-cm gap at the apex. Flies enter from the bottom of the box, then move upward toward the light. The mesh "V" directs them toward the 1-cm gap at the apex, where they are funneled into the trap. For the purposes of this study, we added a hinged panel along the bottom of two opposing sides. The panels formed a floor between the bottom of the inverted "V" and the sides of the trap. The panels could be opened to clean the trap and remove dead flies. To monitor fly abundance, five cubic boxes were placed at random along the island. The flies caught in these boxes were counted weekly from 1984 through 1989, on the same day as the other boxes. Experiment I: Effect of Trap Shape. Two trap shapes were compared. The "cubic" traps had all sides measuring 0.6 m and a volume of 0.2 m 3 (Spencer 1971, Wall & Doane 1980). The "shallow" traps measured 0.9 m by 0.6 m on the two sides and 0.45 m by 0.6 m on the two ends, with a volume of 0.36 m 3 (Olkowski 1985). Both traps were raised above the ground on 0.6-m legs. In 1985, 20 cubic traps were set out in 10 pairs; 6 traps within pairs were separated by an average of 49 m (range, 17-150 m), and pairs separated by an average distance of 433 m (range, 18-2,700 m). Each pair was located in an area of low vegetation either on or adjacent to the Spartina patens marsh. One member of each pair was chosen randomly to be the experimental site. The number of flies collected at each experimental site was compared with the number collected from its mate (control) to determine whether the members of each pair of traps caught comparable numbers of flies. The following summer (1986), one cubic trap was left in place at each control site, and a shallow trap replaced the cubic trap at each experimental site. All traps were checked weekly from June to September and the numbers
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Results In Virginia, the greenhead fly complex is composed of Tabanus nigrovittatus Macquart and Tabanus conterminus Walker. The two species usually can be separated by body size, but precise differentiation requires electrophoretic analysis (Sofield et al. 1985). Most of the flies we collected fell within the 10.47 ± body length (10.47 ± 0.71 mm) reported for T. nigrovittatus (Sofield et al. 1985). There were a few larger flies observed in August about the time of a secondary population peak (Fig. 3); this may indicate the presence of a small population of T. conterminus emerging late in the summer. Because there is no known difference in the nuisance levels or trap attraction by these two species, flies were not separated to species in this study. Only adult females were found in the traps; however, hostseeking females were the portion of the population of interest to us because other portions do not bite man.
1400
1 2 3 JUNE
4
1
2 3 4 1 2 3 4 JULY AUGUST DATES (WEEKS)
Fig. 3. Adult female greenhead fly populations at Wallops Island, Va., averaged over the years 19841989.
The fly population exhibited a sharp increase in late June (Fig. 3). The weekly count at each trap increased rapidly; e.g., from 12 (6 June 1987) to 3,980 flies (29 June 1987) at the southernmost control site. On a specific date, however, catch size varied markedly among sites; e.g., catches on 29 June 1987, ranged from 15 flies (southern trap, Layer 2) to 3,980 flies (southernmost control trap). Catches at a given site remained consistent in relation to other traps; i.e., sites experiencing a low catch in June always had among the lowest catches, whereas the traps with a high catch in June continued to have the highest counts within and among years. Experiment I: Effect of Trap Shape. Before shallow traps were introduced, there was no significant difference in the mean number of flies caught at the control (x = 42.38) or the experimental (x = 34.83) sites (F = 0.65; df = 1,9; P > 0.05). The number of flies caught varied weekly, whether shallow traps were present (range x = 4.45-331.07; F = 11.79; df = 12,108; P < 0.0001) or absent (range x = 1.05-155.67; F = 33.36; df = 16,144; P < 0.0001). When the shallow traps were introduced, they caught significantly fewer flies (x = 25.44) than did the cubic traps (x = 58.61) (F = 11,22; df = 1,9; P < 0.01). When shallow traps were present, differences among years were not significant (3c1986 = 44.70, x1987 = 37.16; F = 3.37; df = 1,9; P > 0.05), but the year by trap type (F = 9.91; df = 1,9; P < 0.02), week by trap type (F = 8.17; df = 16,144; P < 0.0001), and year by week by trap type (F = 3.03; df = 16,144; P < 0.0005) interactions were significant. Experiment II: Effect of Trap Placement. The mean number of flies caught per trap in each of the layers is presented in Table 1. Analysis of variance indicated a seasonal effect (F = 83.38; df = 16,336; P < 0.0005) and yearly (F = 6.90, df = 1,21; P < 0.025) differences. There was also a
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aging 52 m apart, and (5) six controls (not shown in Fig. 2) located at distances varying from 0.25 to 2.0 km from the Aegis site (three traps located south of the test site and three north), in locations as similar to the test site as possible. Data were recorded and transformed as described above. The effect of trap placement on fly capture was tested using a repeated-measures ANOVA with trap location as the between-subjects factor and week and year as within-subject factors. Calculations were performed using SPSS procedure MANOVA (SPSSX [SPSS, Inc. 1988]). Two additional shallow traps were originally included in each of layers 1 and 2, but we excluded these during analysis because of the results of Experiment I. We predicted several possible patterns of differences among the layers of traps based on the hypothesized patterns of fly movements and trap arrangements. If flies move around the marsh, then fly out in a seaward direction toward the building sites, then traps located in or near the marsh should catch more flies than those located near buildings on the side nearest the marsh or the sea. Under this hypothesized arrangement, traps should capture linearly decreasing numbers of flies when Layers 1-4 are compared. Alternatively, traps in Layers 1-4 could have functioned as inner (Layers 2 and 3) and outer (Layers 1 and 4) layers. In this case, the inner layers would be expected to capture fewer flies than either of the outer layers. In either case, control traps should have caught more flies because of the greater spacing among them and because they were located in the marsh. We tested these possibilities using three orthogonal planned comparisons (Winer 1971). Unless identified by specific date, all reported means are backtransformed.
March 1992
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Table 1. Adult female greenhead flies captured in the trap layers on Wallops Island, Va. Averages are backtransformed means for data used in the ANOVA.
Location Marsh, outer Marsh, inner Site, inner Site, outer Controls No. weeks
Designation Layer Layer Layer Layer Layer
1 2 3 4 5
—
Avg. no. flies/trap (June-Sept.) 1987 1986 94.3 21.2 43.4 81.2 319.0
62.7 13.9 41.2 70.9 269.0
18
17
Discussion Our results support the conclusion that cubic box traps catch more greenhead flies than shallow box traps and thus should be more effective in reducing annoyance from tabanids. The cubic box is deeper than the shallow box, and the cubic box lid is smaller than the shallow box lid. The lids are constructed of transparent wire mesh. As a result, the interior of the cubic box appears darker when viewed from below than does the interior of the shallow box. It may be that the insects are more attracted to the darker box. We found a seasonal difference in the relative effectiveness of the two trap shapes. The significant interactions among factors indicated that the difference in trap efficacy is at times less marked. When fly populations are at their peak (see Fig. 3), there is a large difference in the number of flies caught by the two trap types. When flies are less abundant, there is less difference in the number of flies caught by the different trap shapes. This effect may be an artifact of the changing size of the population. Although the two trap types may retain the same proportional efficacy, when the population drops, the catch sizes begin to approach similar absolute quantities. Trap placement also appears to be a factor influencing the capture of flies by box traps. We tested three hypotheses concerning fly capture: (1) whether traps around the site of interest would capture different numbers of flies than more widely spaced traps placed in the marsh, (2) whether traps closer to the marsh would catch more flies than those farther from the marsh, and
(3) whether traps in the outer layers caught more flies than those in the inner layers. The traps surrounding the work site caught fewer flies than controls placed in the marsh. It is impossible to separate the influence of intertrap distances from that of location within the marsh. Perhaps traps in close proximity to one another act competitively in collecting flies. On the open marsh it would be possible for flies to see more than one trap. Flies attracted to the traps then would have to choose among those visible, thus reducing the probability of capture in any individual trap. Because the controls were located on the marsh where the insects breed and hunt, success in this group of traps would be expected to be high. It had been suggested that the traps at the work site might reduce the populations of flies throughout the island. Although the controls can not indicate changes in absolute population sizes, it can safely be stated that the traps at the site did not eliminate flies from other areas of Wallops Island. Our results failed to support the hypothesis that the flies emerge from the marsh and travel seaward unidirectionally. There was not a decreasing linear trend in catch size from marsh to sea. As they were arranged, the traps appeared to function as inner and outer layers. Because there were significantly fewer flies in the traps in the inner layer, it would appear that the outer layers, both on the marsh side of the site and on the ocean side of the site, provided protection to the inner layers of traps. It thus would appear that a barrier of traps between the marsh and the work site will not provide sufficient protection. A site must be entirely surrounded by traps or dense vegetation to achieve maximum protection from the flies. Box traps are being used with increasing frequency in the resort areas near Wallops Island (personal observation). They do not pollute the environment and appear to reduce adult fly populations in protected areas. Any improvement in the efficiency of the traps will result in better greenhead fly control. Increased use of mechanical controls such as the box traps will result in the reduced use of potentially harmful chemicals. Our results indicated that future traps should be built in the cubic rather than in the shallow configuration. The traps should be placed in a layer around the entire perimeter of the site to be protected, except where dense vegetation protects some portion of the perimeter. Acknowledgment The authors express their gratitude to the National Aeronautic and Space Administration, Goddard Space Flight Center's Wallops Flight Facility, for permission to conduct the study on lands under their jurisdiction, and for the generous help and support offered by NASA
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significant difference among the layers of traps (F = 14.70; df = 4,21; P < 0.005). The results of the orthogonal comparisons indicated that control traps caught more than those in the layers (F = 52.38; df = 1,21; P < 0.0005). There was no decreasing linear trend for Layers 1—4 (F = 0.11; df = 1,21; P > 0.05). Traps in Layers 2 and 3 caught fewer flies than traps in Layers 1 and 4 (F = 6.27; df = 1,21; P < 0.025) and appeared to function as inner and outer layers.
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personnel. This study was supported by the Naval Surface Warfare Center, Dahlgren, Va.
References Cited
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Dixon, W. J. [ed]. 1985. BMDP Statistical Software Version 6. Univ. Calif. Press, Berkeley, Ca. Hansens, E. J., E. M. Bosler & J. W. Robinson. 1971. Use of traps for study and control of salt marsh greenhead flies. J. Econ. Entomol. 64: 1481-1486. Morgan, N. O. & R. P. Lee. 1977. Vegetative barriers influence flight direction of salt marsh greenheads. Mosq. News 37: 263-267. Olkowski, W. 1985. A better horsefly trap. The IPM Practitioner 7: 10-11. Pechuman, L. L. 1973. Horse flies and deer flies of Virginia (Diptera: Tabanidae). The Insects of Virginia No. 6. Res. Div. Bull. 81, Virginia Polytechnic Institute and State University, Blacksburg. Sofield, R. K., E. J. Hansens & R. C. Vrijenhoek. 1985. The size and distribution of the sibling species Tabanus nigrovittatus and Tabanus contermi-
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