Deer flies are already out in large numbers in some parts of the state and it will not be long before horse flies add to the summertime mayhem throughout much of North Carolina. These biting flies (collectively called tabanids) routinely ambush people and livestock, inflicting painful bites and otherwise harassing their victims. Tabanids have also been shown to transmit infectious equine anemia and hog cholera. Anyone who has had much experience with these pests knows that repellents and insecticides are not overly effective. What's left is avoidance (stay indoors) and trapping. If the truth be known, avoidance is the most effective method of tabanid control, but who wants to stay indoors all summer. Traps provide an inexpensive alternative that can at least satisfy our need for revenge against these biting pests. Some types of traps catch tabanids by the thousands, but there's no firm evidence they can markedly reduce tabanid populations in a given area. That's not to say they will not. There just hasn't been a lot of research on the approach. Since horse flies will be the dominant tabanid from June to first frost in many parts of the state, I thought it might be useful to offer plans for the construction of a rigid canopy (Manitoba or pyramid) trap that has been shown to trap a number of horse fly species in large numbers. The trap also catches a variety of deer fly species. It may be worth the effort to construct some of these traps and place them around the perimeter of yards or pastures. When properly placed (more about that later in the article) the traps will catch horse flies all day long, week after week, month after month. Once placed, all that's needed is to remove dead flies from the collection jar every few days. Successful trapping of large numbers of horse flies may gradually reduce the overall pressure in a localized area during the course of the summer.
The trap design is taken from an article published by R. C. Axtell, T. D. Edwards and J. C. Dukes in the Journal of The Georgia Entomological Society (Vol. 10, No. 1) in 1975. The trap can be modified somewhat depending on the type of materials available and the ingenuity of the builder so long as the dimensions and contrasting surfaces remain approximately the same. I suggested a similar but smaller design to horse owners last summer.
The trap frame consists of four uprights (legs) made from wood (2 x 2 x 84 inches) or 1.5-inch diameter schedule 40 PVC pipe (84 inches long) and four 57-inch long braces made from 1 x 2-inch wood or 3/4 inch PVC pipe. Cut one end of each of the four legs at a 45 degree angle. Each end of the braces should be cut at a 45 degree angle if wood, or notched to fit if PVC pipe. Four 2 x 2 x 2 x 3/8-inch brackets cut at a 45 degree angle from aluminum channel is recommended for wood construction. The brackets should be attached to the legs 31 inches from the top, and serve as attachment points for the four braces.
A top platform is constructed from two 9-inch square pieces of 1/2 inch plywood. A circular hole is cut in each square so that when sandwiched together, the lower square forms a ledge where the collection device (a canning jar with a lid) rests. My preference is for a single 9-inch square of 3/4 inch plywood with a hole diameter slightly smaller than the jar lid so that the jar lid will fit snugly. The simplest collection device consists of a canning jar and a plastic funnel sized so that its narrow end extends into the jar when held in place by the threaded ring of the jar's canning lid. Find the largest canning jar you can lay your hands on to reduce the frequency with which they must be emptied of dead horse flies. Modify the funnel by cutting off a portion of its narrow end. The opening should be large enough to allow horse flies to enter the jar, but not easily escape. More elaborate collection devices are possible, but the jar seems to work reasonably well. Nail or screw four metal brackets (135 degree angle) at the corners on the bottom side of the top platform. The angle will allow the brackets to extend down and out from the top platform for attachment to the outer side of each leg. Assemble the frame by first attaching the braces to the leg brackets. Then, attach the angled end of each leg to its respective bracket on the bottom of the top platform.
The sides of the canopy trap are attached once the frame is assembled. Each side consists of a trapezoidal piece of light-colored (white is preferable) hardware cloth (screen) or clear plastic. The dimensions of each trapezoid will be 9 x 54.5 x 62 inches (top x side x bottom). A single piece measuring 36 x 54.5 x 248 inches may also be cut by my calculations, unless there is some secret tailor's measurement I don't know about. The latter approach will limit the amount of stapling, tying or gluing needed to attach the cloth, but may make it more difficult to align. Once the cloth is attached, the bottom of the canopy will be about 5 feet square and about 2 feet above the ground. Each of the legs may be attached to stakes driven into the ground for greater stability. Alternatively, the legs may be cut 4 to 6 inches longer and buried to that depth when the trap is placed.
The last two assembly steps are important. First, paint a wide band onto the bottom 18 inches of the canopy sides. A high gloss exterior enamel is recommended. Black plastic may also be used. The high contrast created by painting the bottom of the trap black is a key part of its attractiveness to horse flies. Movement is also important in attracting the horse fly's attention, and may be added to the trap by using nylon twine to suspend a black-painted ball from the top platform of the trap. The twine should be only long enough to allow the ball to extend below the bottom of the canopy by 1/2 to 2/3 of its diameter. A 16-inch diameter beach ball works well for this. I have also seen black-painted, gallon milk jugs (use two) work well. The suspended device must be light enough to be kept in motion by a light breeze. This last addition is not absolutely necessary, but will increase the effectiveness of the trap.
A few words about placing the traps are needed. Tabanids typically rest and hide on low hanging vegetation such as brushy shrubs and tree limbs. Once a host is within a 10 or so feet of their hiding place, the hungry female tabanids use visual cues. They launch themselves toward high contrast targets (cattle, horses and people) moving through open areas (pastures or trails). This is what the canopy trap is designed to mimic. It is helpful to walk or ride 10 to 20 feet inside the perimeter of a yard or pasture and note where horse fly activity seems to be most severe. These areas may indicate fly ways or concentrations of resting horse flies. Place traps at these locations. Small areas may need only one or two traps. Large pastures may require more. Watch the traps closely after they have been placed. If the catch doesn't seem very large (25 or fewer horse flies a week), try moving the trap to a new location. It may take several moves to find the best location(s). Of course, another possibility for poor performance may simply be that the species of horse flies in your area are not strongly attracted to this type of trap. Different species of horse fly are active at different times of the summer as well. By way of comparison, this trap collected an average of 189 lined horse flies (widespread in North Carolina) and 143 black striped horse flies over a twenty-four-week period. Other species were trapped in greater or lesser numbers. The black horse fly, for example, was trapped in very low numbers (an average of 8) at the salt marsh locations used in the study. A salt marsh tabanid called Tabanus nigrovittatus was trapped in very high numbers (1,308). It should be noted that the trap didn't catch only one species at a time. One trap placed where lined, black striped and salt marsh horse flies were active caught 2,261 of these flies. This amounts to about 95 horse flies per week.
One final note: A different kind of tabanid trap may be commercially available in the next few years. Research and observation indicate that this design is a horse fly magnet. Unfortunately, patent and marketing negotiations are ongoing. It may be some time (if ever) before the trap is widely available. You will also find they're more expensive than the homemade canopy trap.
There have been few changes in the types of cattle ear tags available for horn fly and face fly control over the past few years. If you are interested in a list of most of the tags available this year, refer to the May 9, 1997 issue of the North Carolina Pest News (Volume 12, Number 3).
Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Last modified on May 24, 1999 by Stephen J. Toth, Jr.
