- Elton J. Hansens
- Research Professor of Entomology and Economic Zoology
- Stuart R. Race
- Extension Specialist in Entomology
- For a number of years Rutgers research has been directed toward
control of the salt marsh greenhead in southern New Jersey. Now we can
recommend a trap which will greatly reduce greenhead annoyance in many
areas. Further research is planned to develop even better controls which
are effective and have no harmful side effects.
The Greenhead Problem
- The salt marsh greenhead fly, Tabanus nigrovittatus, is an abundant
and bothersome summertime pest along our coastal marshes. Because the
females bite during daylight, and because they occur in large numbers,
have a long flight range, and attack persistently, they interfere with
the enjoyment of coastal areas throughout much of the summer.
- To anyone who has not visited the New Jersey coastal areas during
“fly season,” the impact of these flies on daytime activities is hard to
imagine. We have collected in traps over 1000 greenhead flies per hour
all seeking a blood meal. Greenhead fly populations reach peak numbers
during July, but extend from late June into September.
- Conventional methods of biting fly control, such as those used for
mosquitoes, are either environmentally undesirable or economically
impractical. Both adults and larvae of greenhead flies are large in
comparison to other, non-target organisms. Generally, more insecticide
is needed to kill larger insects. The higher concentrations or greater
amounts of toxic material needed to obtain greenhead control have un-
desirable effects on other insects and animals. Marsh water management
by ditching may actually enhance greenhead production. Although high-level
impoundments reduce the numbers of developing greenhead larvae, this is a
costly and impractical approach to fly control for much of our coastal
- In studying the life history and habits of the salt marsh greenhead,
we have developed several types of traps to capture greenheads in large
numbers. These traps show promise as an inexpensive yet effective means
of reducing the number of biting flies during midsummer.
Where Greenheads Come From
- Greenhead flies are produced from our coastal marshes. We have found
as many as 70 larvae in a single square yard of marsh sod. Developing
larvae concentrate along the upper vegetational zone reached by daily high
tides. Foraging through wet thatch, surface muck, and vegetation, the
predaceous larvae attack and devour a variety of invertebrates, including
some of their own kind. Larvae overwinter and form a pupa after a brief
period of spring foraging. The adult emerges from the pupa in late spring.
- Adult flies mate on the open marsh. Within a few days and without
seeking a blood meal, the female lays her first egg mass, consisting of
100 to 200 eggs. To produce additional egg masses, the female needs a
blood meal. Among biting flies, blood serves as a rich protein source
necessary for egg development. In the case of the salt marsh greenhead,
protein for the first egg mass is obtained when the predaceous larva eats
other insect larvae or small animals, but to lay additional egg masses, she
must obtain a blood meal.
- Older female greenheads move from the salt marsh to nearby wooded ór
open areas along the marsh edge to seek suitable blood sources. There
they await and attack wildlife, livestock, and people that venture close
enough for them to detect.
- Females live for three to four weeks in the uplands before they become
too weak to bite. Because of this long life, large numbers of blood-
hungry flies build up in areas near salt marshes. The physical removal of
large numbers of flies can reduce this buildup and thus decrease the
greenhead fly problem locally.
Traps for Greenheads
- Traps were developed originally to measure fly populations during
Rutgers research. In fact, the traps capture large numbers of blood—seeking
flies and, if such traps are located at the edge of a marsh or in adjacent
uplands where flies concentrate, they serve as a partial control for
- Where single traps capture hundreds of flies per day, a marked reduction
in greenhead annoyance results. In Delaware, the use of three to five traps
near isolated human dwellings has resulted in almost complete reduction in
- After several years of refinement in trap design as well as the study
of optimal trap location, we are confident that such traps will capture
flies in numbers great enough to decrease the salt marsh greenhead problem
in local areas. What we don’t know is whether or not the continual
removal of large numbers of these flies over several seasons will reduce
the size of the total fly population. To put the results of these studies
to good use, we are encouraging all interested coastal dwellers to build
and maintain one or more of these simple trapping devices.
Building the Box Trap
- The basic box trap design is diagrammed in Fig. 1. Essentially the
trap is a four-sided box having a screen top and open bottom. This box
stands on legs so that its bottom is about 2 feet above the marsh surface.
Flies enter the trap from below and move into secondary traps on the top of
the box. The design is simple. The sides of the box can be made of a
number of materials including plywood, cardboard, or plastic sheeting
tacked to wooden framing. The trap dimensions have been developed
experimentally and we urge the wise builder to pay strict attention to the
- 1. Build a box 16 x 32 inches on a side fastened to corner posts. We
use ¼” plywood and 1 x 2 inch furring strips, but other materials can
be used. Also nail a strip to the top of each side for later attachment
of the screen top. The bottom of the box remains open. The
optimal size for each side of the box is about 16 x 32 inches. In
our tests, larger and smaller box traps were less efficient. Furthermore,
these dimensions allow nine sides to be cut from a standard
4 x 8 foot panel. This means that the sides for nine traps can be
cut from four 4 x 8 sheets of material.
- 2. We use separate legs 40 inches long which are attached to the trap
when it is placed for catching flies. The box is fastened so that
its lower edge is 24 inches above the ground surface. This is
important because the greenhead fly usually flies at about this
- 3. The trap should be painted a glossy black to contrast with its
surroundings and to absorb heat from the sun. Either shiny or dull
black plastic sheeting attached securely to a frame is also
- 4. The top of the trap should be made of a metal insect screen. Plastic
screening or sheeting will be damaged readily by birds seeking to get
at the trapped flies and should not be used.
- 5. Take care in building the box and attaching the screen. Be sure there
are no holes for escape of trapped flies. Once inside the box, most
flies move to the top of the trap, through the screen cones and into
the collectors, described in Step No. 8.
- 6. Cut two holes in the screen roof of the box at diagonal corners.
These holes should be 2 to 3 inches from the sides of the trap and 2½
inches in diameter. Make a cone of insect screen with a base of 2½
inches diameter and 2 to 2½ inches high and with a hole ½ inch in
diameter at the top. This can be done from the template shown in
Fig. la. After the piece of screen is cut, roll it into a cone and
securely cement, staple or sew it with wire.
- 7. Cement the cones around the holes in the screen roof, using an all—
purpose or epoxy cement.
- 8. The two collectors can be any type of clear plastic container such as
a shoe box or cake box. We have found that plastic bags are unsatisfactory
because crows and other birds tear them open to feed on
the trapped flies. We use round containers 10 inches in diameter
and 3 to 4 inches high, but square or rectangular boxes of similar
size can also be used. Smaller collectors may require frequent
emptying of flies and, therefore, are inconvenient.
- 9. Cut a 2½ inch diameter hole in the bottom of the collector so it will
fit easily over the screen cones attached to the top of the trap.
Then make two more cones as described in No. 6 and cement them around
the holes in each plastic container. The collector will then have a
cone which will fit over the one on the trap and prevent loss of
flies. (See Fig. 1 and lb).
- 10. Place the screen cones in the collectors over the matching cones on
the corners of the trap so that flies have a clear path through
matching ½ inch holes from the inside of the box into the inside of
the collector. Install a wire or cord across each collector to hold
it securely in place.
- 11. The trap is now ready for placing in a suitable collecting site on
the marsh or along a “fly path”. After attaching the legs, drive
stakes beside two of them and attach the legs to the stakes. This
will prevent wind from upsetting the trap.
- 12. Trap effectiveness can be increased by hanging a decoy beneath the
trap. A beach ball 14 to 16 inches in diameter and painted shiny
black helps attract flies when suspended beneath the trap. The decoy
should clear the ground by four to six inches so it moves with the
- 13. The cost of materials for a box trap is reasonable. Even without the
use of salvaged or less durable materials, a trap can be build for a
Box Trap Use and Maintainance
- Traps should be set out when the first greenheads appear on the marsh
(mid- to late June) and kept in operation through August. Generally,
maintenance is a simple matter. Traps should be inspected at least weekly
during the peak of the fly season. At each inspection dead flies should
be emptied from the plastic containers and discarded, and tears or holes
in the screens or sides should be patched or plugged. Trapped flies usually
die in less than 24 hours and soon dry up and decompose. Traps perform
best when the secondary collector is not clogged with flies, obscuring the
light through the screen funnel. Thus frequent disposal of dead flies
results in a more effective trap. Before storage traps should be cleaned
of flies and dirt. If stored indoors, they will last much longer.
- Selection of trap location is important. Great variation in trapping
success exists depending on location. The following suggestions will help
you find the best location for your trap. Traps should be placed on the
marsh edge near the upland or along the open edge of wooded or shrubby
areas. The best locations are at breaks, or openings of low vegetation in
screening stands of trees or tall brush near the marsh. We call these
breaks “fly-paths” because most of the fly traffic from marsh to upland
passes through these points. Clusters of two or three traps in a fly-path
tend to capture more flies than the combined totals of isolated traps.
Traps can also be placed on beaches and should be located where flies seem
to be most abundant. If the beach is near the marsh, best results can be
expected by trapping flies in the marsh and “fly-paths”.
- Vegetation beneath and around the trap should be kept low (four to six
inches high) for about a 6-foot radius.
- This box trap gives an ecologically safe, inexpensive, and effective
means of greenhead fly control available to anyone with the energy and
manual dexterity to build one. Why not try to build and operate one or
more of these traps this summer. Remember, FOR EVERY FLY YOU TRAP, THERE
IS ONE LESS FEMALE FLY TO BITE.