STATE LLANT BOARD ET-29
August 1950 ET-29
United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant Quarantine
A PORTABLE FIELD CAGE i/
By Marion W. Pedersen, / Frank E. Todd, and Frank V. Lieberman /
Several common disadvantages of portable field cages have been
overcome by using plastic screen for the covering and standard electrical
conduit for the framework. The utilization of these materials has made
it possible to construct a cage that is (1) light, (2) portable intact,
(3) easily assembled and taken apart, and (4) conveniently stored. In
addition, meteorological factors are not seriously modified.
The cage described in this paper was designed to confine or exclude
bees in pollination experiments (fig. 1). The dimensions were chosen to
fit immediate requirements. Other sizes might be more practical. The
cages have held up well through two seasons of use, largely because of
sturdiness supplied by rigid corner construction. However, the top
screen in some of the coverings used from spring to fall has become
brittle. The life of these covers depends upon careful handling so as
to avoid creasing*
The cage covers an area of 247 square feet -- 11 feet wide, 21i
feet long, and 6 feet high. A plastic screen covering (12 by 12 mesh) is
supported on a framework of -inch electrical conduit, which is anchored
to the ground with steel stakes and wire. A 6-foot zipper is installed
in each of the four corners to permit entrance, and to facilitate the
handling and folding of the cover for storage.
1./ Cooperative legume seed investigations conducted at Logan, Utah,
by the Bureau of Plant Industry, Soils, and Agricultural Engineering, the
Bureau of Entomology and Plant Quarantine, and Utah Agricultural Experi-
ment Station. Photographs were taken by W. P. Nye. Clarence Austin
aided materially in the cage construction.
2 Division of Forage Crops and Diseases, Bureau of Plant Industry,
Soils, and Agricultural Engineering.
1/ Division of Bee Culture Investigations of this Bureau.
4/ Division of Cereal and Forage Investigations of this Bureau.
The weight of the assembled cage is approximately 115 pounds, the
cover weighing 32 pounds and the framework 83 pounds. Two men can
easily pick it up and move it for short distances.
Comparative tests were made of the relative humidity, temperature,
light, end wind movement inside and outside these cages. The results,
shown in table 1, indicate no significant differences in temperature and
humidity but a significant reduction in light and in wind movement. No
differences in alfalfa growth have been observed that could be attributed
to a csge effect.
Table 1. Comparison of meteorological conditions inside and outside
of 12- by 12-mesh plastic screen cages covering alfalfa.
of humidity 11 Temperature I/ Light 2j Wind
Percent OF. Candles M.P.h.
per square foot
In cage 43.3 70.5 158 5.27
In open 39.7 70.3 193 6.70
Difference 3.6 0.2 -35 i/ -1.43 0 /
1/ Average of 1 reading in each of 6 pairs of plots taken with a
2/ Average of 2 readings in each of S pairs of plots taken with a
Vj Average of 20 readings in each plot of 1 pair taken with an
/ Highly significant (1% level).
/ Significant (5% level).
Construction of Conduit Framework
The boxlike framework is made from suitable lengths of conduit
coupled to welded corner units. Short lengths are cut from standard
10-foot pieces with a hacksaw. Two or more pieces must be coupled to
form lengths longer than 10 feet. A conduit bending tool is essential
for making bends.
Corner units (fig. 2):
The corner pieces are 4-foot lengths of conduit bent at right angles.
One of these corner pieces is welded to each end of the corner posts.
The 2k-foot braces, which support the corner pieces, are then welded into
place as shown. A wooden form made to hold the pieces of conduit in
place while starting the welds speeds up the operation and assures uniform
corner units. Ends of the braces can be more easily welded if depressed
on one side with a ballpeen hammer.
Sides and ends:
The upper and lower end pieces are 7* feet long. The upper and
lower side pieces are 17z feet long (a 10-foot length coupled to a 7-
Cross members (fig. 5):
Three cross members are used in the top of the framework to prevent
the covering from sagging. The center one is a regular 10-foot length
attached to 6-foot posts by means of side supports. The other two are
made by coupling together a 10-foot piece and a 1*-foot piece. A small
hole is drilled in each end of the 11-foot cross members for use in
wiring them to the main part of the frame.
Side supports (fig. 3):
The connector piece for the center cross member is made by bending a
11-foot length at a right angle. The piece is then welded to the 6-foot
post so that the bottom of the horizontal arm is level with the top of
the post. A small hole is drilled through the post 1 inch from each end
and in the same plane as the connector piece.
Construction of Screen Covering
The covering is made as follows:
1. Cut the strips of plastic screen from the rolls.
2. Sew the strips together in the design indicated (fig. 4a), using
i-inch cloth tape to strengthen the seams. Use double stitching. Cotton
thread is satisfactory.
3, Install the zippers. The edges of the screen (c) are folded
back for 1 inch and bound between the zipper and 1-inch cloth tape, using
a double seam.
4. Bind the remaining edges (b). Sew 2-inch webbing along the four
sides, folding unfinished edges back. A double thickness of canvas can
be substituted for webbing.
5. Install the grommets at 18-inch intervals along the webbing.
Assembly and Anchorage
Although one person can assemble the framework (figs, 5 and 6) and
put the covering in place, it is much more convenient for two people to
do it together. After both ends are assembled on the ground, they are
raised to their upright positions and joined with the side pieces. The
center cross member is next coupled directly to the connector piece of
the side supports. This U-shaped unit is then wired in place, being
fastened to the upper and lower side pieces. Care should be exercised
to twist the wire in such a way that it cannot catch in the covering. An
auxiliary cross member is then wired in place halfway between the center
cross member and each end of the cage.
After the covering is pulled over the framework the zippers are
closed. The covering is then securely fastened to the bottom of the
framework by wiring each grommet to the conduit, Por some purposes more
positive closure may be necessary, in which case the bottom can be
banked with soil. Canvas that is to be kept covered for extended periods
of time with moist soil should be treated to make it rot resistant.
Anchorage is the final step. The bottom of the cage is secured to
the ground with open-end steel stakes (fig. 3). These stakes are driven
into the ground until the hook comes into place over the conduit. Guy
wires from the corner units and side supports are attached to closed-end
stakes in the ground (fig. 3). Guy wires can be placed either inside or
outside the cage and have been used both ways satisfactorily. Convenience
and local wind conditions should govern the arrangement. The outside
anchorage is preferable against strong winds, since holes may form where
the wires go through.
Two men can dismantle and store a cage in 30 minutes. The flexible
covering may be folded or rolled into a compact bundle. The four corner
units partially nest.
Materials and Cost of Cage
Units Length (feet)
Conduit (4-inch galvanized)
Plastic screen (3-foot width)
Zippers (6 feet)
Stakes (3/S-inch iron)
Sewing (including webbing, grommets,
tape, end thread)
Cutting and bending
FIRM : !
Figure 2. --View of upper half of a corner unit.
"."' ". ," .. .. .... I ,. f,' :.( .. .
w- "-" .** *
Figure 3. --Side support showing welded, wired,
and coupled connections. The two types of LIBRARY
anchorage stakes are shown at the right. T f pLu.4T BOARD
6' 22' 1 61-
Figure 4. --Diagram.showing shape and seams of cage covering:
a, Double seams with 1/2-inch cloth tape; b, 2-inch webbing or
canvas with 3/8-inch grommets at 18-foot intervals; c, zipper sewed
on 1-inch tape with double seams.
Figure 5. --Diagram of 1/2-inch conduit framework for supporting cage:
a, welded joint; b, watertight coupling; and c, wired joint.
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