May 1942 ET-193
UNITED STATES DEPARTMENT OF AGRICULTURE
BUREAU OF ENTOMOLOGY AND PLANT QUARANTINE
SWEEPING WITH AN INSECT-COLLECTING NET TO DETERMINE
WHEN AND WHERE TO DUST FOR PEA WEEVIL CONTROL
By T. A. Brindley and F. G. Hinman,
Division of Truck Crop and Garden Insect Investigations
The farmer who dusts to control the pea weevil can do an
effective job only if he has some knowledge of the number of weevils
present and where they are located in his fields. He must know if
they are numerous enough to warrant the expense of dusting, and he
will certainly be interested in checking the control after dusting
and in determining whether a later dusting is necessary. This
information can be readily obtained only by sweeping the peas with
an insect-collecting net,
Description of the Net
A net of the type used successfully in the northwestern pea-
growing areas is shown in figure 1. This net consists of a hard-
wood handle 40 inches long, a hoop 15 inches in diame or made of
No. 9 spring steel, a metal ferrule 4 inches long whichh fits over
the handle and holds the steel hoop firmly in place, and a collect-
ing bag made of a durable grade of curtain scrim. The parts of the
net are assembled as shown in the figure. The hoop should fit
tightly; one that wobbles at the end of the handle catches fewer
weevils and is less easy to use.
Other materials may be used, but the ones listed have proved
the most satisfactory.
How to Use the Net
The net is swung through the peas in front of the sweeper in
an arc about 4 feet long and dipped down into the vines about
8 or 10 inches. The lower edge of the hoop should be an inch
or two in front of the upper edge so that weevils which drop from
the vines are scooped up. Each stroke across the vines is called
a "sweep." A step or two is taken after each sweep; that is, the
sweeper walks as he swings the net. Sufficient force should be put
into each sweep so that the tips of a few vines are collected in
each 25 sweeps.
How to Sample the Weevil Population in the Field
The amount of sweeping needed to obtain the necessary infor-
mation for the effective use of control measures depends upon the
size of the field and the infestation that can be tolerated in the
final product. Fields up to 10 acres in size usually should be
dusted completely if weevils are found in numbers sufficient to
warrant treatment in any portion. Larger fields should be surveyed
carefully in order to determine where to dust. The green-pea field
must be kept as nearly weevil-free as possible, but it may not be
economically practical to dust an entire seed-pea field with a
very light infestation.
The method of sampling may be outlined as follows: Go into
the field in several places on each of the four sides. If the field
is irregular in shape, sweep at intervals of at least every quarter
mile around it. Make two or more 25-sweep collections at each
place swept; examine the catch and count or estimate the number
of weevils in each collection. After the population at any point
along the edge has been determined and if it is a green-pea field,
start at the edge and sweep at intervals toward the center until
no more weevils are found. On the other hand, if it is a seed-pea
field, examine toward the center until the weevil population drops
below the number for which it is considered profitable to dust.
If large populations are present, the number of sweeps and
the number of localities swept can often be reduced. But if no
weevils or very few weevils are collected, the surveyor should
sample more carefully, for the distribution of the weevil may be
very scattered. A weevil-free area along one side of a field or
sometimes even around the entire field may not mean that the central
portions are also weevil-free.
It is advisable to follow a systematic procedure in making
the survey in order that portions of the field may not be over-
looked. A record of the sweepings made and the number of weevils
collected should be made and kept for future reference. A record
sheet of the type used for large fields by several operators in
the Northwest is shown in figure 2.
When to Sample Weevil Populations
Sweep in the field first before dusting is necessary; that
is, soon after the first blooms appear and before pods form. The
effect of control operations may be determined by sweeping in a
similar manner 18 to 24 hours after dusting. Once control measures
have been applied, sampling should be continued at 3- to 4-day
intervals to determine if any reinfestation occurs. Many good
control jobs have been nullified by the failure of the operators to
redust reinfested areas.
Weevil Populations and Resulting Infestations
The infestation at harvest resulting from the presence of a
given number of weevils, as determined by sweeping with a collecting
net, varies greatly in different fields and depends on the stand
of peas, weather conditions, and other factors. The same number
of weevils, as determined by sweeping, results in a greater infesta-
tion in the later fields than in the early fields. Very roughly,
approximately the following infestations may be expected from an
average population of 1 weevil in 25 sweeps in the Palouse and
Blue Mountain areas of Idaho, Washington, and Oregon:
Early varieties harvested
for the cannery 1
Midseason and late varieties
harvested for the cannery 7
Alaska peas harvested for seed 3
RECORD-WEEVIL COUNTS By
(Date of count)
(Time of count)
(Acres in field)
(1) Early blc
(2) Full bloc
(3) Full bloc
(Name of grower)
(Check one only)
(1) before dusting
(2) after dusting
(3) before 2nd dusting
(4) after 2nd dusting
anf. No pods.
M. Few pods.
m. Mar pods.
WEEVIL COUNTS: NUMBER IN 25 ET SWEEPS.
Side of Approximate feet from field edge.
Field Border 90 150 200 270 Center
4 ____ ____ ____
map of field on reverse of page.
Figure 2.--A page from a field notebook designed for recording weevil
populations determined by sweeping with an insect-collecting net.
UNIVERSITY OF FLORIDA
I111111III IIV1 11111 tN11 JII I III 1111111111l
3 1262 09240 9365