6 pp, 6 photos, 1 figureThe pecan weevil is a key pest of pecan in portions of Texas. This publication discusses the biology of the pest and how to use integrated pest management practices for control. Photographs illustrate adult and larval stages and signs of pecan nut damage

Harris, Marvin

Knutson, Allen E.

Ree, Bill

Texas A&M University

Controlling the Pecan Weevil
Bill Ree, Allen Knutson and Marvin Harris*
E-343
3-05
*Extension Agent-IPM(Pecans), Extension Entomologist
and Entomology Professor, The Texas A&M University
System.
Figure 1. Pecan weevils are found in 131 counties in Texas.
The pecan weevil (Curculio caryae [Horn])is a key pest of pecan in the UnitedStates and portions of Texas. Pecan
weevils, which also feed on all species of North
American hickory, can be found  from New York
to Iowa, south to Oklahoma, and across the
southeastern states from Florida to west Texas.
As of 1999, pecan weevil had been found in 131
Texas counties (Fig. 1).
Description
The adult is a brownish weevil about 3/8 inch
long. The female’s snout is as long as its body;
the male’s snout is somewhat shorter (Fig. 4).
The larvae are cream colored grubs with
reddish heads. When fully grown, larvae reach
a length of 3/5 inch (Fig. 5).
Biology
Adult weevils, both
males and females, dam-
age pecans by feeding on
and/or laying eggs in nuts.
Even though damage can occur
from the time of adult emergence
to shuck split, the key to weevil
control is to prevent egg lay or
oviposition.
Pecan weevil activity starts in early
August. Adults emerge from the soil where
they have spent  2 or 3 years in soil cells
located 4 to 12 inches beneath the soil surface.
The emergence of adult pecan weevils is
directly related to the type of soil and soil
moisture conditions. Drought conditions and
clay soils delay emergence of adults by a month
or more.
Under normal soil  conditions, approxi-
mately 80 percent of adult weevils emerge
between August 20 and September 10 (Fig. 2).
This emergence pattern is typical across the
pecan belt. However, soils hardened by
drought delay adult emergence. During
drought conditions in 1977-78, almost 30
percent of the adult weevils emerged after an
October 19 rain. Drought-delayed emergence
can only be broken when the soil is softened
by moisture, either from irrigation or rainfall.
Hard soil acts as a physical barrier to
adults moving from their soil cells to the soil
surface. Irrigation can be used to ensure adult
weevils emerge at the normal time (Fig. 2).
Application of 2 to 3 acre inches of water in
mid-August should prevent most soils from being
drought-hardened during the critical August 20 to
September 10 emergence period. Irrigation can
prevent drought delay but will not accelerate weevil
emergence earlier than normal. Schedule irrigation
to ensure weevil emergence at the normal time, but
allow time for soils to dry enough to allow pesticide
sprayers access before emerging weevils can begin to
lay eggs in nuts. Orchards under drip irrigation will
have normal weevil emergence near water emitters
but, under drought conditions, also will have delayed
emergence from hard soils under the canopy but
away from emitters.
Testing for Soil Hardness
Hard soils prevent weevils from emerging
normally. Soil hardness can be measured by
applying pressure to a dowel rod inserted into the
soil. Soil hardness should be tested throughout an
orchard.
Soils with a mechanical impedance equal to or
greater than 60 kg/cm2 in the top 5.5 inches (14
cm) of soil inhibit weevil emergence. To determine
if your soil is that hard or harder, take an 8-inch
length (20 cm) of 1/2-inch dowel rod fitted with a
handle. Press the flat surface of the dowel rod (a
1/2-inch diameter dowel rod equals about a square
centimeter of surface area) onto the soil surface
and apply up to 132 pounds (60 kg) of force to the
handle. If the dowel rod penetrates the soil to a
depth of 6 inches (15 cm), weevils should emerge
during the normal time (Figure 2). If the rod does
not penetrate the soil, emergence of some weevils
will be delayed until the soil becomes softer.
As soon as they emerge from the soil cells, adult
pecan weevils move to the nearest tree. Research
indicates that 77 percent of adults fly to the tree
trunk at a height of 6 to 8 feet, 5 percent walk to the
tree trunk and 15 percent fly directly to the canopy.
Once in the canopy, the tasks of feeding and finding
a mate begin.
Feeding activity of adult weevils, both males and
females, before nuts enter the gel stage can cause
nut drop. After shell hardening, males only feed on
the shuck and this will not cause nut drop. A close
inspection of damaged pecans will reveal a puncture
the size of a straight pin that can be traced through
shuck and shell to the liquid endosperm area of the
nut (Fig. 6). Often this feeding puncture or egg
laying site will be surrounded by a circle of tracks
created by the adult (Fig. 7). The presence of punc-
tures and tracking confirm weevil presence.
Feeding rates for males and females prior to shell
hardening are low. Most weevils emerge just at or
shortly after gel stage, so nut losses caused by adult
feeding are small compared to those caused by egg
laying. The adult emergence period lasts for several
weeks so time sprays to prevent oviposition rather
than to protect a few nuts that will be lost to the
small number of early emerging adults that feed
during the water stage.The key to managing the
pecan weevil is to prevent the laying of eggs.
Female weevils do not begin laying eggs until 5
days after they emerge from the soil. For egg lay to
be successful, nuts must be in the gel stage or later.
Nuts are susceptible to oviposition from the gel stage
up to shuck split.
To deposit eggs in pecans, a female feeds through
the shuck and shell to the kernel where she exca-
vates a small cavity in the developing kernel. She
turns around and, with her ovipositor, places three to
four eggs per nut on the developing kernel (Fig. 8).
Figure 2. Cumulative percent of adult pecan weevil emergence
when soil hardness is not a factor.
Figure 3. Cumulative percent of adult pecan weevil emergence
from drought hardened soils and subsequent adult emergence
after soils are sufficiently softened by rain. Graph reprinted
from The Pecan Quarterly, Vol. 14, No. 2
100
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Rain
Natural drought
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She will avoid any pecans in which eggs already
have been laid by other females. A female will lay
approximately 75 eggs in her life at a rate of 2.6 to
3.8 eggs per day. Each female will oviposit in ap-
proximately 30 nuts during her 3- to 4-week life.
Female weevils cannot lay eggs in nuts after shuck
split.
Larvae hatch and feed in the kernel. When fully
developed, larvae chew a single hole, rarely two,
through the shuck and shell (Fig. 9), exit the nut and
drop to the ground. The time period from egg lay to
larval emergence is approximately 42 days.
Once larvae have dropped to the ground, they
burrow into the soil to a depth of 4 to 12 inches
where they create a cell. A year later, about 90
percent of these larvae pupate during a 3-week
period. After pupation, the adult pecan weevil
remains in the soil for another year. These adults
emerge the next year, resulting in a 2-year life cycle.
The remaining 10 percent delay pupation until the
second year. These adults remain in the soil for
another year, which results in a 3-year life cycle.
Integrated Pest
Management Practices
The objective in a pecan weevil integrated pest
management (IPM) program is to prevent female
weevils from laying eggs in nuts. Because the larvae,
pupae and adults are covered with 4 to 12 inches of
soil and pesticides cannot reach larvae inside the
nuts, management of these life stages is not practi-
cal. The only possible time to manage infestations is
after adults have emerged from the soil and before
egg laying starts. To prevent weevils from laying
eggs, pecan producers must do the following:
• monitor kernel development to determine when
the earliest maturing cultivars reach a stage
susceptible to oviposition;
• monitor adult emergence from the soil; and
• apply an effective insecticide at the proper time
to prevent emerging weevils from laying eggs in
susceptible nuts.
Monitoring Kernel Development: Pecan kernel
development must be in the gel or dough stage for
oviposition, larval hatch and development to be
successful. It is important to detect this early devel-
opmental stage so the onset of oviposition can be
determined. To monitor kernel development, cut the
distal or tip end of the nutlets of the earliest matur-
ing varieties and check for the beginning of the gel
stage. Pecans mature from the distal end to the stem
end.
Monitoring Adult Emergence: There are
several monitoring techniques to detect adult pecan
weevil activity. They include inspecting dropped nuts
for punctures and using knock down sprays, sticky
bands, limb jarring and assorted traps. Of these
techniques, the wire cone trap, pyramid or “Tedders”
trap, and the Circle trap are commonly used.
Wire cone traps have been used for many years to
monitor adult pecan weevil emergence. Wire cone
traps are placed on the ground beneath pecan trees
with a known history of pecan weevil infestations.
These traps are durable and can be used for many
years with limited maintenance. However, these
traps are expensive, labor intensive and cannot be
used with livestock unless the trap area is fenced.
The pyramid or “Tedders” trap is built of two
triangular-shaped pieces of 1/2-inch hardboard. Each
piece is slotted in the center so that one piece slips
over the other at a 90-degree angle. The result is a 4-
foot high vertical pyramid. The trap is painted a
dark color and fitted with a boll weevil trap to collect
adults. A single trap is placed beneath a tree canopy,
approximately 8 to 10 feet from the trunk. It is
recommended that one trap should be used per tree;
no more than a total of 15 traps are needed for 100
acres. Since this trap works as a visual preference,
the tree trunk next to the trap should be painted
white to a height of 6 to 8 feet. Trap efficiency is
enhanced by keeping grass and weeds mowed around
the monitoring trees. These traps require little effort
to put up and are relatively inexpensive. Like the
wire cone traps, these traps cannot be used in
conjunction with grazing unless the trap area is
fenced.
A fairly recent trap that has only been tested for a
couple of years in Texas is the Circle trap. This trap
is a wire cone trap that is placed on the side of a
tree. Trap placement on the tree allows use with
grazing livestock.
Trap construction plans or sources for the pur-
chase of traps can be obtained from your local county
Extension office.
Regardless of the type of trap used, place traps on
or under trees with known weevil infestations. If an
orchard has several different soil types (sandy, loam,
clay), place traps under infested trees growing on the
different soils. Place traps in the orchard 1 to 2
weeks before the earliest maturing varieties reach
the gel stage and monitor the traps every 2 to 3 days.
Adult weevils collected in the traps should be
counted and removed with each inspection.
Applying Insecticide: If adult pecan weevils are
found in an orchard, economic damage will occur if
the orchard is not treated. A treatment program for
the pecan weevil requires at least two properly timed
insecticide applications. In some cases, three or four
applications may be needed to prevent an economic
loss.
Unlike treatment for the pecan nut casebearer,
where infestations can vary from year to year by
several weeks or more, the initial insecticide applica-
tion for pecan weevil is applied around August 20 to
22. The initial treatment should be made when the
earliest maturing nuts of the early varieties are in
the gel stage at the distal end of the nut. Even in
years where drought delays emergence and traps fail
to collect adults, some pecan weevil adults will
emerge through soil cracks before gel formation and
this justifies the initial treatment.
The initial insecticide application should be
effective for 5 days. Adult female pecan weevils do
not begin to lay eggs until 5 days after emergence, so
the earliest time to retreat will be 10 days after the
initial application. If adult emergence traps are
collecting adult weevils on or after the sixth day
following the initial insecticide application, a second
application is needed. The second application is
needed no sooner than 10 days after the first treat-
ment. If no adult weevils are trapped 6 days after
the initial treatment, then the second application
can be delayed until traps again detect adult activity.
At least two sprays are needed to control established
pecan weevil infestations. If drought has caused soil
to become hard and adults continue to be trapped
more than 5 days after the second application, a
third insecticide application will be needed. Any
pecan that has not reached shuck split will be
susceptible to pecan weevil and should be protected
with an insecticide application.
In Texas, the recommended insecticide for pecan
weevil control is carbaryl (Sevin® 80S) at a rate of
1.25 to 3.0 pounds per 100 gallons. Do not use
spreader stickers with the treatment. Other formula-
tions of carbaryl also can be used. Read and follow
label directions.
The pecan weevil is an insidious pest that infests
nuts that could otherwise be harvested and used as
food. Routine management of the weevil calls for two
or three well-timed, late-season applications of
insecticide. This prevents weevils that live in the
infested orchard from causing economic damage.
Adults do not fly far and natural spread of infesta-
tions occurs over distances of a mile or less. Move-
ment over longer distances results from transport of
infested nuts by humans.
Pecan Weevil Eradication
Integrated pest management (IPM) practices in
pecan are designed to keep pecan weevil populations
below economic levels. The goal of eradication,
however, is to eliminate a pest species from an area.
Occasionally, efforts to eradicate pecan weevil are
attempted when isolated populations are detected.
An eradication program is typically conducted under
the regulatory authority of the Texas Department of
Agriculture. Eradication efforts include an intensive
insecticide treatment program, destruction of in-
fested nuts and discarded trash, and strict quaran-
tines to prevent movement of infested nuts into the
eradication area.
To eradicate pecan weevil, insecticides are applied
just before gel stage and continue at 7- to 10-day
intervals. Several applications that blanket the
presumed adult emergence period are made in an
effort to kill every emerging weevil. This eradication
program requires a minimum 3-year period when no
larval infestations appear in the harvests. The
program probably should be conducted for a fourth
season to ensure eradication success. Eradication is
considered complete when no pecan weevil larvae are
found for four consecutive seasons.
Additional Information
These publications are available at the Texas Coop-
erative Extension Bookstore, http://tcebookstore.org.
E-215, Managing Insect and Mite Pests of Commer-
cial Pecans in Texas.
E-341, Field Guide to the Insects and Mites Associ-
ated With Pecans.
CD-ROM, Pecan Pest Management. Available from
the Texas Pecan Growers Association. Send check for
$30.00 plus $3.00 shipping (Texas residents add
$4.95 [8.25 percent sales tax]) to: Olde Pecan Book
Store, P.O. Drawer CC, College Station, TX 77841.
Figure 4. Pecan weevil adults (male, left; female, right). Figure 5. Pecan weevil larvae.
Figure 6. Pecan weevil puncture of pecan shell. Figure 7. Pecan weevil circular tracking pattern around
puncture.
Figure 8. Female pecan weevil ovipositiing eggs in pecan.
Courtesy of Oklahoma State University Entomology
Department.
Figure 9. Pecan weevil larvae emergence holes.
The information given herein is for educational purposes only. Reference to commercial
products or trade names is made with the understanding that no discrimination is intended
and no endorsement by Texas Cooperative Extension Service is implied.


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Controlling the Pecan Weevil

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