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WEEVILS ON
STORED GRAINS
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 A
complex of weevils, the rice (Sitophilus oryza), granary (Sitophilus
granarius), and maize (Sitophilus zeamais) weevils, are among
the most destructive pests of grains, seeds, and grain products stored
in elevators and bins. They probably are not native to North America,
but entered in seeds carried by settlers through ports. These weevils
are pests of grain throughout the world.
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| DESCRIPTIONS
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Rice Weevil:
| The rice weevil is a small
snout beetle which varies in size, but it averages about three thirty-seconds
inch in length. It varies from a dull red-brown to black, and is usually
marked on the back with four light red to yellow spots. The rice weevil
has fully developed wings beneath its wing covers and can fly readily.
The thorax is densely pitted with somewhat irregularly shaped punctures,
except for a smooth narrow strip extending down the middle of the
back. The larval stage of this insect is a soft, white, legless, fleshy
grub which feeds on the interior of the grain kernel. When mature,
the grub changes to a naked white pupa and later emerges as an adult
beetle. |
Maize Weevil:
| The maize weevil is a
small snout beetle which varies in size, averaging about three thirty-seconds
inch in length. It varies from dull red-brown to nearly black and
is usually marked on the back with four light reddish or yellowish
spots. The maize weevil has fully developed wings beneath its wing
covers and can fly readily. The thorax is densely pitted with somewhat
irregularly shaped punctures, except for a smooth narrow strip extending
down the middle of the dorsal (top) side. An egg hatches in a few
days into a soft , white, legless, fleshly grub which feeds on the
interior of the grain kernel. The grub changes to a naked white pupa
and later emerges as an adult beetle. The rate of development is slightly
slower for the maize weevil than for the rice weevil. A minimum of
thirty days is required for passing through the egg, larval and pupal
stages. |
Granary Weevil:
| The adult granary weevil
is a somewhat cylindrical beetle about two-tenths of an inch (two
to three mm) long. The head is prolonged with a distinct snout extending
downward from the head for a distance of about one-fourth the length
of the body. The weevil is polished red brown to black with ridged
wing-covers and a well-marked thorax with oval pits. Unlike the rice
and maize weevils, the granery weevil cannot fly. The egg hatches
in a few days into a soft, white, legless, fleshy grub which feeds
on the interior of the grain kernel. The grub changes to a naked white
pupa and later emerges as an adult beetle. |
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| LIFE
HISTORIES |
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Rice Weevil:
| Adult rice weevils live
for four to five months and each female lays 300 to 400 eggs during
this period. The female uses her strong mandibles to chew a hole in
the grain kernel where she deposits a single egg and seals the hole
with a gelatinous fluid. During hot weather, the development period
for egg to adult may be as few as twenty-six days. This period is
greatly prolonged during cool or cold weather. Rice weevils are capable
of flight, and infestations may develop in the field prior to harvest.
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Maize Weevil:
| Maize weevils, for a long
time were referred to as a larger strain or race of the rice weevil,
but are now recognized as a distinct species. The maize weevil is
slightly larger, up to one-eighth inch (four mm) long, and darker
than the rice weevil; the degree of variation within each species
makes them difficult to tell apart. The thorax of the maize weevil
is densely and uniformly pitted with round punctures. An egg hatches
in a few days into a soft, white, legless, fleshy grub which feeds
on the interior of the grain kernel. After the larval stages are completed
the grub changes to a white pupa and later emerges as an adult beetle.
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Granary Weevil:
| Adult granary weevil live
an average of about seven to eight weeks. Each female lays 50 to 200
white eggs during this period. The female uses her strong mandibles
to chew a small hole in the grain kernel, where she deposits a single
egg in the hole and seals it with a gelatinous fluid. In warm weather,
the granary weevil can develop from egg to adult in about five weeks.
Cold weather prolongs development. The granary weevil cannot fly and
so is most likely to be found where grain is stored, and moves with
infested grain. |
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| DAMAGE
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These weevils are very destructive grain pests. Of the three, the rice
weevil is probably the most insidious, owing largely to the ability of
flight. All three weevils develop as larvae within the grain kernels.
They frequently cause almost complete destruction of grain in elevators
or bins, where conditions are favorable and the grain is undisturbed for
some length of time. Infested grain will usually be found heating at the
surface, and it may be damp, sometimes to such an extent that sprouting
occurs. Wheat, corn, macaroni, oats, barley, sorghum, Kaffir seed, and
buckwheat are just some of the grains and products on which these weevils
feed.
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| CONTROL |
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Prevention is the best strategy to avoid insect problems in stored grains.
Proper bin sanitation before introduction of new grain minimizes the need
for pesticides. Good sanitation involves the removal of old grain and
dust in and around the grain bin. This includes removal of old grain from
corners, floors, and walls and grain that may have spilled on the exterior
of the bin. Any grain remaining when a bin is emptied can harbor insect
infestations which will move into the new grain. After the bin is cleaned,
and all needed repairs have been made, the floor and wall surfaces both
inside and outside the bin should be treated. Take special care to treat
all cracks, crevices, and areas around doorways and other places where
insects could hide or enter. Spray the bins about four to six weeks prior
to storing grain.
Before grain is placed in a
bin, it should be screened to eliminate fine materials and broken kernels.
Grain placed in a clean bin should be checked at two week intervals during
warm months and at one month intervals during cooler months for the presence
of hotspots, moldy areas, and live insects. If any of these conditions
exist, the grain should be aerated to lower the moisture level and temperature.
Grain that is to be stored
for longer than six months may need a protective application of an approved
insecticide. Treatments can be applied as the grain is loaded into the
bin through the use of a metering device calibrated to apply the proper
amounts. After the grain is binned and leveled, a surface dressing can
be applied to prevent insects from entering the grain on the surface.
If infestation occurs in spite of these precautions, fumigation of the
grain will be necessary. Because of the high toxicity of registered fumigants
and technical knowledge needed for their proper use, a qualified pesticide
applicator should be contacted to perform the fumigation.
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| WARNING |
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Pesticides are poisonous. Read and follow directions and safety precautions
on labels. Handle carefully and store in original labeled containers out
of the reach of children, pets, and livestock. Dispose of empty containers
right away, in a safe manner and place. Do not contaminate forage, streams,
or ponds.
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