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Grain Weevils
Description
Order: Coleoptera (‘sheath-wings’)
Characteristics:
Forewings hard and leathery, meeting along
mid-line of dorsal surface; hindwings
membranous, sometimes lacking; biting
mouthparts; well developed thorax; complete
metamorphosis with egg, larval, pupal and
Grain weevil
adult stages.
3.5mm long
Family: Curculionidae.
Insects with cylindrical bodies and a
pronounced rostrum (snout) equipped with
Distribution Significance
Grain weevils are encountered in all temperate Grain weevils are important pests of farm-stored
mouthparts which is used by females as a
and warm-temperate climates and are widely grain. They are frequently regarded as primary
boring tool; elbowed, clubbed antennae set on
distributed in grain stores throughout Europe. pests of grain since they are able to infest
rostrum; 4-segmented tarsi.
Infestations arise as a result of their regular otherwise undamaged grain. Grain weevils will
importation in grain and cereal products and also attack other hard cereal products, e.g.
Species characteristics:
from the fabric of vehicles or buildings used for macaroni and spaghetti. Fine cereal products
the transportation and storage of these and are unsuitable for breeding purposes unless they
Grain Weevil (Granary Weevil)
other vulnerable commodities. Sacks, too, are an become caked.
(Sitophilus granarius) Adults, 3-4mm long;
important source of infestation, the insects
colour: dark brown, nearly black, with shiny
finding harbourage in the seams and weave. The following damage may be caused:
appearance; 8-segmented antennae; prothorax
with distinct oblong/oblong-oval punctures;
Being well suited to temperate conditions the I) Reduction in the weight and quality of grain
hindwings absent.
species will breed readily in Europe and there as a result of the larvae feeding on the
may be three or more generations per year in endosperm. The germ is not always attacked so
Rice Weevil (Sitophilus oryzae)
unheated conditions. Both adults and larvae are germination may take place, producing a weak
Adults, 2-3.5mm long (average 2.5mm); colour:
cold-hardy. seedling which is vulnerable to attack by
dark brown, nearly black, with four clearly
moulds, bacteria and other insects. Both larvae
defined reddish spots on the elytra; less shiny
Rice and Maize weevils are widely distributed in and adults will feed upon grains.
than the Grain weevil; 8-segmented antennae;
tropical and sub-tropical areas and will be
prothorax with round or irregular punctures;
carried to temperate areas on imported Il) Tainting with white, dusty excreta which
hindwings present.
commodities. The Maize weevil will breed on contaminate the product as well as rendering it
maize in the field, but the Rice weevil only unpalatable.
Maize Weevil (Sitophilus zeamais)
breeds in stored grain. Both insects are less
Very similar in appearance to the Rice weevil
cold-hardy than the Grain weevil and will not Ill) Heating of the grain, accelerating
with the characteristics described above, except
normally overwinter in unheated premises or development of the insects and making the
that the insects are longer, adults reaching a
grain stored at normal temperatures. commodity liable to caking, moulding and even
length of 3-3.5mm (average 3mm).
germination. Temperatures may be attained
which actually kill the insects.
Weevil-damaged grain can be readily recognised
by the presence of large holes which are the exit
holes of the emerging adults. Some idea of the
huge numbers of weevils that can be generated
is provided by the results of one study in which,
5 weeks after wheat was infested with larval
forms, adults were seen to be leaving the grain
at a rate of 100 per kg per day.
Life-Cycle Control
The Grain weevil can only breed in grain with a Assessment of infestations
moisture content of more than 9.5%and at A variety of trapping techniques are available
temperatures within the range 13-35°C. The for measuring stored product beetle
female lays about 200 eggs at a rate of 2-3 per infestations. These include pit fall traps, bait
day depending upon temperature and humidity, bags, insect probe traps and adhesive traps.
placing each one in a small hole bored in the Whatever system is employed adequate records
grain and sealing it in with a mucilaginous plug must be kept.
of saliva. At 18-20°Cthe eggs hatch in 8-11 days
to give small, white, legless larvae which feed Hygiene/management
on the endosperm of the grain. Only one larva Stores should be soundly constructed to ensure
develops in small grains such as wheat and rice maintenance of correct storage conditions and
but large grains such as maize will support the allow for easy cleaning. They should be
development of several. Larvae are never free- insulated, well ventilated and damp-proof.
living and develop entirely within the grain. Cracks and crevices, which may provide
They moult four times, finally pupating within harbourages for the beetles, should be kept to a
the grain after 6-8 weeks. The adults emerge minimum.
after a further 5-16 days and will live for about
9 months. If disturbed they will feign death by Commodities should be stacked neatly above
drawing their legs up to their bodies and the floor level using pallets, away from walls
remaining still. At 15°Cand a grain moisture and should not touch the ceiling. A gap between
content of 11.3%the full life-cycle takes 6 stacks will allow for ventilation, regular
months. inspection, cleaning and, if necessary, treatment
with insecticides. Appropriate stock rotation is
The life-cycles of the Rice and Maize weevils important and if possible there should be a
follow a similar course to that of the Grain one-way passage of commodities through the
weevil. premises. The careful choice of packaging can
help to deter insect attack. Generally, thick,
tough materials with a smooth, shiny finish are
preferred. Packs should be strong and well
sealed.
It is important to ensure that there are no food
residues (stored commodities or secondary
sources, e.g. birds’nests) in which beetles can
breed and develop to infest new materials. All
infested commodities should be destroyed or
fumigated. Stores should be kept scrupulously
clean and farm stores should be thoroughly
cleaned before harvest.
All grain taken into store should be dried to a
suitable moisture content and temperature e.g.
<15%MCand <15°Cand maintained in that
condition.
Insecticidal control
Insecticides can be applied to the fabric of
stores concentrating on potential insect
harbourages. Alternatively it may be appropriate
to employ grain protectants.
