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Grain Beetles
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 adult stages.
Family: Silvanidae
Narrow, flattened insects with 11-segmented, clubbed antennae and 5-segmented
tarsi.
Species characteristics:
Saw-Toothed Grain Beetle
(Oryzaephilus surinamensis) Adults, 2.5-3.5mm long; colour: dark
brown; distance from eye to prothorax more than half vertical diameter
of eye; 6 teeth along each side of prothorax (hence the name ‘Saw-toothed’).
Merchant Grain Beetle
(Oryzaephilus mercator) Very similar in appearance to Saw-toothed
grain beetle with the characteristics described above, except that distance
from eye to prothorax less than half vertical diameter of eye.
Foreign Grain Beetle (Ahasverus advena) Adults, 2-3mm long;
colour: reddish-brown; sides of prothorax not toothed but each apical
angle of prothorax has single large tooth.
DISTRIBUTION
As indicated by its species name, which is taken from Surinam, the Saw-toothed
grain beetle probably originated from the Americas. It is now cosmopolitan
in distribution.
The species will infest cereals and a wide range of foodstuffs. Retail
premises and warehouses can suffer infestations and the beetles will frequently
appear in packaged food.
Infestations arise from the presence of insects in the fabric of grain
stores (including sacks), in grain being transferred from one establishment
to another and in vehicles used for transportation. Relatively small numbers
of insects can rapidly give rise to serious infestations.
They are small, active insects which readily exploit cracks and crevices
where they can hide. Being cold-hardy they are able to overwinter in temperate
lands without difficulty. Their life-cycle may be completed in as little
as 20 days and serious outbreaks may occur within a month of harvest.
The Merchant grain beetle is also widely distributed. Like the Saw-toothed
grain beetle, it too will infest a wide variety of foodstuffs. However,
since it is more sensitive to low temperatures and humidities, infestations
only become established in heated buildings where suitable conditions
exist.
The Foreign grain beetle also originated in the tropics. It is capable
of infesting a variety of materials but only in the presence of moulds.
SIGNIFICANCE
Saw-toothed grain beetles are potentially important pests of farm-stored
grain. They also infest cereal products, dried fruit, dried meats, oilseeds,
nuts, rice and even drugs.
In grain, the mere presence of insects may result in its rejection. The
germ may be damaged and when infestations become heavy they cause the
grain to heat. This in turn leads to caking, moulding and even sprouting.
Both the quality and weight of the grain may be reduced.
Malting barley may be rejected because of poor germination, whilst milling
wheat is adversely affected by tainting and discoloration.
The presence of insects in other foodstuffs will render them unpalatable
and cause their rejection.
Merchant grain beetles mainly infest oilseeds and dried fruit whilst Foreign
grain beetles attack cereal products and cocoa as well as these commodities.
LIFE-CYCLE
The female Saw-toothed grain beetle lays up to 400 eggs, either singly
or in small batches, at a rate of 6-10 per day. These are laid in, or
adjacent to, a suitable food supply and at a temperature of 20-23°C hatch
in 8-17 days to give flattened larvae about 0.9mm long. They are yellowish-white
in colour, with brown flecks and a brown head. Typical of coleopterous
larvae, they have a well-developed head, biting mouthparts and 3 pairs
of legs on the thoracic segments. The larvae are active and feed on damaged
grains, so they can be regarded as secondary pests of grain. The larval
stage lasts 4-7 weeks during which the larvae go through 2- 5 moults,
attaining a length of 3mm. They then construct a cell of food particles
and other debris in which to pupate, emerging after 1-3 weeks as adults.
On emergence the adult beetles live for 6-10 months, breeding within a
temperature range of 17.5-40°C. At 20°C the full life-cycle is completed
in 12-15 weeks whilst at 32-35°C it takes only 20 days.
CONTROL
Assessment of infestations
A variety of trapping techniques are available for measuring stored product
beetle infestations. These include pit fall traps, bait bags, insect probe
traps and adhesive traps. Whatever system is employed adequate records
must be kept.
Hygiene/management
Stores should be soundly constructed to ensure maintenance of correct
storage conditions and allow for easy cleaning. They should be insulated,
well ventilated and damp-proof. Cracks and crevices, which may provide
harbourages for the beetles, should be kept to a minimum.
Commodities should be stacked neatly above the floor level using pallets,
away from walls and should not touch the ceiling. A gap between stacks
will allow for ventilation, regular inspection, cleaning and, if necessary,
treatment with insecticides. Appropriate stock rotation is important and
if possible there should be a one-way passage of commodities through the
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%MC and <15°C and 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.
Glossary of terms
Prothorax: First thoracic segment.
ADVICE
Valent BioSciences has an extensive range of products specifically formulated
for the control of flying and crawling insect pests.
Further information on all Valent BioSciences' household insecticide products
is available from:
Valent BioSciences Corporation
870 Technology Way
Suite 100
Libertyville, IL 60048
The greatest care has been taken to ensure the accuracy of the information
contained in this Fact Sheet. Under no circumstance, however, will Valent
BioSciences Corporation be liable in respect of any error or omission.
© Copyright Valent BioSciences Corporation 2001
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