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The greatest damage to stored grain is generally caused by insects, though this may be exceeded by rodents in some countries. A high rate of reproduction and a short development period enable insects to cause important damage by rapidly developing from a small number of individuals to a large mass.
The multiplication factor of Tribolium is 70, for example. This means that under optimum conditions one pair of Tribolium will have the following offspring: after I month: 2 x 70 = 140 after 2 months: 140 x 70 = 9 800 after 3 months: 9 800 x 70 = 686 000
After 4 months the theoretical number would be: 686 000 x 70 = 48 020 000 specimen, but by that time resources for survival and further development become scarce and concurrence for food and overcrowding limit further development.
Insect species are different from one another in terms of their behaviour, their damage caused and their reaction to control measures. It is essential to identify insects found in the store and to know about their biology in order to be able to answer the following questions:
- Is it a storage pest?
Example :
Several species of Bruchus are held pests of
pulses and may be brought into the store where they cannot develop. In this
case, these insects ate no storage pests.
- Is it an important storage pest?
Example:
The Maize weevil (Sitophilus zeamais) is
for example a very important storage pest of different commodities,
especially cereals in tropical and subtropical regions, whereas the Depressed
flour beetle (Palorus subdepressus) generally plays a minor role.
- is it an insect species which reveals problems in storage?
Example:
The Black fungus beetle (Alphitobius
laevigatus) occurs mainly in mouldy stock. If this insect is found, it
indicates moist storage conditions.
- What control measures should be performed?
Example:
Bostrichidae, e.g. the Lesser grain borer
(Rhyzopertha dominica) are most effectively controlled by pyrethroids,
less by organophosphorous compounds.
There are various aids to identifying insects:
· identification keys, which are not suitable for everyday
practical use in stores
· illustrations hi the form of posters,
leaflets, brochures or books
· reference collections of storage pests
for direct comparison with the ones found.
Every storekeeper should have a hand magnifying glass with eight to twelvefold magnification.
By far the largest group of storage pests are beetles (Coleoptera), followed by moths (Lepidoptera). There are still others including dust lice (Psocoptera) which cause little damage to stored produce but may become a hygienic problem if they occur in large numbers.
Beside insects mites (Acarina) occur as pests in grain and particularly in flour. They belong to the order of Arachnida.
Comparable to other insects, beetles and moths pass through several stages of development. The adult insects lay eggs from which larvae hatch. Larvae cause most damage as a result of their intensive feeding on the stored produce. Their development passes through a number of growing stages called instars followed by pupation. The adult emerges from the pupa. This development cycle is referred to as complete metamorphosis. The duration of this cycle varies from species to species and is greatly influenced by external factors (see section 7.4)
There are often also differences from species to species as to where the eggs are laid (in or on grain), where the larvae develop (inside or outside grain) and where pupation takes place (inside or outside grain).
Figure
82
Every species has its optimum temperature and moisture conditions for development (see section 2.2.5). individual stages of development of any particular species may also have different preferences.
The optimum temperatures lie mainly between 25 and 32°C. At temperatures of below 14°C and above 42°C development generally does not take place. Most storage pests die at temperatures of below 5°C and above 45°C.
The optimum relative humidity for most species lies at around 70°/0, the minimum being 25 - 40% and the maximum 80 - 100%. Very few species are able to survive in extremely dry conditions (Oryzaephilus spp. down to 10% r.h., Trogoderma granarium and Tribolium spp. down to 3% r.h.).
Under optimum conditions, the duration of the development cycle from egg to adult is around 18 - 25 days for beetles and 28 - 35 days for moths. Under unfavourable conditions, this period may be extended to several months.
Insects are greatly influenced in their activity and fertility by the changes in light in the course of a day. Especially moths are most active at dawn and at dusk. inspections to cheek for flying insects should therefore be made at these times. This applies also to moth control measures with fogging machines. Artificial light can help to considerably restrict the flying activity and fertility of moths.
Stored product insects have different requirements as to the composition of their food:
· Primary pests are able to teed on whole, healthy
and well-storable grains.
Examples: weevils, lesser grain borer
(Rhyzopertha dominica). Angoumois grain moth (Sitotroga
cerealella)
· Secondary pests can only attack broken grain,
moist, and thus soft grain, grain damaged by primary pests or processed
products, e.g. flour.
Examples: flour beetles
· Mould indicating pests live partially or entirely
on fungi and their presence reveals problems with moisture.
Examples:
Black fungus beetle (Alphitobius diaperinus), Foreign grain beetle
(Ahasverus advena)
· Scavengers live largely on dust, the excrements of
other insects or dead insects. They do not usually feed on the stored produce
itself but often pose a serious hygienic problem.
Examples: dust lice
· Predators live entirely or partially on insects,
mostly on eggs and larvae (see section 10.2).
Examples:
Teretriosoma nigrescens, Wheat beetle (Tenebroides
mauritanicus)
Some storage pests also prey mm the larvae of other species.
Their use in reducing infestation is, however' far less than the damage they
themselves cause by feeding on the stored produce.
Example:
Tribolium castaneum
Whether an insect can make use of stored produce as a source of food depends on a number of factors:
Most storage pests are able to penetrate a stack of bags far more quickly and thoroughly than bulk produce because of the gaps between the bags. The size, the surface texture and nutrients in the grain influence the ability of the pest to attack the commodity. This applies also for packaging material and the state of the store itself
The body of an insect is divided into three parts:
1. The head, which bears the eyes, the antennae and the
mouthparts
2. the thorax, which consists of three segments (prothorax,
mesothorax, metathorax) carries three pairs of legs and the wings or the elytra,
respectively
3. the abdomen, where the reproductive and digestive
organs are located.
In the case of beetles, the forewings (elytra) are thickened and hornlike and protect the abdomen.
Moths have two pairs of membranous wings densely covered with pigmented scales.
Beetle larvae have three pairs of legs. However, in some species which develop inside the grain (e.g. weevils) they are lacking.
Figure 83
Figure 84
Moth larvae have three pairs of thoracic legs and additionally four pairs of prolegs, located at the 3rd, 4th, 5th and 6th segment of the abdomen. The final segment of the abdomen has a further pair of prolegs.
- Pictorial key for the most important stored product beetles
The following key only refers to the most frequently found stored product beetles and is not a comprehensive tool for determination. Any identification should be confirmed by comparing with other illustrations, descriptions or specimens from reference collections.
Figure 85
Figure 86
Figure 87
Sitophilus oryzae
Common name: Rico weevil
Family: Curculionidae
Description
size: 2.5 - 3.5 mm
shape: more or less cylindrical
colour: black-brown with four reddish spots on the elytra
recognition:
well defined snout: elbowed and clubbed antennae; circular punctures on the
prothorax; can fly
Distribution: cosmopolitan
Life history
range of temperature: 17 - 34°C
optimal temperature:
28°C
range of rel. humidity: 45 - 100%
optimal rel. humidity: 70%
eggs laid: up to 150 separately deposited inside the grain
life cycle:
35 days at optimum
110 days at sub-optimal conditions
Damage
Adults and legless larvae are primary pests of cereals, rice and dried cassava. Larvae spend their lives inside the grain.
Similar species
S. zeamais (Maize weevil): larger, but almost
indistinguishable externally; with similar distribution, biology and behaviour.
Good flyer.
S. granarius (Granary weevil): without spots on
elytra, punctures on prothorax oval-shaped. A pest of cereals (especially wheat
and barley) in temperate regions.
Figure 88
Tribolium castaneum
Common name: Rust-red flour beetle
Family:
Tenebrionidae
Description
size: 3 - 4 mm
shape: elongate body, more or less parallel
sided
colour: red brown - dark brown
recognition: antennae are inserted
under the sides of the head (frontal ridge) and form a three-segmented club;
elytra with finely punctured lines
Distribution: throughout the tropics and the subtropics
Life history
range of temperature: 22 - 40°C
optimal temperature:
35°C
range of rel. humidity: 1 - 90%
optimal rel. humidity: 75%
eggs laid: up to 500
life cycle: 20 days under optimum conditions
Damage
Larvae and adults are secondary pests and attack cereals and cereal products, groundnuts, nuts, spices, coffee, cocoa, dried fruit and occasionally pulses. Infestation leads to persistent disagreeable odours of the products.
Similar species
T. confusum (segments of antennae gradually broaden towards the tip), cosmopolitan.
Figure 89
Rhyzopertha dominica
Common name: Lesser grain borer
Family:
Bostrichidae
Description
size: 2 - 3 mm
shape: slim, cylindrical
colour:
red-brown to black-brown
recognition: head concealed beneath prothorax
(typical for the Bostrichidae); prothorax bears marginal rows of teeth; elytra
with well defined rows of punctures
Distribution: mainly in tropical and sub-tropical regions
Life history
range of temperature: 18 - 38°C
optimal temperature:
34°C
range of rel. humidity: 25 - 70%
optimal rel. humidity: 60 -
70%
eggs laid: 300 - 500 life cycle:20 - 84 days
Damage
Primary pest of cereal grains, other seeds, cereal products, dried cassava, etc. Damage is done by adults and larvae, which develop within the grain.
Similar species
Dinoderus spp. bearing two slight depressions at the base of the pronotum. Found on dried cassava and incidentally on other commodities.
Figure 90
Figure 91
Prostephanus truncatus
Common names: Larger grain borer
Greater grain borer
Description
size: 3 - 5 mm
shape: cylindrical
colour: dark brown
recognition: similar to Rhyzopertha, but elytra apically flattened,
steeply inclined, curved ridges at the sloping part; elytra look like cut off:
Distribution
Central America, accidentally introduced to East and West African countries
Life history
range of temperature: 18 - 40°C
optimal temperature:
32°C
range of rel. humidity: 40 - 90%
optimal rel. humidity: 80%
eggs laid: up to 400 life cycle: 27 days at optimum
Damage
Primary pest.
Adults and larvae attack maize as well as dried
cassava and yams. Causes severe losses of farm-stored maize in African
countries.
Figure 92
Figure 93
Trogoderma granarium
Common name: Khapra beetle
Family: Dermestidae
|
Description |
adult |
larva |
|
size: |
2 - 3 mm |
5 mm |
|
shape: |
oval |
spindle-shaped |
|
colour: |
dark brown, often with blurred, reddish markings |
yellowish brown to golden brown |
|
recognition: |
body covered with fine hairs |
reddish-brown hairs with two tail-like tufts |
Distribution
In hot, dry areas, especially in the near and middle east and Africa
Life history
range of temperature: 22 - 41°C
optimal temperature: 33
- 37°C
range of rel. humidity: 2 to 50%
optimal rel humidity: 25%
eggs laid: 50 - 80 life cycle: 25 days at 37°C and 25% r.h. larval
diapause up to 4 years
Damage
Primary pest
Damage is done only by larvae on cereal grains
and products, oilseed cakes, nuts, pulses, etc.
Figure 94
Oryzaephilus surinamensis
Common name: Saw-toothed grain beetle
Family:
Silvanidae
Description
size: 2.5 - 3.5 mm
shape: slender
colour: dark brown
recognition: six toothlike projections along each side of the prothorax
Distribution: cosmopolitan
Life history
range of temperature: 18 - 37°C
optimal temperature: 30
- 35°C
range of rel. humidity: 10 - 90%
optimal rel. humidity: 70 -
90%
eggs laid: up to 150 life cycle: 20 - 80 days
Damage
Secondary pest of cereals and cereal products, also on copra, spices, nuts and dried fruit. Damage is done by larvae and adults.
Similar species
O. mercator in the warmer temperate and tropical regions. Less tolerant to extremes of temperature and humidity than O. surinamensis. More common on oilseeds, also on copra, spices, nuts and dried fruit.
Figure 95
Cryptolestes ferrugineus
Common name: Rust-red grain beetle
Family:
Cucujidae
Description
size: 1.5 - 2.5 mm
shape: tiny, flat and slender, elongate
colour: reddish brown
recognition: head and prothorax account for half
of the body length: prothorax bearing two longitudinal ridges; antennae without
club and half the length up to the length of the body
Distribution: cosmopolitan
Life history
range of temperature: 21 - 43°C
optimal temperature: 33°C
range of rel. humidity: 50 - 90%
optimal rel humidity: 70%
eggs laid: 100 - 400
life cycle: 17 - 100
days at optimum conditions; mean duration of life cycle: 23 days
Damage
Secondary pest on all types of grain and grain products, also on nuts, dried fruit. oilseed cakes, cocoa and cowpeas. Adults and larvae attack stored products and are often causing "hot spots".
Figure 96
Callosobruchus chinensis
Common name: Cowpea weevil
Family: Bruchidae
Description
size: 3 - 4.5 mm
shape: mote or less triangular
colour:
pale brown with blackish patches on the elytra
recognition: body clothed in
short hairs; last abdominal segment visible; antennae slightly serrated: each
hind femur bears a tooth; large emarginated eyes
Distribution: throughout the tropics and subtropics
Life history
range of temperature: 18 - 35°C
optimal temperature:
30°C
range of rel. humidity: 25 - 90%
optimal rel. humidity: 80%
eggs laid: up to 100 glued to surface of pod or seed life cycle: 23 days at
optimal conditions
Damage
Larvae, which develop within the seed, feed as primary pests on cowpeas, pigeon peas, lentils and other pulses. Infestation begins in the field.
Similar species
C. maculatus (originated in Africa, now
distributed throughout the tropics and subtropics)
Caryedon serratus
(Groundnut seed beetle, size 4 - 7 mm)
Figure 97
Acanthoscelides obtectus
Common name: Dried bean weevil
Family:
Bruchidae
Description
size: 3 - 5 mm
shape: oval
colour: grey and reddish
brown with yellowish and dark brown patches of hairs on the elytra recognition
hind femur with one large tooth and two small teeth; elytra do not completely
cover the abdomen; antennae serrated
Distribution: cosmopolitan
Life history
range of temperature: 17 - 35°C
optimal temperature:
30°C
range of rel. humidity: 30 - 90%
optimal rel. humidity: 70%
eggs laid: 40 - 50 laid on ripening pods or among stored seeds life cycle:
21 days at optimum conditions
Damage
Larvae are primary pests of common beans. Infestation may begin in the field.
Similar species
There are various other species of Bruchidae attacking pulses, which cannot easily he identified
Figure 98
Ephestia cautella
Common name: Tropical warehouse moth
Family:
Pyralidae
|
Description |
adult |
larva |
|
size: |
15 - 20 mm (wing span) |
15 - 20 mm |
|
colour: |
grey; fore wing greyish-brown with an indistinct pattern |
white, sometimes pinkish or greyish |
|
recognition: |
|
setae (hairs) arising from dark brown pigmented spots |
Distribution: throughout the tropics; less common in arid areas
Life history
range of temperature: 10 - 33°C
optimal temperature:
30°C
range of rel. humidity: min. near 0%
optimal rel. humidity: 40
- 75%
eggs laid: 200 - 500 life cycle: 30 days at optimum conditions
Damage
Larvae are found as primary pests in a wide range of commodities, especially cereal flours and other milled products, but also in whole grains, mainly feeding on the germ. Webbing and frass produced in infested products are nuisance factors.
Similar species
E. kuehniella (15 - 25 mm wing span, mainly in countries with temperate climate)
Figure 99
Plodia interpunctella
Common name:
lndian-meal moth
Family: Pyralidae
|
Description |
adult |
larva |
|
size: |
14 - 20 mm (wing span) |
up to 17 mm |
|
colour: |
basal third of the fore wing cream coloured, rest of the wing copper with dark grey markings |
yellowish-white, sometimes reddish or greenish |
|
recognition: |
|
base of setae without pigmented spots |
Distribution: cosmopolitan
Life history
range of temperature: 16 - 36°C
optimal temperature: 28
- 32°C
range of rel. humidity: 30 - 90%
optimal rel. humidity: 75%
eggs laid: 60 - 400 life cycle: 27 days at 30°C and 70% r.h.
52 days
at 20°C and 70% r.h.
Damage
Larvae are primary pests of cereal grain and flour, groundnuts and dried fruit. Webbing and frass produced m the infested commodities are nuisance factors.
Figure 100
Corcyra cephalonica
Common name: Rice moth
Family: Pyralidae
|
Description |
adult |
larva |
|
size: |
15 - 25 m (wingspan) |
15 mm |
|
colour: |
fore wings mid-brown; uniformly coloured |
yellowish-white |
|
recognition: |
|
spiracles thickened on their posterior rims: |
Distribution
Throughout the humid tropics
Life history
range of temperature: min. 18°C
optimal temperature:
30- 32°C
range of rel. humidity: min. 20%
optimal rel humidity: 70%
life cycle: 26 - 27 days at optimum conditions
Damage
Larvae are primary pests of cereal grain and flour, nuts, groundnuts, dried fruit, cocoa, copra and many other commodities. The dense white cocoons of the pupae, which are very tough are often seen attached to the bag surfaces. Infestation is characterized by aggregations of kernels, frass, cocoons and dint caused by webbing
Figure 101
Figure 102
Sitotroga cerealella
Common name: Angoumois grain moth
Family:
Gelechiidae
Description
|
size: |
10 - 18 mm (wing span) |
|
colour: |
fore wings buff often with a small black spot in the distal half, hind wings greyish |
|
recognition: |
hind wings with a long fringe of hairs, sharply pointed at the tip |
Distribution: cosmopolitan
Life history
range of temperature: 16 - 35°C
optimal temperature: 26
- 30°C
range of rel. humidity: 20 - 80%
optimal rel. humidity: 75%
eggs laid: up to 200 life cycle: 28 days at 30°C and 80% r.h.
Damage
Larvae are primary pests of whole cereal grains as paddy, sorghum, maize and wheat. Larval development takes place inside the grain. Damage is very similar to that caused by weevils.
Figure
103
DELOBEL., A. & M. TRAN (1993)
Les Coléoptères des
denrées entroposées dans les regions chaudes. CTA/ORSTOM, Paris, 424 pp.
DOBIE, P., C.P. HAINES, R.J. HODGES & P.F. PREVETT (1991)
Insects and Arachnids of Tropical Stored Products Their Biology and
Identification, TDRI, Slough, 273 pp.
FREEMAN, P., ed. (1980)
Common insect Pests of Stored Food
Products, BMNH, London, 69 pp.
SCOTTI, G. (1978)
Les insectes et les acariens des céréales
stockées, AFNOR/ITCF, Paris, 238 pp.
WEIDNER, H. & G. RACK (1984)
Tables de determination des
principaux ravageurs des denrées entreposées dans les pays chauds, GTZ,
Eschborn, 148
pp.
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