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close this bookManual on the Prevention of Post-harvest Grain Losses (GTZ)
close this folder7. Important pests in storage
View the document(introduction...)
View the document7.1 Identification of pests
View the document7.2 Classification of storage pests
View the document7.3 Development of insects
View the document7.4 The effect changes in climate on development
View the document7.5 The use of various sources of food by pests
View the document7.6 Morphological features of insects
View the document7.7 Storage pest species
View the document7.8 Further literature

(introduction...)

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.

7.1 Identification of pests

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.

7.2 Classification of storage pests

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.

7.3 Development of insects

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

7.4 The effect changes in climate on development

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.

7.5 The use of various sources of food by pests

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

7.6 Morphological features of insects

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.

7.7 Storage pest species

- 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

Family: Bostrichidae

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

7.8 Further literature

DELOBEL., A. & M. TRAN (1993)
Les Colts des denr entropos 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 cales stock, AFNOR/ITCF, Paris, 238 pp.

WEIDNER, H. & G. RACK (1984)
Tables de determination des principaux ravageurs des denr entrepos dans les pays chauds, GTZ, Eschborn, 148 pp.