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Stored Grains Insect Pest

1) Grain weevils 

Grain weevils are a very important and serious pest of stored grains. Some grain weevils are; Rice weevils, Sitophilus oryzae; Coleoptera: Curculionidae 

Maize weevils, S. zeamais; Coleoptera: Curculionidae 

Granary weevils, S. granaries; Coleoptera: Curculionidae 

Sitophilus oryzae is an elongated dark brown weevil having four reddish brown patches on its  elytra. It has a circular puncture on its prothorax. S. granarium has oval shaped punctures on its  prothorax. S. zeamais is a small brown weevil darker than rice weevil. 

Damage: Eggs are laid anywhere on grains. Immediately after hatching, grub feeds by  burrowing the grain. Developing grubs live inside the grain and feed by hollowing the grain.  Both grubs and adults cause serious damage to grain by feeding and contaminating grain lots. The  level of damage on grains caused by these weevils depends on grain moisture content,  temperature and duration of storage. 

Life cycle 

Adults live 2-3 months. 

Larvae generally not seen – they feed and develop inside single grains. Life cycle completed in four weeks at 30°C, 15 weeks at 18°C, breeding stops below 15°C. 

Eggs 4-6 days Larva 3-4 weeks Pupa 5-12 days Adult

Management 

  1. Maintain storage hygiene, store grain in airy shady and dry rooms. 
  2. Reduce grain moisture content i.e. <12%. 
  3. Harvest completely mature grains. 
  4. Ensure timely harvesting followed by sun drying before storage. 
  5. Admix Neem seed powder @10gm / kg of grains thoroughly with grains before storage. 6. Apply chemical fumigants such as Aluminium Phosphide (trade name Phosphos) in between  the piles of grain sac under an air tight room @ 20 tablets / 30 m3
  6. Disinfect the store room and container by 0.005% solution of Malathion 50% EC. 8. Use moisture, rodent and insect proof bins made from galvanized iron sheets. 

2) Angoumois Grain Moth, Sitotroga cerealella; Lepidoptera: Gelechiidae 

A pest of whole cereal grains which only infests surface layers of bulk-stored grains. Infestation  of standing maize crops before harvest is quite common, occasionally in other cereal crops. 

Key features 

Silvery grey to grey brown wings which taper to a point. 

Wings have a long fringe of fine hairs along the posterior edge. 

Adults (5 – 7mm long) are unable to penetrate grain, therefore only infest surface layers of  bulk grain. 

This pest does not create webbing. 

Life cycle 

Adult moths do not feed but lay 150 – 300 eggs on or near the grain surface. Life cycle takes around 5 – 7 weeks in warm conditions. 

Eggs 4-6 days Larva 2-3 weeks Pupa 5-7 days Adult 

Damage: Larva of Angoumois moth causes damage by feeding on and hollowing the grains.  Larval development takes inside the grain. Hole on the grain gets partly filled with pellets of excreta.  Infested grains give out an unpleasant smell and dirty appearance. Larvae burrow into a single  grain and feed and develop until the adult moth emerges in 10 – 14 days through a visible hole.  Take regular monthly samples and look for moths near the grain surface. When adults emerge, pupal  cases are often found protruding from grain. 

Management  

  1. Maintain storage hygiene, store grain in airy shady and dry rooms. 
  2. Reduce grain moisture content i.e. <12%. 
  3. Harvest completely mature grains. 
  4. Ensure timely harvesting followed by sun drying before storage. 
  5. Admix Neem seed powder @10gm / kg of grains thoroughly with grains before storage.
  6. Apply chemical fumigants such as Aluminium Phosphide (trade name Phosphos) in between  the piles of grain sac under an air tight room @ 20 tablets / 30 m3
  7. Disinfect the store room and container by 0.005% solution of Malathion 50% EC. 16. Use moisture, rodent and insect proof bins made from galvanized iron sheets. 

3) Bruchids: Cowpea weevils or Pulse Beetle, Callosobruchus chinensis;  Coleoptera: Bruchidae 

Callosobruchus spp are pests of most pulse crops, including mung beans, cowpeas, field peas,  chickpeas, soybeans and lentils. 

Key features 

Adults (up to 4mm long)emerge through perfectly round holes in the seed. Globular, tear-shaped body is reddish brown with black and grey markings. Wing covers (elytra) do not fully cover the abdomen. 

Adults have long antennae, climb vertical surfaces (glass jar) and are strong flyers. 

Life cycle: Adults do not feed, but lay 100 white eggs clearly visible on the outside of seed.  Adult lifespan is 10 –12 days. Unlike most storage pests, adults may also lay eggs on mature  seed pods in a standing crop. 

Eggs 4-6 days Larva 2-3 weeks Pupa 5-6 days Adult 

Damage: Larvae feed and develop within individual seeds and emerge as adults leaving a neat  round hole. The appearance of eggs cemented to the surface of the pulses is the early stage  symptom of pulse infestation by bruchids. 

Management 

  1. Cultural control can be effective, growing vulnerable crops at least half a mile (pulse crop)  away from farm stores. 
  2. Admix sweet flag powder @ 5 gm /kg of grain before storage. 
  3. Use of chemical fumigants such as aluminium phosphide in between piles of grain sacs under a partial air tight room. 
  4. Surface treatment; disinfect the store room and container by 0.05% solution of malathion  50%EC. 
  5. Use domestic metals beans which have moisture, rodents and insect proofs. 

4) Rust-Red Flour Beetle, Tribolium castaneum; Coleoptera: Tenebrionidae 

Commonly found in stored cereal grain, processed grain products, oilseeds, nuts and dried fruit. 

Key features 

Adult beetles (3 – 4.5mm long) bright reddish-brown in colour when young and a darker  brown when older.

Three larger segments on end of antennae. 

Similar species: Tribolium confusum – confused flour beetle, more common in cool,  temperate regions. 

Life cycle 

Life cycle completed in 4 weeks at 30°C, 11 weeks at 22°C and reproduction stops below  20°C. 

Adults live from 200 days to 2 years and fly in warm conditions. 

Up to 1000 eggs per female loosely scattered throughout the commodity. Eggs 7- 12 days Larva 21-90 days Pupa 5-6 days Adult 

Damage: Cream coloured larvae feed externally on damaged grain. Beetles infest whole grain,  but breed more successfully on processed products (i.e. flour). Beetles move quickly and are  strong flyers. Infested flour turns grayish, moldy and possesses pungent disagreeable odour making  it unfit for human consumption. Prefered habitat for flour beetles is around storage areas with  poor hygiene, broken grain, gradings or bulk cottonseed. 

Management (same as Grain weevil) 

5) Lesser Grain Borer, Rhyzopertha dominica; Coleoptera: Bostrichidae 

A serious pest of most stored grains: The Lesser Grain Borer has developed resistance to a  number of grain insecticides. 

Key features 

Dark brown cylindrical shaped beetle (up to 3mm long) with club-like antennae. Viewed from the side the beetle‟s mouth parts and eyes are tucked underneath the thorax  (chest) 

Adult beetles are strong flyers. 

Life cycle 

Life cycle completed in four weeks at 35°C and seven weeks at 22°C. Breeding stops below  18°C. 

Females lay between 200 – 400 eggs on the grain surface. Young larvae (white with brown  heads) initially feed outside then bore into the grain. 

Adults live for 2 – 3 months. 

Eggs 7- 12 days Larva 21-90 days Pupa 5-6 days Adult 

Damage: Lesser grain borer is a serious pest of every stored grain. Their habit is to remain hidden  in grain. Both adult and grub causes damage to stored grains. Larvae feed from inside of grain  and hollow them while adults consume grains from the outer side of grain. Larvae eat the flour and  adults bore the grain and chew up. 

Management (same as Grain weevil)

6) Khapra beetle, Trogoderma granarium; Coleoptera: Dermestidae 

Attacks most stored grains. 

Larvae are covered in fine hairs. 

Looks identical to the warehouse beetle to the naked eye. 

Causes grain loss in storage. 

Larvae skins contaminate grain and cause allergies on consumption. 

Life cycle 

Eggs 7- 12 days Larva 21-90 days Pupa 5-6 days Adult Management (same as Grain weevil)

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