Pest:Rhizome Weevil

Banana weevil is known from all banana growing countries of the world.

The adult weevil is stout 10-16 mm long and dark brown in colour (Fig 106). It is hard shelled with a long curved snout. They are free living and commonly seen at the base of the plant or associated with crop residues. They feed on dead banana plants, newly cut stems and other decaying plant materials near the base of the plant. They may live for 1-2 years and can also live without food for more than 6 months but are very sensitive to desiccation and will die within 48 hours if kept in dry substrate. They are active at night .The adults are sluggish, not strong flyers .and only cover short distances. They commonly walk over the soil surface and vegetation. The beetles feign death when disturbed.

The female weevil lays its eggs in small burrows made at the ground level at the base of pseudostem in the upper part of the corm. It also lays eggs in roots and at the end of the cut stems (stumps). The eggs hatch in 5-8 days. The grub is apodous, creamy white with red head and with strong mouth parts. The full grown grubs are stout, about 15 mm long (Fig 107) . The grub becomes full grown within 25 days. Pupation takes place inside the pupal chamber constructed by full grown grubs near the surface of the corm. The pupal period lasts for 5-6 days

Click on the image to enlarge

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Symptoms

  • It prefers decaying banana corm material. Spent stems (cut or left standing), residual corms left after the stem has been cut, underground stubs of corm tissue left after de-suckering, uprooted suckers or stems, and any corm tissues that are large enough to dry slowly are good targets for this pest
  • Dispersal within a banana field occurs when adult weevils walk from plant to plant or when infested plants containing eggs and larvae are moved.
  • Injury is caused by grubs (larvae), which tunnel through the corms. Tunnels are circular in cross section, become wider as the grub grows and are filled with dark-coloured debris.(Image1)
  • Extensive feeding damage by grubs results in root destruction, slowed plant growth (Image2), reduced fruit production, and, sometimes, toppled plants.
  • The tunneling by the grubs makes the corms susceptible to invasion by secondary decay organisms. Production and growth of suckers reduced when parent plants are heavily damaged.
  • Affected sucker plants can be recognized by their dull, yellowish-green withered leaves . Relatively little damage is caused by adults(Image3) feeding on plant tissues.
  • The life cycle (egg to adult) of the weevil requires 30 to 40 days.
  • Eggs are deposited singly in cavities chewed by the adult females in the corm or pseudostem at ground level or between leaf sheath scars on the crown of the banana corm.
  • The stout, creamy white, legless grubs have reddish-brown heads and fleshy white bodies. The grubs are normally found in the pseudostem upto 2 feet above the ground.
  • Grubs pupate within chambers that are usually close to the surface of the corms.
  • The black, hard-shelled weevils are reported to live for as long as 2 years.
  • The nocturnal adults hide under debris or in the soil around banana plants during the day and are active at night.

Procedure for Observation

Select 5 plants from 10 cent plot at random. Assess the adult population at the base of the pseudostem.

ETL

10 beetles from 5 plants

Control Measures

  • Adopt strict field sanitation. Remove crop residues and keep the field clean.
  • Select only healthy planting materials free from weevil infestation.
  • Plough or dig the land deep so as to expose the inner soil layer to sun .
  • Cut and remove the outer layers of rhizome and sundry for 3-4 days after smearing it with cow dung slurry and ash.
  • Dip suckers in Beauveria bassiana @ 20 g /l for 30 min. before planting.
  • Prepare traps using pseudostem of approximately 50 cm long, which are split lengthwise and treated with B.bassiana . Keep about 100 traps per ha.
  • Apply 50 g of powdered neem seed kernel per plant in the baby pits in which suckers are planted.
  • Apply one kg of neem cake in two equal split doses at one and two month after planting.
  • Cosmolure (an aggregation pheromone) can be set at the rate of 4 traps per ha. The pheromone will last for 2-3 months. Pit fall trap is a metal basin of 15 cm diameter and 12 cm height filled with detergent water up to 3 cm level from the bottom. This is placed in such a way that the brim of the basin is at the ground level and the remaining part inside a pit. A small wooden stand is kept at the middle of the basin on which cosmolure is suspended. The trap should be placed on a 20 x 100 m grid throughout the area. After a month the trap is moved to 20 m thereby the whole area will be covered with in a cropping season at 4 traps per ha.