Pest:Pseudostem Weevil

Pseudostem weevil is a monophagous pest of banana and plantain, cosmopolitan to Asia.

The adult beetle is reddish brown to black in colour measuring 17 x 6 mm (Fig 109). The life cycle occupies 3-5days for eggs,20-26 for grubs and 20-25 for pupae during summer and slightly longer for winter. Adult weevil keeps their eggs in air chamber through slits cut on leaf sheath. Mating occurs in the mating spaces especially encarved by adults while feeding between leaf sheaths. Frequent mating is observed. Preovipositin period is 15-30 days. Adults mate throughout the day and night. Female makes small slit with rostrum on the outer epidermal layer of leaf sheath down up to air chamber and insert ovipositor to lay a single egg –one egg in one air chamber.There are 5 larval instars . Full grown grub measures 16-20 mm.

Immediately after emergence the neonate larvae start feeding on the tissues around the air chamber of leaf sheaths, while the feeding larvae move across horizontal or in a slight oblique direction and bores in to inner leaf sheath. Third, fourth and fifth instars are voracious feeders and riddle the pseudostem by cutting holes. Sometimes the grubs also make holes on the outer surface, for better aeration of the tunnels and thus cause heavy damage.Pupation takes place inside adjoining leaf sheath of pseudostems in a cocoon formed by plant fibres. The cocoon is formed by winding short pieces of fibrous materials of sheath around the body.

In Kerala total life cycle of the weevil takes about 42 days and adult longevity is 90 -120 days.

Click on the image to enlarge

  • Pest 1
  • Pest 2
  • Pest 3
  • Pest 4

Symptoms

  • This weevil, resembling the rhizome weevil of banana, has become a serious pest in recent years.
  • Adult female weevil (Image 1) punctures and inserts eggs into the pseudostem. Grubs emerging out feed extensively on the pseudostem(Image2) and thereby the entire plant collapse. Colourless jelly like plant sap oozes out(Image3) of from minute boreholes on the pseudostem or leaf bases.
  • This is the first symptom. The ooze later turns deep violet in colour. Chewed up material will also be pushed out through the holes .
  • Leaves turn yellow, dry up and finally, the plant collapses.(Image4)

Procedure for Observation

Randomly select 20 pants from 10 cent plot. Look for symptom of damage and record the number of infested plants.

ETL

One infested plant.

Control Measures

  • Keep the garden clean by properly disposing pseudostem and rhizome after harvest as this is the most important factor in the prophylactic and curative control of the pest.
  • During cropping, periodically remove the dried leaves and leaf sheath.
  • Remove side suckers up to 8 months to prevent harbouring of adult weevils.
  • Remove the affected plants beyond cure along with the rhizome and destroy them by burning the life stages using kerosene or by burying the same in deep pits
  • Fill the leaf axils with neem seed kernel powder @ 50 g per plant when the plant attains 135 days after planting.
  • Beauvria bassiana and Metarrhizium anisopliae have shown efficacy as control agent of the pest. Spray either of this at 20 g per lit of water or 5 ml /l covering the leaf axil and pseudostem at 5 months after planting.
  • Cowdung ash slurry can be brushed on the pseudostem
  • Prepare traps using pseudostem of approximately 50.cm long, which are split lengthwise and treated with B.bassiana . Keep about 100 traps per ha.
  • The disc on stump trap swabbed with B.bassiana can also be used to monitor and reduce weevil population. This is more effective due to higher exudation of plant fluid.
  • In endemic areas use any of the recommended insecticide viz quinalphos 0.05 %,chlorpyriphos 0.05%,or carbaryl 0.2 %. The spry fluid should reach the leaf axil ,rhizome, surrounding soil and all round the entire pseudostem inserting the nozzle through bore holes made by the larvae, if any, and also within the outer sheath raising the same at different spots. Before spraying remove the dry outer sheath of the pseudostem of all infested and uninfested plants. Repeat the treatments after 3 weeks if infestation persists.
  • Nanma and Menma (a tapioca leaf based organic pesticide) developed by CTCRI have been found to be very effective in managing the pest. Spray Nanma 5 % (50 ml/l) covering leaf axil and pseudostem when the plant attains 4-5 months after planting. Nearly 100 ml of spray fluid will be required per plant. Before spraying clean the pseudostem of the dried and drooping leaves. Once the infestation is noticed inject undiluted Menma just below the bore hole made by the weevil. Injections are to be made at 3 locations around the pseudostem near the infestation point @ 5 ml per injection.