BANANA PESTS AND THEIR CONTROL
The banana weevil, cosmopolites sordidus and plant nematodes are the most destructive pests of bananas in Uganda. These pests may result in severe yield loss if uncontrolled.
Banana Weevil
Damage and Symptoms
- It results from larvae feeding and tunneling into banana corms and pseudostems. The adult weevil lays its eggs near the corm and on hatching, the larva attacks the underground part, boring tunnels in it.
- As weevil larvae grow in size, they make large tunnels 115 cm in diameter and may extend 60 to 100 cm up the pseudostem.
- The damage weakens the plant and interferes with uptake of nutrients and water.
- Weevil infestation of young plants causes stunting, disruption and delay of fruiting and sometimes leads to plant death.
- Heavily infested plants produce small bunches and have reduced resistance to drought and strong winds, leading to snapping and toppling of large or matured flowered plants.
- The banana weevil causes more damage to the cooking types than it does to the beer cultivars.
- Weevil damage may increase when plants of reduced vigor are attacked. Low vigor is as a result of plants being grown in soils of low moisture and fertility levels, weeds or intercrop competition for nutrients.
- The absence of banana weevil from higher elevations suggests that the weevil has a lower temperature threshold for larva development or adult survival.
CONTROL MEASURES
- Cultural methods
Manipulation of weevil habitat and oviposition site provides the first line of defense against the banana weevils.
It is cheap and doesn’t entail extra inputs.
Various cultural practices are used and they include
- Use of clean planting materials
This minimizes the spread of the weevils which are mainly carried to new sites with infested suckers.
Clean suckers may be obtained from non-infested fields by paring the corm to remove eggs and larvae or by use of tissue cultured plants.
- Good husbandry
It involves clean weeding, de-suckering, pruning, manuring and mulching which produce vigorous plants that are more tolerant to weevil damage.
- Destruction of post-harvest residues
Removal and splitting of harvesting stems into small strips and spreading them out to dry quickly reduces hiding and breeding sites for the weevil.
It also exposes weevil eggs and larvae to desiccation.
- Trapping
There are two types of traps including pseudo stem and disc-on-stump.
- Pseudo stem trap is made from pseudo stem pieces split longitudinally into halves and placed a banana plant with split surface on the ground.
- The disc-on-stump traps are made by cutting harvested stump 15 to 25 cm above ground level and then placing a pseudo stem sheath or banana leaves on top of the stump. The weevils attracted to these traps are collected and destroyed. Traps remain effective for about 1 or 2 weeks and are renewed whenever ample supply of pseudo stem pieces are available.
- Use of mixture of ash, urine and insecticidal plants
Traditionally, farmers have used ash in banana fields for soil nutrient enhancement and weevil control. Farmers are now using mixtures prepared by adding various levels of ash, urine, tobacco, capsicum, phytolacca and other weed species. The method and rates of application vary from farmer to farmer. But the most common one is use of a 14 day fermented mixture at 1 to 2 cups (500ml to 1000ml) per banana stool. While the practice is being recommended and encouraged, its field efficacy is still unclear and work is underway to validate the practice.
- Chemical control
At planting, a certain amount of insecticide such as Furadan, Pimicid, Mocap and Dursban should be systematically applied around the suckering of the planting hole. In the case of established plantations, the insecticides should be applied to the soil around the base of the banana stool.
The chemicals can also be applied with pseudo stem or disc-on-stump traps in mature banana plantations to kill weevils that get attracted to the traps.
It is important for a farmer to seek advice from the relevant services to assess the level of damage and whether it is economical to apply and which chemicals are to be used.
Monitoring of the weevils is required before chemical application (more than two weevils per trap might require chemical control). As is the case with herbicides, utmost care should be taken when using insecticides.
- Biological control
Research is ongoing to develop a biological control strategy for the banana weevil using entomophathogenic fungi. Beauveria bassiana seems more promising and is being tested to be inter-grated with other methods of weevil control.
Nematodes
Damage and symptoms
Nematodes are miniature worms which live in the soil and infest plant roots.
- During their process of feeding, nematodes tunnel through the root cortex leading to expansion of the lesions around.
- The lesions collapse into large necrotic patches reddish-purple in color, a symptom known as root necrosis.
- It results in premature root death or root fracture at points where necrosis girdles across.
- This necrosis interferes with the water and nutrient exchange passage.
- The most obvious symptom of nematode damage is the toppling over of the entire plant, particularly in fruiting ones.
- In general, damage to the banana root system results in stunted growth, premature leaf drop, reduced vigor, and delayed ratooning and increased susceptibility to water deficiency.
