The black banana weevil (or the banana root borer) is one of the most serious insect pests of bananas. This beetle is found throughout the banana growing regions of the world. Corms or rhizomes are used for propagation of the banana plants and eggs and larvae are easily transported therein. The infected plant parts when used for commercial cultivation, result in drastic decrease in the yield. Today it is present in southern Asia, Africa, many Pacific islands, Australia, northern South America, Central America and the West Indies.
Description of the insect responsible for the disease –
Scientific name – Cosmopolites sordidus
Phylum - Arthropoda
Kingdom – Animalia
It is an insect belonging to the largest phylum of the animal kingdom, namely Arthropoda which is responsible for this deadly disease. The adult insects have well-developed wings, but can rarely fly. One of the most important features of the weevils is their excellent capability of camouflage. They pretend dead whenever they sense disturbance or arrival of predators. This activity saves them from many potential threats. During the day time, they hide under plant residues or weeds or in the soil specifically around banana plants. But, at night they walk short distances over soil and vegetation in search of food. Spreading is done mostly by means of infested planting materials. The weevils live up to the age of 2 years maximum. The invasion of this pest can occur at any time of the year, but damp and humid conditions of the rainy season typically accelerate the process of multiplication of the insects.
Detection and Inspection of the Insects –
The weevils are very small insects and are thus hard to locate as individuals. But, when present in colonies, they can be located and identified easily. The larvae and their tunnels should be searched for in the corms just above and below ground level. Colonies may even occur below the soil also. Careful and gradual inspection of the plants is the only alternative available for the detection of the insects.
10 cm thick discs should be cut from freshly harvested banana stems and within a plantation, 50 discs should be kept in one hectare land. Each disc should be placed on the soil at the base of the banana stool and should be covered with leaves to prevent drying out of the discs.After 5 days, the number of weevils on the discs should be calculated. More than 4 weevils in a disc indicate that the plantation requires treatment.
Stages of growth susceptible to infection –
The weevils in bananas can appear in all stages of the life cycle namely flowering stage, fruiting stage, post-harvest stage, pre-emergence stage, seedling stage and vegetative growth stage.
The weevils damage the plants mainly by -
The grubs or larvae do the primary damage. They bore into the corm below ground to feed. When there are many weevils, tunnels occur throughout the corm, and into the pseudostem. As a result, deep bores or holes can be found when examined carefully. Infestation by the banana weevil begins at the base of the outermost leaf-sheath and in injured tissues at the lower part of the pseudostem. The larvae are able to penetrate to adjacent inner leaf-sheaths and then they bore into the pseudostem base and corm, but also into the base of suckers and into the roots. Larval tunnels may run for the entire length of fallen pseudostems and can be clearly observed by means of naked eyes. Infested plants have dull yellow green and unhealthy foliage. Young infested suckers often wither and fail to develop.
Damages caused by weevil attack-
Invasion of weevils in banana plants is one of the most deadly and fearful attacks that cause irreparable damage for the farmers. When the stems are affected, the yield becomes highly limited and thus the income generated for the farmers decreases drastically. Also, the spreading capability and the capacity of forming colonies of this insect is very high which results in difficulty in controlling the spread of the pest once the attack is initiated. Affected stems are responsible for less yield from the plants since the food and water supply through the vascular tissue system (xylem and phloem) is disrupted. Sometimes, large colonies of weevils can even kill banana plants, but this is a rare phenomenon.
Some of the additional deadly damages include -
- Appearance of rots in the tunnels formed by the larvae and thus the flow of nutrients to the leaves is stopped so they appear unhealthy and die in a premature condition.
- They feed on the plants from inside and thus make the plants extremely weak which result in easy uprooting of the plants even in strong winds.
- The suckers associated with the plants begin to wither and start dying.
- The fruits become gravely undersized and the size of the bunch is also greatly reduced.
Life cycle of the weevils -
Female weevils lay 1-3 eggs per week. The eggs are oval in shape and are about 2 mm in length. They are laid individually in pits chewed between the leaf scars on the corms; near or just above the ground level. They are also laid on plants blown over in the wind, where the corm and leaves meet, and on the particular side facing the soil. The eggs are very hard to find as they become covered in plant sap and are heavily camouflaged. Eggs hatch in 5-7 days, and the stout, legless grubs/larvae that emerge out are milky-white with reddish-brown heads. After 20 days of tunnelling and eating through the corms, they are fully-grown. They then make oval chambers near the corm surface and start the process of pupation. After completion of eight days, on the ninth day, adults emerge. They are reddish brown at first, black at the later stage and are about 12 mm long, with a typical weevil snout. They feed on plant tissues or crop residues, and the damage done is slight.
The life cycle of weevils completes in almost 50 days.
Prevention & Control –
The attack of weevils is deadly, but it can be prevented by means of certain steps such as –
1. Use of cultural practices
Cultural practices are more important than all the other methods for preventing this disease. They include -
2. Chemical means of control –
Chemicals like chlorpyrifos, fipronil, bifenthrin and imidacloprid can be used for effective control of the weevil grubs. An aggregation pheromone (sordidin) that attracts both sexes can be used for monitoring (4 traps/ha) and mass trapping (20 traps/ha). And afterwards, they can be captured and killed. Using neem oils (Azadirachta indica) also proved to be successful for controlling the pest.
3. Biological means of control (Use of natural enemies) –
Predatory ants such as the big-headed ant (Pheidole megacephala) and Tetramorium spp. are important predators of the banana weevil. Although these ants feed on a wide variety of food materials such as nectar, sugar, honeydew and other insect substances with high fat content, high populations in banana stands make them very efficient. They will even enter crop residues and living plants in search of weevil eggs, grubs and pupae.
Some fungi like Beauveria bassiana and Metarhizium anisopliae have shown efficacy as control agents of this pest. Some of them caused weevil mortality of over 90%.
Some nematodes like Steinerma and Heterorhabditis spp. attack both adults and grubs in the field, but extremely high cost and their limited efficiency limit their use on a large scale.Some beetles like Plaesius javanus are also found to be extremely efficient predators for weevils.