Attack of aphids in banana plants is one of the most common insect invasive damages found in nearly all the banana-growing regions of the world. It is highly dangerous and damages almost all parts of the banana plants including leaves, fleshy stems and fruits as well. The plant experiences death only in the rarest cases but the invasion of the colony of aphids can severely decrease the yield of the plants when susceptible varieties of the plants are used and the weather is also favorable for rapid multiplication of the insects. The aphids generally attack older plants but can also attack plants from all ages in some cases. It is also a very alarming problem for lettuce, ginger and apple plants.
Description of the insect responsible for the disease –
Scientific name – Pentalonia nigronervosa
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 Aphids tend to live where they eat, and what they eat depends on the species of the aphid. They can be found on most fruit and vegetable crops, mostly on the banana species and on some flowering plants like roses or chrysanthemums, on trees, and in some bushes too. Also, it can already be present in the soil from the previous batch of harvested crops or it can arrive with the newly purchased suckers or seedlings. The invasion 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 aphids are very small insects and are thus hard to locate individuals. But, when present in colonies, they can be located and identified easily. The colonies should be searched for near the midrib on the underside of the leaves, or under the sheath that wraps around the stem, or on the fruit. Brown-black insects with prominent black wing veins are the confirmatory identifying traits of aphids. However, excessive and abnormal invasion of ants can also indicate the presence of aphids on the plant since the ants are attracted to the aphids’ honeydew.. 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.
Stages of growth susceptible to infection –
The aphids 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 aphids damage the plants mainly in three ways -
- They rob the plants off their nutrients and water by feeding on their sap and thus damaging the vascular tissues of the plants.
- The aphids produce a specific type of honeydew that attracts white ants which damage the roots and a species of fungus as well known as sooty moulds. This dual attack of ants and the fungal infection further degrades the health of the plants.
- The aphids act as vectors of many deadly viruses and bacteria. While feeding on the banana plants, they spread those pathogens into the plants. In this case, the aphids are the vectors of the deadly banana bunchy top virus.
Damages caused by Aphid attack-
Invasion of aphids in banana plants is one of the most deadly and fearful attacks that cause irreparable damage for the farmers. When the fruits 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 infection once it is initiated. Affected leaves and stems are also responsible for less yield from the plants. Sometimes, large colonies of aphids can kill banana suckers, but this is a rare phenomenon. Large colonies may sometimes mark the fruits, but this too is uncommon. Sooty mould fungi growing on aphid honeydew very slightly damages the plants. However, there are reports of moulds at the base of leaves causing leaf rots.
The most deadly and major damage is caused by the spread of Banana bunchy top virus (BBTV). In some parts of the world, the disease caused by this virus has completely stopped commercial banana production.
Life cycle of the aphids -
In the Pentalonia species, the occurrence of male individuals is absent. As a result, propagation occurs asexually without fertilization and is termed as “parthenogenesis”. Eggs are also unknown in this particular species and the adult females directly give birth to the young ones known as viviparity.
Two to four young ones take birth every day. The nymphs (young insects with no difference in the form with the adults) go through four moultings. The adult species are up to 1.5 mm long, shiny red-brown or almost black, usually wingless. Winged adults appear after 7-10 generations of continuous reproduction. The wings have characteristic dark veins. Aphids are not strong flyers but can travel long distances with the flow of winds. They are most abundant in the warm and humid (high moisture content in the air) season.
The nymphs are more efficient at spreading the virus. But, the winged adults start new infections. Pentalonia has long mouthparts called stylets that make them able to pierce plants to suck sap from the phloem (part of the vascular bundle carrying food from leaves to other parts of plants). Aphids pick up the virus after feeding on infected bananas for about 18 hours, and keep it for the rest of their lives (15-20 days).
Prevention & Control –
The attack of aphids is deadly, but it can be prevented by means of certain steps such as –
1. Use of resistant varieties or cultivars
Nowadays with the gradual advancement of science, there are a number of genetically engineered disease resistant banana cultivars available in the market. Biotechnology has been able to incorporate disease resistant genes into some improved varieties of banana. However, resistant varieties are not immune to aphids, but are immune to the infection caused by banana bunchy top virus (BBTV). A few common varieties with BBTV resistance are Gros Michel, Veimama, etc.
2. Use of cultural practices
Cultural practices are more important than all the other methods for preventing this disease. They include -
- Use of pathogen-free seeds and collection of seeds from disease-free plants.
- The practice of crop rotation.
- Controlling susceptible weeds and keeping the fields completely weed-free. Since the weed patches serve as the breeding ground or host for the aphids.
- Maintenance of proper fertilization for optimum plant growth. Overfertilization and under fertilization both are harmful.
- Other alternative hosts for aphid population should be checked and eliminated accordingly.
- Drip irrigation should be used instead of traditional irrigation for keeping the foliage dry.
- Regular pruning of the banana mats should be carried out and the unwanted suckers should be got rid off since the aphids prefer to feed on young and delicate suckers.
3. Chemical means of control –
The aphids on bananas should be exterminated as soon as symptoms are seen. This is the most important task that is to be carried out for gaining proper yields. The old leaf sheaths from around the base of the plants should be stripped away and the aphids on the plants should be killed by spraying the stem thoroughly with insecticide, paying special attention to the "throat" and funnel leaves. The methodology of the spray on the suckers should be as follows -
- Derris or Pyrethrum should be used.
- Any types of soap or oil can be used. (white or horticultural)
- A synthetic pyrethroid should be used.
- Kerosene or a mixture of diesel and mineral oil should be used.
After 1 week, the infected plants should be sprayed with herbicide (glyphosate). The dead plants therefore should be removed and burned eventually.
4. Biological means of control (Use of natural enemies) –
Indigenous lacewings and syrphid fly larvae (the adults are hoverflies) can attack aphid colonies and prevent them from further spreading the damage.