Attack of skippers in banana plants is one of the most common insect affected damages found in South-East Asia, Oceania, Hawaii and Papua New Guinea, which are some of the most important banana growing regions of the world. It is highly dangerous and gravely damages almost all the leaves present in that particular tree. The plant experiences death only in the rarest of cases but the invasion of colony of skippers can severely decrease the yield of the plants by destroying the leaves when susceptible varieties of the plants are used and the weather is also favourable for rapid multiplication of the insects. They generally attack fully matured plants but can also attack plants from all ages in some cases. It is also a very alarming problem for hemp, bamboo, coconut and palm plants.
Description of the insect responsible for the disease –
Scientific name – Erionota thrax
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 banana skipper, also known as the palm redeye is a butterfly belonging to the family Hesperiidae. It originated from India and travelled through south-eastern Asia to Papua New Guinea. In the north, it is found up to southern China. They can be found on most fruit and vegetable crops, mostly on the banana species and on some fruit trees like palms 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 skippers are butterfly-like insects and thus the adult insects can be spotted easily. But, it is very hard to locate the larvae and the eggs of the skippers individually. Therefore, one should search for the distinctive leaf rolls, which are upto 15 cm long, made exclusively by the skipper caterpillars. After careful inspection, the caterpillars in the rolls can also be seen with shiny blackheads and covered in wax. They should be searched for near the midrib on the underside of the leaves, or under the sheath that wraps around the stem. Wings are chocolate-brown in colour; the anterior wings have three discal pale ochraceous spots, situated one on the crossing cell, another beneath the cell and between the two lower median nerves, and the third and smallest is in between the first and second median nerves. The posterior wings are greyish-ochraceous. Body and legs are more or less concolourous with wings.
Stages of growth susceptible to infection –
The skippers 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 growing stage. But most commonly, they appear on the mature green leaves of the plants in the fully grown stage.
The only symptom available for the confirmation of a skipper attack in the banana plants is untimely shredding of the young and green leaves and vicious rolling of leaves measuring as long as 15 cm in length. The caterpillars of the skippers do this damage by shredding the leaves and making the rolls of the leaf blade in order to hide and feed safely inside them. For this reason, they are also termed as banana leaf roller.
Damages caused by Skipper attack-
Invasion of skippers in banana plants is one of the most deadly and fearful attacks that cause irreparable damage for the farmers. When the leaves are affected, the yield becomes highly limited due to an overall decreased rate of photosynthesis and respiration. Thus, the income generated for 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. Sometimes, large colonies of skipper caterpillars can kill banana suckers, but this is quite a rare phenomenon. Damage of up to 60% of the leaf area of banana plants has been recorded during massive outbreaks. If the outbreak comes at the time of fruiting, yields are lower due to delayed fruit maturity and the bunch size is greatly reduced. Pruning experiments have concluded that a 50% defoliation at fruiting stage can result in nearly 30% loss in fruit weight and overall yield.
In an outbreak in Papua New Guinea in the late 1980s, 70% defoliation of banana plants was recorded in some parts of the country, with up to six rolls per leaf. In some extremely rare cases, loss of leaves was absolute and not a single leaf lived.
The life cycle of the skippers -
The eggs laid by the adult insects are 2 mm in diameter and are laid singly or in groups of up to 25, mostly on the underside of the banana leaves. The eggs hatch after a period of 5-8 days. The newly hatched larvae, the caterpillars, which are pale green with a distinctive shiny blackhead, make their way to the edge of the leaf to feed and to make the characteristic rolls off the leaves. If a caterpillar mistakenly rolls the leaf near to the roll of another caterpillar, it abandons the roll, and starts again on another part of the same leaf or makes way to another leaf. In a span of 30 days, five larval stages are attained. All except the first stage is covered in a white waxy powder; this and the fact that older caterpillars close their rolls more securely than younger ones, allows them to survive heavy rains. Caterpillars grow upto a length of 6 cm.
The act of formation of pupa occurs inside the leaf roll and takes about 10 days to complete. The pupae, too, are covered in a protective white wax. The banana skipper butterfly spreads as eggs on the leaves of banana suckers used as planting material, and, possibly, longer distances through rivers, air and so on.
Prevention & Control –
The attack of skippers 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 -
- Use of egg-free seeds and collection of seeds from disease-free plants, if propagated sexually.
- The practise 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 skippers.
- Maintenance of proper fertilization for optimum plant growth. Over fertilization and under fertilization both are harmful.
- Other alternative hosts for the skipper population should be checked and eliminated accordingly.
- Drip irrigation should be used instead of traditional irrigation for keeping the foliage dry. Since damp and moist surroundings attract the insects more.
- The leaves containing rolls as well as the caterpillars and the eggs are handpicked carefully and destroyed immediately for preventing further spread of the disease.
- The eggs and caterpillars are collected and placed in screened boxes close to banana plants with rolled leaves. The screen allows the parasites to escape.
2. Chemical means of control –
If pesticides are necessary, botanical (plant-derived pesticides) sprays should be used first, as these may cause less harm to the natural enemies, and also are less expensive than synthetic commercial products.
- Neem extracts, derris, pyrethrum or chilli extracts can be used. If these are used, soap should be added to help the chemicals reach the caterpillars within the rolled leaves.
- Alternatively, commercial biopesticides like spinosad (product name - Success) or Bt - Bacillus thuringiensis var. kurstaki.
- While using Bacillus, the following should be noted:
- Caterpillars will only die if they eat the Bt crystal proteins. So, it should be ensured that Bt covers all portions of the leaves.
- Eggs are not susceptible to Bt. So, if the plant is affected by eggs of the skippers, using Bt will be futile.
- Bt should be used as soon as damage is seen.
- Small larvae are more susceptible to Bt than the fully grown ones.
Bt works by entering the guts of the insects or the larvae and getting activated by the alkaline pH of the gut and thus resulting in releasing proteins which makes holes or bores on the gut wall.
- Synthetic pyrethroids are likely to be effective, but will also kill natural enemies of the insects thus resulting in decreasing the efficiency of biological control of the pest..
3. Biological means of control (Use of natural enemies) –
There are several parasitic wasps and flies that attack the banana skipper. The egg parasite, Ooencyrtus erionotae, and the larval parasite Apanteles erionotae are found effective for controlling skippers. Heavy rains also bring about death of the young caterpillars, and wind reduces populations by shredding the banana leaves so that the caterpillars cannot make their protective rolls.