Erionota torus: Range extension and extent of crop damage in Western Ghats, India

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Erionota torus (Hesperiidae: Lepidoptera) : Range extension and extent of crop damage in Western Ghats, India

ABSTRACT:

Erionota torus Evans (1942); was described originally from Sikkim, India in the year 1942. The species occurs from Sikkim till Taiwan and the Malay peninsula. E. Torus wasn’t reported in India since 1956. It was discovered in Ernakulam, Kerala in 2013. It is a known pest species which feeds on the commercially important Musa paradisiaca. Field surveys indicated that E.torus is distributed all across Kerala and is spreading northwards. About 3 % of total plant damage in 2013 in Kerala and 29% of the total plant damage in Kodagu was infested by E. Torus. In this paper, we give a detailed account of its range extension, damage and the potential of its sister taxa to evolve into serious pests.

INTRODUCTION:

Insects are the most abundant group of organisms in the world with approximately 1 million species described as of now. They exhibit great diversity in terms of morphology, habitat , reproduction and nutrition. (1) Phytophagous insects identify plants based on chemical signals from the plants. The plants in turn and hence can potentially become pests when they feed on commercially important plants. There are many insects which have become destructive worldwide and cause substantial damage to economy and agriculture. For instance, the desert locust (Schistocerca gregaria) a well studied pest species which occurs from Northern Africa to Western India and causes billions of dollars worth of agricultural damage every year. They feed on almost all the aerial parts of the plants including stem, Similarly, the Khapra beetle; which originated from India is one of the world’s deadliest pest which feeds on dried crops.

Lepidoptera is one of the most important orders of insects which are known to cause substantial agricultural damage. The larvae of Lepidoptera are phytophagous in nature and hence can become potential pests. Helicoverpa armigera (Noctuidae : Lepidoptera) ,6 a cosmopolitan species, is a pest on many commercially important crops. Annual damage amounts to more than $5 billion worldwide.

There are several lepidopteran pests in India which are known to cause substantial damage to economically important crops. DBT (Fam:) which feeds on plants belonging to the family Brassicasese causes damage upto 16million$ per year. Another pest DMT(Fam:)

The genus Erionota is spread across South and South east Asia. The genus feeds primarily on the members of Musaceae and Arecaceae. There are two species of Erionota which occurs in the Indian subcontinent i.e Erionota thrax and Erionota torus. Both species are confined to the North eastern Region from Sikkim to Arunachal Pradesh. However, Erionota thrax has been reported in A&N . In this paper, we report the infestation of banana by Erionota torus in the western ghats, South india, along with its damage and possible control.

Materials and Methods:

Field surveys were conducted from August 2013 to December 2013 in Kerala (8.5074° N , 76.9730° E) and in August 2014 and September 2014 in Karnataka (12.9702° N , 77.5603° E). Field observations were carried out between 0800 hours to 1600 hours every day during the survey period. Sampling was carried out both in cultivated plantations and in the wild. After the establishment of infestation in the region, the following data were recorded: the number of plants infected per plantation, the life stage of E. torus, the total number of eggs/larvae/pupae infesting each plant, the larval instars present on each plant, history of E. torus damage and pesticide usage. The level of damage to infested plants was determined on a scale of 1-5; where 1 = minor damage to the plant where the infestation was seen only on few leaves without substantial damage ; 2 = substantial damage to few leaves with many or few larvae; 3 = substantial damage to the plant where few leaves have been completely eaten; 4 = massive damage to the plant where all the leaves have been eaten ; 5 = death of the whole plant due to E. torus infestation along with removal of plant by the farmer.

Phylogeny:

Larval Host Plant Data:

The larval host plant data for Erionota and its sister taxa were primarily obtained from Lepidoptera Host Plant database (HOSTS) which provides systematic information with literature references along with published literature and personal communications. (kunte, winter blyth, evans, swinhoe)

Pest species:

Erionota torus The adults are crepuscular in nature. The larvae of E .torus is known to feed on Musa paradisiaca and on members from Arecaceae.

Egg: The eggs are laid in batches ranging from 4- 40 on either side of the leaf (per. Obs.). The eggs are hemi spherical and white in colour when laid and gradually turns red.

Larvae: The larvae feed on Musa paradisiaca and on Palms. The newly hatched larvae are green in color with a black head. They feed on the egg shell after hatching and form leaf rolls in which they rest and feed. The early instars are yellow in colour with a black head. The middle and late instars are white in colour with a black head. They also have a white powder on their body and the quantity of the powder increases with advancement to the next instar. The larvae reach a length of approximately 50 mm (per. obs.) The larvae roll the leaf and feed from inside the roll. Pupation occurs inside the roll.

Pupae: The pupae are formed inside the cell (leaf roll) and are brown in color. They vibrate vigorously when handled.

The entire life cycle takes 30 – 35 days from the day of oviposition (per. obs.). (SOM Fig.1a-p).

More images of the species can be found at http://www.ifoundbutterflies.org/293-Erionota/Erionota-torus

Results:

Historical distribution and range extension:

E. torus was described by Evans W H in the year 1941 with Sikkim (India) as the type locality. Its distributional range extends from Uttarakhand in India till Taiwan in the east along with the entire Malay region. Its historical range in India was confined to the Western Himalayas from Uttarakhand to Arunachal Pradesh in the east (Fig.1a). E. torus was discovered in Ernakulam, Kerala in August 2013. This sighting from Kerala is the first report of this species in Southern India. It is a range extension from its previously known range in North East India. Subsequent surveys indicated the presence of E. torus across Kerala in 2013 and its infestation was spread to Kodagu in Karnataka in 2014. (Fig.1a)

Level of Damage:

Our field surveys in Kerala established the infestation of E. torus in 7 districts. This contributes to 50% of the districts being infested with the hesperiid in 2013. Of these 7 districts, Ernakulam was the most affected followed by Palakkad (Fig.1c). 991 banana plants were infested with E. torus which accounted to 3% of the total plant damage in Kerala. Most of the plants had minor damage due to infestation with a damage level of 1. The damage extent was high in Ernakulam a s a significant proportion of the plants were destroyed completely (Fig.1c).

E. torus was spotted in August 2014 at Kodagu in Karnataka. It was the only infested district in Karnataka from our sampling. 1946 plants were infested and constituted a significant 29.9% of the total damage. The damage extent was high with a damage level of 4 and 5 across

Larval Host Plant relations in Erionota clade:

The phylogenetic relationships of the Erionota clade is represented in the Fig. 2A. All taxa in the clade feed primarily on monocotyledons belonging to the orders Poales, Arecales, Zingiberales and Asparagales. The families Arecaceae and Poaceae are the most used hosts by the clade. The genus Erionota feeds exclusively on Musaceae and is known to occasionally feed on Arecaceae.

Discussion:

Our record of Erionota torus is the first report of this species in recent times. Erionota torus if often confused with E. thrax as both species are similar morphologically. Out record confirms the presence of Erionota torus in the Western Ghats. Since majority of the banana plantation is grown in South India, this species can pose as a potential threat to the banana marketing in the future. The caterpillars feed exclusively on the leaves. This in turn hampers the overall fruit production. The leaves are also commercially important and therefore, the damage caused by Erionota torus is economically significant.

Our results show that the infestation of E. torus increased from 3% to 29% over a period of one year. This indicates that E. torus is spreading at a faster rate across South India. It is uncertain to determine the path and period of its infestation in South India. Even though E. torus was discovered in August 2013 in Kerala and August 2014 in Karnataka, it is evident from the range of its infestation that E. torus might have been present in South India before it was discovered in August 2013. Insects are known to travel to new places through the transportation of food materials and it is highly possible that E . torus caterpillars reached South India by similar methods.

The damage level caused by E. torus during our survey in Kerala was minimal on majority of the plants. This can be attributed to the fact that majority of the plants had young caterpillars and therefore hadn’t caused much damage. The damage level was however severe in ernakulam due to the presence of late instar caterpillars in the plantations. This is due to the random sampling done at different times. The level of damage was severe in Kodagu, Karnataka due to the same reason. Late instar caterpillars feed gregariously and also make larger leaf rolls, thus inflicting more damage to the plant. Eggs and early instars are difficult to spot by farmers as they are small in size and the damage looks like a small cut which is mistaken for some other reason. Ignorance by the farmers helps in increasing their population and also in spreading the pest species to newer areas.

Along with the larvae of E. torus, several other moth larvae were seen feeding on the banana plantations in our study (SOM fig3). We however couldn’t determine the identity of those caterpillars and were excluded from our study.

Evolution of host plant use by E torus

The Erionota clade feeds on Areaceae, Poaceae and Zingiberaceae families which are economically important.

Control of E torus in South India:

The genus Erionota has x species which originated from South and South East Asia and has spread as far as Australia in the South and Hawaii in the west. Of the x species , E. thrax and E. torus have known to cause substantial damage to Banana production.

Mau et al (1980) have mentioned that the damage caused by E. thrax in Hawaii was 80% and had affected the banana industry to a great extent. E. torus also seems to have similar potential as both species share identical properties. Control of E. torus is not necessary in areas with less population. When the

References:

1 Grimaldi, David, and Michael S. Engel.Evolution of the Insects. Cambridge University Press, 2005.

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