Introduction

Thuja orientalis L. (Cupressaceae), is an evergreen species, which grown naturally in China, Korea, Japan, Iran and India2. Also, this species is widely cultivated as a common ornamental garden plant in India and other countries. A yellow dye is obtained from the young branches. The wood is used in Buddhist temples, both for construction work, and chipped, for incense burning.

Futher the species is an important medicinal plant. This plant is commonly used in Chinese herbalism. Both the leaves and the seeds contain an essential oil consisting of α-pinene, α-cedrol and ß-phellandrene8. The leaves are antibacterial, antipyretic, astringent, diuretic, emollient, expectorant, febrifuge, haemostatic, and stomachic. The root bark is used in the treatment of burns and scalds. Leaf powder has molluscidal property9. Insect may cause a considerable damage to economically important gymnosperms. Fossil record of insect-induced damage in gymnosperms date back to Middle- Upper Triassic3. Cecidomyiid leaf gall in Pinus longifolia6, Cecidomyiid stem gall in Ephedra trifurca5, defoliation of cycads by caterpillar7, mining of Podocarpous leaves by Plutellid4 and extensive leaf damage of pines by pine tube moth1 are the few reported cases of insect infestation in the extant gymnosperms.

The present paper reports an interaction between Thuja orientalis and its bagworm pest from Lower Gangetic Plains of West Bengal wherein the host plant suffered from extensive defoliation damage by the insect.

Materials and Methods

The present study was conducted in a garden located at Boalia, some 13 km south of the city of Kolkata (220 34′ N 88024′ E) during November 2009 to November, 2010. Thuja orientalis which was cultivated in the garden was kept under observation for visit of moths. Some of the caterpillars were caught alive while they are feeding. The bags or cases along with the pupa were reared and the adult was caught. Faecal pellets of the caterpillar were collected and shaken with a small amount of water. Drops of suspension were mounted in glycerine jelly and examined under microscope for any residual undigested plant parts.

Results and Discussion

Injury: The caterpillars were found feeding voraciously on the young foliage of Thuja orientalis. The principal harm done by the insect was the destruction of foliage by the caterpillars. Infested trees exhibit increasingly damaged foliage as the infestation increases until the leaves are stripped bare. The most notable sign of bagworm infestation was the presence of protective bags attached to a branch. The cases or bags were constructed from small pieces of foliage by fastening the pieces together with silk. Only one larva was found in each brown coloured bag. On average 950 eggs were found inside the bags. Faecal pellets were black in colour, rectangular in shape deposited at the bottom of the bags. The bags are elongated, measuring 4.2 cm x 2.1 cm. Within 4-5 days of bagworm infestation the host became almost completely defoliated. The adult moths were emerged after 28 days of pupation.

Insect appearance:

a) Caterpillars: Full grown caterpillars are stout, 1.9 cm to 2.54 cm long, tan brown with black spots on the head and thorax. Setae are sparsely present and they appeared naked. The larvae hatch in the spring and construct their cases and carry them about during feeding.

b) Adults: Adult moths are identified as Thyridopteryx ephemeraeformis (Lepidoptera: Psychidae). Males are small, with well developed wings, hairy, and wasp like. Females are wingless, legless and worm-like and usually never leave the bag but remain inside the bags during their short adult life.

Insects have always played an important role in the life of plants. They may be a means of pollination and also a dangerous troublesome pest as well. Most injury to plants caused by insects occurs when they use various parts of the plants for food. The most troublesome insect are those that have mouthparts which are adapted for chewing the solid, woody plant parts. The newly hatched larvae of Thyridopteryx ephemeraeformis randomly cut leafy branches of the host plant for making their bags. When fragmented parts are found unsuitable for making their bags they reject them and are either dropped to the ground or remain entangled to the plants as dry skirts. Such extreme defoliation gives the plant a nude appearance that is sufficient to reduce their aesthetic value for which they are planted. Further, it is worth mentioning that such defoliation damage will reduce the economic value of the plant to a great extent.

Presence of undigested tissue remains like tracheids, mesophyll tissue and fragments of leaf cuticle in the faecal matter of the insect are the direct evidences of feeding of the insect on both the leafy as well as woody parts of the host plant.

The bagworm’s best defense is its camouflage bag, worn throughout its life cycle. As bagworm usually infests evergreen trees, the brown bags may be overlooked at first, appearing like seed cones. The bag allows otherwise vulnerable larvae to move freely from place to place in protected condition. The bagworm lives its entire life cycle inside the safety of its bag. Only the adult male moth leaves the protection of its bag when ready to mate.

Conclusion

From the above result and discussion we can conclude that the bagworm Thyridopteryx ephemeraeformis is a serious pest of Thuja orientalis growing in the plains of West Bengal and cause severe defoliation damage to the plant that may substantially reduced their ornamental as well as economic value and it becomes a great concern to the gardener.

Fig. 1: The infestation of evergreen bagworm on Thuja orientalis.

a. Initial stage of bagworm infestation

b. Severe defoliation and bare branches with hanging bags

c. A sealed pupal bag

d. An abandoned bag

e. An open bag with a larva. Note the presence of plant twigs in the bag

f. The adult male

g. Faecal pellets of the caterpillar along with uneaten foliage

h. Tracheids recovered from faecal pellets of the caterpillar.