Silk worms on cashew
Silk, ‘queen of textiles’ stands for livelihood opportunity for 7.6 million Indians owing to its high employment oriented, low capital intensive and remunerative nature of earning.
Indian tasar silkworm: Antheraea mylitta Drury (Lepidoptera: Saturniidae)
It is one of the commercially exploited silkworms reared outdoors and distributed all along central India. Tasar cocoons are the largest among all the silk-producing insects in the world and their silk fibre has its own distinctive colour, higher tensile strength, elongation and stress-relaxation values than the mulberry silk fibre; tasar silk also protects from bad effects of ultraviolet rays. Primary host plants include Terminalia arjuna, Terminalia tomentosa and Shorea robusta while, more than 15 plants were reported as secondary hosts that includes cashew also. The larva is found in low intensity (1-3 Nos/tree) in random trees, feeds voraciously on the cashew leaves and pupates in the shoot itself.
Seasonal occurrence
A. mylitta occurs initially on young cashew plants during the monsoon season from June onwards. The population is present up to February – March on old trees as well. The eco-race of A. mylitta recorded in Puttur, Karnataka was KE-02 eco-race. The eco-race is also reported in Kozhikode, Kannur and Palakkad regions of Kerala.
Pest appearance
- Creamy flat ellipsoidal eggs are laid individually by female moths on the lower side of semi-mature cashew leaves. The part of the chorion is eaten away by the larvae upon hatching. Later, the larvae moved to tender shoots for feeding.
- In the infested trees, tender cashew leaves including midribs were eaten away by silkworms and a lot of faecal pellets can be noticed on the ground.
- Matured silkworms spin light yellow or whitish-grey cocoons with peduncles on the cashew shoots itself. Cocoons are single-shelled, pendent and oval.
- Female moths are yellowish in colour- bigger, and male moths are yellowish-brown and smaller compared to females.
Biology
- The incubation period of eggs is 6.43 days on average. Mortality of 1st instar is less, and the larvae moulted into the second instar in 4.71 days.
- The larvae could complete all five instars successfully in 26-31 days on cashew leaves, and in 25-29 days on Terminalia paniculata.
- Fecundity varied from 165 to 193 eggs/female and the egg hatchability recorded was 96.8 %.
Silk potential
- Each larva fed 35-41 tender cashew leaves of varying sizes before spinning, and feeding was voracious during the fifth instar (23-27 leaves). Fully grown tasar silkworm weighed 17 to 18 g and measured 8 to 9 cm in length.
- Upon reeling, the quality of commercial silk was found superior bestowed with comparable properties.
- The first crop of KE-02 eco-race can be raised as a seed crop during July-August, and the commercial crops during September-February.
Photo credit to tasar silk worm: K. Vanitha, ICAR-DCR, Puttur