Insect growth regulators in sericulture- towards self sufficiency in farm level manufacture

C.M. Sreejit^1,3,  Sasi Kuthannur², Chinchu Bose¹ and A. Banerji¹

¹School of Biotechnology, Amrita Viswa Vidyapeetam, Amritapuri, Kollam, Kerala, 690525.

² Kerala Sericulture Farmers Association, Nedungode kalam, Kuthannur, Palakkad, Kerala.

 ³Present address: Research Department of Botany, S. N. M. College Maliankara, Maliankara PO, Ernakulam, Kerala, 683516.

Technology innovations have helped Indian sericulture in taking giant strides ahead. Probably there are few agricultural enterprises in India, as technology intensive as sericulture. However there is a down-side to this. Some of these technologies are very costly and hence literally not accessible by farmers. The IGR technologies used in sericulture is an example. IGRs can help sericulturists a great deal by regulating larval duration for economic benefit. As most of these bio-molecules are either being imported or being replaced with costly synthetic mimetics / analogues; they are less accessible to farmers. The research group lead by Prof. A. Banerji at School of Biotechnology, Amrita Viswa Vidyapeetam, Amritapuri, Kollam, Kerala has been toiling over past few years screening candidate phyto chemical molecules with potential IGR properties and in fine tuning protocols for extraction and preparation of them at farmer level. Read on further about this work, which hold promise to sericulture farmers across developing economies.

Insect growth regulators (IGR) are routinely used in countries like, China, Japan, France for the management of sericulture and increasing silk-production. IGR’s have been comparatively recently introduced in the Indian scenario. Difficulties in procuring the reliable sources of IGR’s have been major impediment in popularizing IGR’s in India. As a part of our bio prospection of flora of Wayanad District, Kerala for bioactive molecules, several sustainable sources of insect growth regulators (IGR) were identified. Plants such as Diploclisia glaucescens (Blume) Diels, Coscinium fenestratum (Gaertn.) Colebr, Cyathula prostrata (L.) Blume, Sesuvium portulacastrum (L.) L. etc. were found to be good sources of phytoecdysoids (PEC). Eco-friendly methods of extraction and isolation of PEC’s have been developed . We also isolated and characterized several juvenoids (JHA) from indigenous plant sources.  These discoveries prompted us to undertake the field trials on application of IGR’s in sericulture. The aim of the trials was to develop and transfer technology for efficiently utilising the local plants for improving the management and increasing the yield of silk fibre for the benefit of farmers.

The field trials were initiated in the rearing facilities of one of the authors, Sasi Kuthanur Palakkad. Another farmer, Sivan from Kannadi Panchayath, Palakkad District Kerala also offered his farm for conducting trials. Both the farmers have long hands- on experience in sericulture. The study was conducted during 6 months from July 2012 to December 2012 in their farms.

Bivoltine double hybrids (FC₁ x FC₂) were brought in the second instar stage from Chawki Rearing Centre, and grown under the field conditions. Temperature was maintained at 23 + 3^oC and the relative humidity was adjusted to 75 + 5 %. 12 hours day/ night period was used during the study period. Mulberry leaves of ‘Victory 1’ genotype was obtained from a periodically watered garden and was fed to the larvae liberally three times a day. Isolation of PEC was carried out in the Phytochemistry Laboratory of Amrita School of Biotechnology, Amrita Vishwa Vidyapeetham, Kollam District, Kerala.

Amongst several sources, Diploclisia glaucescens (Blume) Diels, and Coscinium fenestratum (Gaertn.), both collected from Wayanad District , Kerala were used for the present trials. PEC’s isolated from these species have not been used in sericulture experiments earlier. PEC’s were fully characterized by their physico-chemical properties (UV, IR, NMR, TLC-UV,HPLC-DADS and LC-MS/MS).

The time and dose of application of PEC’s are very crucial. Aqueous solutions of PEC of appropriate concentrations were sprayed on to fresh V1 genotype mulberry leaves and was allowed to dry in shade for 30 minutes. The treated leaves were fed to larvae as the last feed. Mounting of larvae is a cumbersome process and require continuous care and labour. If the cocooning is not synchronised, the larvae which are late in cocooning may damage the already cocooned shells by their excretion by changing their colour to brown and reducing their marketability. PEC induces speedy and synchronised maturation of larvae when applied at the proper time. In our experiments also, PEC at 10 ppm level hastened cocooning. After 18 hours from the time of spray most of the larvae (90%) completed maturation and started spinning.

Like phytoecdysteroids,there were many plant extracts which showed juvenile hormone-like activity . One of the compounds was characterized as bakuchiol (BK) by chemical and physical (UV,IR,TLC, HPLC-DAD, NMR, MS) properties. A sample of BK was prepared as an emulsion in water. BK emulsions with different concentrations were  applied  Bombyx mori L. larvae 48 hours after the first feed in fifth instar stage. The concentration range was selected in the broad range of 1-10 ppm. The emulsions of appropriate concentrations were directly sprayed on to the larvae using a garden sprayer. After 30 minutes they were provided with fresh feed.

The effective concentration range for BK was 1-3 ppm. Above this range, linear increase in economic parameters was not obtained. Hence it was concluded that concentration of 2-3 ppm was optimum for this variety of silkworm in this geographical region.

A 12 to 20 % increase in larval weight on the sixth day of fifth instar stage was recorded within the effective concentration range of BK for the silkworm larvae from our studies in the two farms. A 15 to 25% increase was obtained within this range of BK concentration for individual cocoon weight. Shell weight also showed a similar pattern of increase with that of cocoon weight. Within the effective concentration range, 15 -30% increases were obtained with BK from the two field -levels trials. Medium control did not show significant increase over the general control with respect to cocoon weight. In our experiments linear increase of economic parameters continued till 3 ppm. Improvement in cocoon weight and shell weight on administration of synthetic JH analogues have been reported earlier. In the present trial no extension in larval period was observed when compared to medium and general controls.

Using combination of both JHA and MH on the same batch at appropriate times and dose, both increase in economic parameters and hastening of maturation occurred simultaneously with synchronisation of spinning.

As a concluding note it can be said that juvenoid, BK can be used in sericulture for yield improvement as it induces a favourable hormetic action in silkworm. In the concentration range (1-3 ppm) it brings about considerable increase in economic traits with respect to this geographical region and the bivoltine variety studied. Phytoecdysoids (PEC) is highly active in hastening the larval maturation and reducing the mounting time without any significant reduction in economic parameters. Both these compounds can be recommended for sericulture in this region. The plants are readily available and simple economic isolation procedures have been developed. The protocol for preparation of the IGR’s for spray is very simple and economical. The farmers can be trained to prepare their own IGR extracts without any significant expenditure. To our knowledge, only one or two companies supply JHA preparations which are based on imported JHA. So indigenous development of IGR’S from local flora will go long way in supplementing the income of farmers.

References

A.Banerji, A monograph :Alternate Strategies of Insect control - natural products as potential biocides. Bhabha Atomic Reserch Centre, Bombay, BARC-1416 (1988).

 N KJoshi, H. B. Mansukhani, D. R. Suryavanshi and A. Banerji, JH effects of a weed plant, Psoralea corylifolia on the metamorphosis of red cotton bug, Dysdercus koenigii. Symposium on Biological approach to problems in medicine, Industry and Agriculture, BARC, Bombay, 36 (1974).

G.R. Shivakumar, K.V.A . Raman, S.B. Magadum, R. K. Datta , S.S. Hussain , A. Banerji, S.K.Chowdhary, 1996. Effect of phytoecdysteroids on the spinning, cocoon and reeling parameters of the silkworm Bombyx mori L., Allelopathy Journal, 3, 71-76.

K. S.Nair , J.S. Nair , K. Trivedy, V.A. Vijayan, 2003. Influence of backuchiol, a JH analogue from Bemchi (Psoralia corylifolia) on silk production in silkworm Bombyx mori L.(Bombycidae: Lepidoptera). J Appl. Sci. Environ. Mgt., 7(2), 31-38.

Read previous articles on IGRs by i) A. Banerji  ii)N. Chandramohan